22844859
A Functional Homing Endonuclease in the Bacillus anthracis nrdE Group I Intron
ABSTRACT The essential Bacillus anthracis nrdE gene carries a self-splicing group I intron with a putative homing endonuclease belonging to the GIY-YIG family. Here, we show that the nrdE pre-mRNA is spliced and that the homing endonuclease cleaves an intronless nrdE gene 5 nucleotides (nt) upstream of the intron insertion site, producing 2-nt 3′ extensions. We also show that the sequence required for efficient cleavage spans at least 4 bp upstream and 31 bp downstream of the cleaved coding strand. The position of the recognition sequence in relation to the cleavage position is as expected for a GIY-YIG homing endonuclease. Interestingly, nrdE genes from several other Bacillaceae were also susceptible to cleavage, with those of Bacillus cereus, Staphylococcus epidermidis (nrdE1), B. anthracis, and Bacillus thuringiensis serovar konkukian being better substrates than those of Bacillus subtilis, Bacillus lichenformis, and S. epidermidis (nrdE2). On the other hand, nrdE genes from Lactococcus lactis, Escherichia coli, Salmonella enterica serovar Typhimurium, and Corynebacterium ammoniagenes were not cleaved. Intervening sequences (IVSs) residing in protein-coding genes are often found in enzymes involved in DNA metabolism, and the ribonucleotide reductase nrdE gene is a frequent target for self-splicing IVSs. A comparison of nrdE genes from seven gram-positive low-G+C bacteria, two bacteriophages, and Nocardia farcinica showed five different insertion sites for self-splicing IVSs within the coding region of the nrdE gene.
20425871
Self-splicing of group I introns.
PERSPECTlVES . ... . .... .. ..... ... . . ........ . .... ...... .... . ... 543 REACTION PATHWAY ... . . ... ...... . ....... .. ..... . ... . ..... .. . . ...... ... . . . 545 Splicing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 545 Intron Cyclization . . . . . . ... . . . . . . .... . . . . . . . . . . . . ... . . . . . . . . . . . . . ... . ... .. . . . . . . . . . . . .. ... .. . . . . . . . 547 Splice SIte Hydrolysis ... ..... .. ...... . . ... ...... . . .. ...... . ... .... ... .. . 547 GROUP I INTRON STRUCTURE ....... . . ..... ....... ... ... . . 547 Secondary Structure and the Catalytic Core . . . .. . .. ..... . . . . .. . . . . . .. .. . .. . . . . . . . . . . . .. . . . . 548 Choosing the Reaction Sites . . . . . . . . . . . _ _ . . _ ..... . . . .... . _ . . . ... . . . ... . . . . . ... . . _ . . 550 G-Binding Site 552 Tertiary Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . . . . .... . . . .. . . . . . . . 555 ENZYMATlC REACTIONS OF THE Tetrahymena INTRON . . . ........ ..... . .. 556 CHEMISTR Y .. . ... . .... . . . .... . ... . ... . . . ...... . .. . . . ... . . . ..... .... . .. . . ... 558 ReversibUiry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 558 Stereochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 559 Metal Ion Requirements 559 Competitive Inhibition by Arginine 560 PROTEIN FACILITATION ....... ... .. . . ... .. .. .. . . . . . . ..... . 561 INTRON MOBILITY ..... ... .... . .... . . .. . . . ..... .. . .... .... . ... . . ...... . ... . . ... ...... . . . .. . . . .. . . . 563
30944271
The natural history of group I introns.
There are four major classes of introns: self-splicing group I and group II introns, tRNA and/or archaeal introns and spliceosomal introns in nuclear pre-mRNA. Group I introns are widely distributed in protists, bacteria and bacteriophages. Group II introns are found in fungal and land plant mitochondria, algal plastids, bacteria and Archaea. Group II and spliceosomal introns share a common splicing pathway and might be related to each other. The tRNA and/or archaeal introns are found in the nuclear tRNA of eukaryotes and in archaeal tRNA, rRNA and mRNA. The mechanisms underlying the self-splicing and mobility of a few model group I introns are well understood. By contrast, the role of these highly distinct processes in the evolution of the 1500 group I introns found thus far in nature (e.g. in algae and fungi) has only recently been clarified. The explosion of new sequence data has facilitated the use of comparative methods to understand group I intron evolution in a broader context and to generate hypotheses about intron insertion, splicing and spread that can be tested experimentally.
43502361
Distribution, Sequence Homology, and Homing of Group I Introns among T-even-like Bacteriophages
Self-splicing group I introns are being found in an increasing number of bacteriophages. Most introns contain an open reading frame coding for a homing endo-nuclease that confers mobility to both the intron and the homing endonuclease gene (HEG). The frequent occurrence of intron/HEG has raised questions whether group I introns are spread via horizontal transfer between phage populations. We have determined complete sequences for the known group I introns among T-even-like bacteriophages together with sequences of the intron-containing genes td, nrdB, and nrdD from phages with and without introns. A previously uncharacterized phage isolate, U5, is shown to contain all three introns, the only phage besides T4 found with a “full set” of these introns. Sequence analysis of td and nrdB genes from intron-containing and intronless phages provides evidence that recent horizontal transmission of introns has occurred among the phages. The fact that several of the HEGs have suffered deletions rendering them non-functional implies that the homing endonucleases are of no selective advantage to the phage and are rapidly degenerating and probably dependent upon frequent horizontal transmissions for maintenance within the phage populations. Several of the introns can home to closely related intronless phages during mixed infections. However, the efficiency of homing varies and is dependent on homology in regions flanking the intron insertion site. The occurrence of optional genes flanking the respective intron-containing gene can strongly affect the efficiency of homing. These findings give further insight into the mechanisms of propagation and evolution of group I introns among the T-even-like bacteriophages.
19965685
Subspecies-specific distribution of intervening sequences in the Bacillus subtilis prophage ribonucleotide reductase genes.
A collection of 212 gram-positive bacilli isolated from natural habitats was screened for the presence of intervening sequences (introns and intein-coding sequences) in the SPbeta prophage-related ribonucleotide reductase genes bnrdE and bnrdF. Three novel configurations were identified on the basis of the presence of (i) intervening sequences in bnrdE and bnrdF, and (ii) an ORF in the bnrdE-bnrdF spacer. Analysis of the cell wall genetic determinants as well as of the incorporation of radio-labelled glycerol into cell wall allowed newly and previously identified B. subtilis strains with different configurations of bnrdE/bnrdF intervening sequences to be assigned to one of two subspecies. Strains apparently belonging to the subsp. subtilis contain three intervening sequences many of which are associated with the putative homing endonuclease activity. Strains of the subsp. spizizenii contain only one or two ORF-less group I introns. Introns occupying bnrdF are confined to the subspecies subtilis.
6087662
Group I Self-Splicing Intron in the recA Gene of Bacillus anthracis
ABSTRACT Self-splicing introns are rarely found in bacteria and bacteriophages. They are classified into group I and II according to their structural features and splicing mechanisms. While the group I introns are occasionally found in protein-coding regions of phage genomes and in several tRNA genes of cyanobacteria and proteobacteria, they had not been found in protein-coding regions of bacterial genomes. Here we report a group I intron in the recA gene of Bacillus anthracis which was initially found by DNA sequencing as an intervening sequence (IVS). By using reverse transcriptase PCR, the IVS was shown to be removable from the recA precursor mRNA for RecA that was being translated in E. coli. The splicing was visualized in vitro with labeled free GTP, indicating that it is a group I intron, which is also implied by its predicted secondary structure. The RecA protein of B. anthracis expressed in E. coli was functional in its ability to complement a recA defect. When recA-negative E. coli cells were irradiated with UV, the Bacillus RecA reduced the UV susceptibility of the recA mutant, regardless of the presence of intron.
6288196
The Bacillus cereus group: novel aspects of population structure and genome dynamics
Aims:  To provide new insights into the population and genomic structure of the Bacillus cereus group of bacteria.
42886038
Ribonucleotide reductases.
Ribonucleotide reductases (RNRs) transform RNA building blocks to DNA building blocks by catalyzing the substitution of the 2'OH-group of a ribonucleotide with a hydrogen by a mechanism involving protein radicals. Three classes of RNRs employ different mechanisms for the generation of the protein radical. Recent structural studies of members from each class have led to a deeper understanding of their catalytic mechanism and allosteric regulation by nucleoside triphosphates. The main emphasis of this review is on regulation of RNR at the molecular and cellular level. Conformational transitions induced by nucleotide binding determine the regulation of substrate specificity. An intricate interplay between gene activation, enzyme inhibition, and protein degradation regulates, together with the allosteric effects, enzyme activity and provides the appropriate amount of deoxynucleotides for DNA replication and repair. In spite of large differences in the amino acid sequences, basic structural features are remarkably similar and suggest a common evolutionary origin for the three classes.
23141883
Efficient growth inhibition of Bacillus anthracis by knocking out the ribonucleotide reductase tyrosyl radical.
Bacillus anthracis, the causative agent of anthrax, is a worldwide problem because of the need for effective treatment of respiratory infections shortly after exposure. One potential key enzyme of B. anthracis to be targeted by antiproliferative drugs is ribonucleotide reductase. It provides deoxyribonucleotides for DNA synthesis needed for spore germination and growth of the pathogen. We have cloned, purified, and characterized the tyrosyl radical-carrying NrdF component of B. anthracis class Ib ribonucleotide reductase. Its EPR spectrum points to a hitherto unknown three-dimensional geometry of the radical side chain with a 60 degrees rotational angle of C(alpha)-(C(beta)-C(1))-plane of the aromatic ring. The unusual relaxation behavior of the radical signal and its apparent lack of line broadening at room temperature suggest a weak interaction with the nearby diiron site and the presence of a water molecule plausibly bridging the phenolic oxygen of the radical to a ligand of the diiron site. We show that B. anthracis cells are surprisingly resistant to the radical scavenger hydroxyurea in current use as an antiproliferative drug, even though its NrdF radical is efficiently scavenged in vitro. Importantly, the antioxidants hydroxylamine and N-methyl hydroxylamine scavenge the radical several orders of magnitude faster and prevent B. anthracis growth at several hundred-fold lower concentrations compared with hydroxyurea. Phylogenetically, the B. anthracis NrdF protein clusters together with NrdFs from the pathogens Bacillus cereus, Bacillus thuringiensis, Staphylococcus aureus, and Staphylococcus epidermidis. We suggest the potential use of N-hydroxylamines in combination therapies against infections by B. anthracis and closely related pathogens.
17032703
Homing endonucleases: keeping the house in order.
Homing endonucleases are rare-cutting enzymes encoded by introns and inteins. They have striking structural and functional properties that distinguish them from restriction enzymes. Nomenclature conventions analogous to those for restriction enzymes have been developed for the homing endonucleases. Recent progress in understanding the structure and function of the four families of homing enzymes is reviewed. Of particular interest are the first reported structures of homing endonucleases of the LAGLIDADG family. The exploitation of the homing enzymes in genome analysis and recombination research is also summarized. Finally, the evolution of homing endonucleases is considered, both at the structure-function level and in terms of their persistence in widely divergent biological systems.
27841011
Homing endonuclease structure and function
Homing endonucleases are encoded by open reading frames that are embedded within group I, group II and archael introns, as well as inteins (intervening sequences that are spliced and excised post-translationally). These enzymes initiate transfer of those elements (and themselves) by generating strand breaks in cognate alleles that lack the intervening sequence, as well as in additional ectopic sites that broaden the range of intron and intein mobility. Homing endonucleases can be divided into several unique families that are remarkable in several respects: they display extremely high DNA-binding specificities which arise from long DNA target sites (14–40 bp), they are tolerant of a variety of sequence variations in these sites, and they display disparate DNA cleavage mechanisms. A significant number of homing endonucleases also act as maturases (highly specific cofactors for the RNA splicing reactions of their cognate introns). Of the known homing group I endonuclease families, two (HNH and His-Cys box enzymes) appear to be diverged from a common ancestral nuclease. While crystal structures of several representatives of the LAGLIDADG endonuclease family have been determined, only structures of single members of the HNH (I-HmuI), His-Cys box (I-PpoI) and GIY-YIG (I-TevI) families have been elucidated. These studies provide an important source of information for structure–function relationships in those families, and are the centerpiece of this review. Finally, homing endonucleases are significant targets for redesign and selection experiments, in hopes of generating novel DNA binding and cutting reagents for a variety of genomic applications.
22547528
Recurrent invasion and extinction of a selfish gene.
Homing endonuclease genes show super-Mendelian inheritance, which allows them to spread in populations even when they are of no benefit to the host organism. To test the idea that regular horizontal transmission is necessary for the long-term persistence of these genes, we surveyed 20 species of yeasts for the omega-homing endonuclease gene and associated group I intron. The status of omega could be categorized into three states (functional, nonfunctional, or absent), and status was not clustered on the host phylogeny. Moreover, the phylogeny of omega differed significantly from that of the host, strong evidence of horizontal transmission. Further analyses indicate that horizontal transmission is more common than transposition, and that it occurs preferentially between closely related species. Parsimony analysis and coalescent theory suggest that there have been 15 horizontal transmission events in the ancestry of our yeast species, through simulations indicate that this value is probably an underestimate. Overall, the data support a cyclical model of invasion, degeneration, and loss, followed by reinvasion, and each of these transitions is estimated to occur about once every 2 million years. The data are thus consistent with the idea that frequent horizontal transmission is necessary for the long-term persistence of homing endonuclease genes, and further, that this requirement limits these genes to organisms with easily accessible germ lines. The data also show that mitochondrial DNA sequences are transferred intact between yeast species; if other genes do not show such high levels of horizontal transmission, it would be due to lack of selection, rather than lack of opportunity.
1923698
Inteins, introns, and homing endonucleases: recent revelations about the life cycle of parasitic genetic elements
Self splicing introns and inteins that rely on a homing endonuclease for propagation are parasitic genetic elements. Their life-cycle and evolutionary fate has been described through the homing cycle. According to this model the homing endonuclease is selected for function only during the spreading phase of the parasite. This phase ends when the parasitic element is fixed in the population. Upon fixation the homing endonuclease is no longer under selection, and its activity is lost through random processes. Recent analyses of these parasitic elements with functional homing endonucleases suggest that this model in its most simple form is not always applicable. Apparently, functioning homing endonuclease can persist over long evolutionary times in populations and species that are thought to be asexual or nearly asexual. Here we review these recent findings and discuss their implications. Reasons for the long-term persistence of a functional homing endonuclease include: More recombination (sexual and as a result of gene transfer) than previously assumed for these organisms; complex population structures that prevent the element from being fixed; a balance between active spreading of the homing endonuclease and a decrease in fitness caused by the parasite in the host organism; or a function of the homing endonuclease that increases the fitness of the host organism and results in purifying selection for the homing endonuclease activity, even after fixation in a local population. In the future, more detailed studies of the population dynamics of the activity and regulation of homing endonucleases are needed to decide between these possibilities, and to determine their relative contributions to the long term survival of parasitic genes within a population. Two outstanding publications on the amoeba Naegleria group I intron (Wikmark et al. BMC Evol Biol 2006, 6: 39) and the PRP8 inteins in ascomycetes (Butler et al.BMC Evol Biol 2006, 6: 42) provide important stepping stones towards integrated studies on how these parasitic elements evolve through time together with, or despite, their hosts.
42611429
Complex group-I introns in nuclear SSU rDNA of red and green algae: evidence of homing-endonuclease pseudogenes in the Bangiophyceae
Abstract The green alga Scenedesmus pupukensis and the red alga Porphyra spiralis contain large group-IC1 introns in their nuclear small subunit ribosomal RNA genes due to the presence of open reading frames at the 5′ end of the introns. The putative 555 amino-acid Scenedesmus-encoded protein harbors a sequence motif resembling the bacterial S9 ribosomal proteins. The Porphyra intron self-splices in vitro, and generates both ligated exons and a full-length intron RNA circle. The Porphyra intron has an unusual structural organization by encoding a potential 149 amino-acid homing-endonuclease-like protein on the complementary strand. A comparison between related group-I introns in the Bangiophyceae revealed homing-endonuclease-like pseudogenes due to frame-shifts and deletions in Porphyra and Bangia. The Scenedesmus and Porphyra introns provide new insights into the evolution and possible novel functions of nuclear group-I intron proteins.
6517131
SegH and Hef: two novel homing endonucleases whose genes replace the mobC and mobE genes in several T4-related phages
T4 contains two groups of genes with similarity to homing endonucleases, the seg-genes (similarity to endonucleases encoded by group I introns) containing GIY-YIG motifs and the mob-genes (similarity to mobile endonucleases) containing H-N-H motifs. The four seg-genes characterized to date encode homing endonucleases with cleavage sites close to their respective gene loci while none of the mob-genes have been shown to cleave DNA. Of 18 phages screened, only T4 was found to have mobC while mobE genes were found in five additional phages. Interestingly, three phages encoded a seg-like gene (hereby called segH) with a GIY-YIG motif in place of mobC. An additional phage has an unrelated gene called hef (homing endonuclease-like function) in place of the mobE gene. The gene products of both novel genes displayed homing endonuclease activity with cleavage site specificity close to their respective genes. In contrast to intron encoded homing endonucleases, both SegH and Hef can cleave their own DNA as well as DNA from phages without the genes. Both segH and mobE (and most likely hef) can home between phages in mixed infections. We discuss why it might be a selective advantage for phage freestanding homing endonucleases to cleave both HEG-containing and HEG-less genomes.
7260461
CLUSTAL W: improving the sensitivity of progressive multiple sequence alignment through sequence weighting, position-specific gap penalties and weight matrix choice.
The sensitivity of the commonly used progressive multiple sequence alignment method has been greatly improved for the alignment of divergent protein sequences. Firstly, individual weights are assigned to each sequence in a partial alignment in order to down-weight near-duplicate sequences and up-weight the most divergent ones. Secondly, amino acid substitution matrices are varied at different alignment stages according to the divergence of the sequences to be aligned. Thirdly, residue-specific gap penalties and locally reduced gap penalties in hydrophilic regions encourage new gaps in potential loop regions rather than regular secondary structure. Fourthly, positions in early alignments where gaps have been opened receive locally reduced gap penalties to encourage the opening up of new gaps at these positions. These modifications are incorporated into a new program, CLUSTAL W which is freely available.
10971295
TreeView: an application to display phylogenetic trees on personal computers
TreeView is a simple, easy to use phylogenetic tree viewing utility that runs under both MacOS (on Apple Macintosh computers) and under Microsoft Windows on Intel based computers, the two most common personal computers used by biologists. Some phylogeny programs, such as PAUP (Swofford, 1993) and MacClade (Maddison and Maddison, 1992) already provide excellent tree drawing and printing facilities, however at present these programs are restricted to Apple Macintosh computers. Furthermore, they require the user to load a data set before any trees can be displayed which is inconvenient if the user simply wants to view the trees. More portable programs, such as DRAWGRAM and DRAWTREE in the PHYLIP package (Felsenstein, 1993) can run on both MacOS and Windows computers, but make little, if any use of the graphical interface features available under those operating systems. TreeView runs as a native application on either MacOS or Windows computers, enables the user to use the standard fonts installed on their machine, their printer, and supports the relevant native graphics format (PICT and Windows metafile) for either creating graphics files or pasting pictures to other applications via the clipboard. The program also supports standard file operations, such as 'drag and drop' whereby dragging a file's icon onto the program opens that file. TreeView can read a range of tree file formats (see below) and can display trees in a range of styles (Fig. 1). Additional information, such as edge lengths and internal node labels can also be displayed. The order of the terminal taxa in the tree can be altered, and the tree can be rerooted. If the tree file contains more than one tree the user can view each tree in turn. The program can also save trees in a variety of file formats, so that it can be used to move trees between programs that use different file formats.
19240465
Modelling of the three-dimensional architecture of group I catalytic introns based on comparative sequence analysis.
Alignment of the 87 available sequences of group I self-splicing introns reveals numerous instances of covariation between distant sites. Some of these covariations cannot be ascribed to historical coincidences or the known secondary structure of group I introns, and are, therefore, best explained as reflecting tertiary contacts. With the help of stereochemical modelling, we have taken advantage of these novel interactions to derive a three-dimensional model of the conserved core of group I introns. Two noteworthy features of that model are its extreme compactness and the fact that all of the most evolutionarily conserved residues happen to converge around the two helices that constitute the substrate of the core ribozyme and the site that binds the guanosine cofactor necessary for self-splicing. Specific functional implications are discussed, both with regard to the way the substrate helices are recognized by the core and possible rearrangements of the introns during the self-splicing process. Concerning potential long-range interactions, emphasis is put on the possible recognition of two consecutive purines in the minor groove of a helix by a GAAA or related terminal loop.
8918668
Ribonucleotide reductase genes of Bacillus prophages: a refuge to introns and intein coding sequences.
The ribonucleotide reductase gene tandem bnrdE/bnrdF in SPbeta-related prophages of different Bacillus spp. isolates presents different configurations of intervening sequences, comprising one to three of six non-homologous splicing elements. Insertion sites of group I introns and intein DNA are clustered in three relatively short segments encoding functionally important domains of the ribonucleotide reductase. Comparison of the bnrdE homologs reveals mutual exclusion of a group I intron and an intein coding sequence flanking the codon that specifies a conserved cysteine. In vivo splicing was demonstrated for all introns. However, for two of them a part of the mRNA precursor molecules remains unspliced. Intergenic bnrdE-bnrdF regions are unexpectedly long, comprising between 238 and 541 nt. The longest encodes a putative polypeptide related to HNH homing endonucleases.
6021935
Improved detection of helix-turn-helix DNA-binding motifs in protein sequences.
We present an update of our method for systematic detection and evaluation of potential helix-turn-helix DNA-binding motifs in protein sequences [Dodd, I. and Egan, J. B. (1987) J. Mol. Biol. 194, 557-564]. The new method is considerably more powerful, detecting approximately 50% more likely helix-turn-helix sequences without an increase in false predictions. This improvement is due almost entirely to the use of a much larger reference set of 91 presumed helix-turn-helix sequences. The scoring matrix derived from this reference set has been calibrated against a large protein sequence database so that the score obtained by a sequence can be used to give a practical estimation of the probability that the sequence is a helix-turn-helix motif.
6517043
CDD: a Conserved Domain Database for protein classification
The Conserved Domain Database (CDD) is the protein classification component of NCBI's Entrez query and retrieval system. CDD is linked to other Entrez databases such as Proteins, Taxonomy and PubMed®, and can be accessed at http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=cdd. CD-Search, which is available at http://www.ncbi.nlm.nih.gov/Structure/cdd/wrpsb.cgi, is a fast, interactive tool to identify conserved domains in new protein sequences. CD-Search results for protein sequences in Entrez are pre-computed to provide links between proteins and domain models, and computational annotation visible upon request. Protein–protein queries submitted to NCBI's BLAST search service at http://www.ncbi.nlm.nih.gov/BLAST are scanned for the presence of conserved domains by default. While CDD started out as essentially a mirror of publicly available domain alignment collections, such as SMART, Pfam and COG, we have continued an effort to update, and in some cases replace these models with domain hierarchies curated at the NCBI. Here, we report on the progress of the curation effort and associated improvements in the functionality of the CDD information retrieval system.
43471646
SWISS‐MODEL and the Swiss‐Pdb Viewer: An environment for comparative protein modeling
Comparative protein modeling is increasingly gaining interest since it is of great assistance during the rational design of mutagenesis experiments. The availability of this method, and the resulting models, has however been restricted by the availability of expensive computer hardware and software. To overcome these limitations, we have developed an environment for comparative protein modeling that consists of SWISS‐MODEL, a server for automated comparative protein modeling and of the SWISS‐PdbViewer, a sequence to structure workbench. The Swiss‐PdbViewer not only acts as a client for SWISS‐MODEL, but also provides a large selection of structure analysis and display tools. In addition, we provide the SWISS‐MODEL Repository, a database containing more than 3500 automatically generated protein models. By making such tools freely available to the scientific community, we hope to increase the use of protein structures and models in the process of experiment design.
22834442
Lightning strikes twice: intron-intein coincidence.
“Multiple introns, and the prospect that these occur within several genes in the same metabolic pathway, suggest a regulatory role for splicing … ” (1). That statement was made more than a decade ago by our colleagues and ourselves in reference to the phage T4 group I introns, all three of which reside in genes involved in nucleotide metabolism. The intervening years were distinguished by a striking absence of any demonstration of a regulatory role for these introns. Furthermore, they seem not to confer any selective advantage to T4 phage, and, despite the discovery of more introns in genes of DNA metabolism, the question of “why exclusively in those genes?” has been all but forgotten. Now we are faced with a report in this issue of the Proceedings of an unrelated phage and a different type of splicing element, an intein; not only does this element coexist with a group I intron in the same gene but also the gene is one of nucleotide metabolism (2). With lightning apparently striking twice in the same place, we are again forced to confront our doubts about pure chance. What might account for the colocalization of two different types of intervening sequence in the same gene on the same metabolic pathway?
12554464
Two self-splicing group I introns in the ribonucleotide reductase large subunit gene of Staphylococcus aureus phage Twort.
We have recently described three group I introns inserted into a single gene, orf142, of the staphylococcal bacteriophage Twort and suggested the presence of at least two additional self-splicing introns in this phage genome. Here we report that two previously uncharacterized introns, 429 and 1087 nt in length, interrupt the Twort gene coding for the large subunit of ribonucleotide reductase (nrdE). Reverse transcription-polymerase chain reaction (RT-PCR) of RNA isolated from Staphylococcus aureus after phage infection indicates that the introns are removed from the primary transcript in vivo. Both nrdE introns show sequence similarity to the Twort orf142 introns I2 and I3, suggesting either a common origin of these introns or shuffling of intron structural elements. Intron 2 encodes a DNA endonuclease, I-TwoI, with similarity to homing endonucleases of the HNH family. Like I-HmuI and I-HmuII, intron-encoded HNH endonucleases in Bacillus subtilis phages SPO1 and SP82, I-TwoI nicks only one strand of its DNA recognition sequence. However, whereas I-HmuI and I-HmuII cleave the template strand in exon 2, I-TwoI cleaves the coding strand in exon 1. In each case, the 3' OH created on the cut strand is positioned to prime DNA synthesis towards the intron, suggesting that this reaction contributes to the mechanism of intron homing. Both nrdE introns are inserted in highly conserved regions of the ribonucleotide reductase gene, next to codons for functionally important residues.
41537423
Insights on Evolution of Virulence and Resistance from the Complete Genome Analysis of an Early Methicillin-Resistant Staphylococcus aureus Strain and a Biofilm-Producing Methicillin-Resistant Staphylococcus epidermidis Strain
ABSTRACT Staphylococcus aureus is an opportunistic pathogen and the major causative agent of numerous hospital- and community-acquired infections. Staphylococcus epidermidis has emerged as a causative agent of infections often associated with implanted medical devices. We have sequenced the ∼2.8-Mb genome of S. aureus COL, an early methicillin-resistant isolate, and the ∼2.6-Mb genome of S. epidermidis RP62a, a methicillin-resistant biofilm isolate. Comparative analysis of these and other staphylococcal genomes was used to explore the evolution of virulence and resistance between these two species. The S. aureus and S. epidermidis genomes are syntenic throughout their lengths and share a core set of 1,681 open reading frames. Genome islands in nonsyntenic regions are the primary source of variations in pathogenicity and resistance. Gene transfer between staphylococci and low-GC-content gram-positive bacteria appears to have shaped their virulence and resistance profiles. Integrated plasmids in S. epidermidis carry genes encoding resistance to cadmium and species-specific LPXTG surface proteins. A novel genome island encodes multiple phenol-soluble modulins, a potential S. epidermidis virulence factor. S. epidermidis contains the cap operon, encoding the polyglutamate capsule, a major virulence factor in Bacillus anthracis. Additional phenotypic differences are likely the result of single nucleotide polymorphisms, which are most numerous in cell envelope proteins. Overall differences in pathogenicity can be attributed to genome islands in S. aureus which encode enterotoxins, exotoxins, leukocidins, and leukotoxins not found in S. epidermidis.
36615524
The complete genomic sequence of Nocardia farcinica IFM 10152.
We determined the genomic sequence of Nocardia farcinica IFM 10152, a clinical isolate, and revealed the molecular basis of its versatility. The genome consists of a single circular chromosome of 6,021,225 bp with an average G+C content of 70.8% and two plasmids of 184,027 (pNF1) and 87,093 (pNF2) bp with average G+C contents of 67.2% and 68.4%, respectively. The chromosome encoded 5,674 putative protein-coding sequences, including many candidate genes for virulence and multidrug resistance as well as secondary metabolism. Analyses of paralogous protein families suggest that gene duplications have resulted in a bacterium that can survive not only in soil environments but also in animal tissues, resulting in disease.
9068245
The Ribosomal Database Project (RDP-II): sequences and tools for high-throughput rRNA analysis
The Ribosomal Database Project (RDP-II) provides the research community with aligned and annotated rRNA gene sequences, along with analysis services and a phylogenetically consistent taxonomic framework for these data. Updated monthly, these services are made available through the RDP-II website (http://rdp.cme.msu.edu/). RDP-II release 9.21 (August 2004) contains 101 632 bacterial small subunit rRNA gene sequences in aligned and annotated format. High-throughput tools for initial taxonomic placement, identification of related sequences, probe and primer testing, data navigation and subalignment download are provided. The RDP-II email address for questions or comments is rdpstaff@msu.edu.
462935
Induction of competence regulons as a general response to stress in gram-positive bacteria.
Bacterial transformation, a programmed mechanism for genetic exchange originally discovered in Streptococcus pneumoniae, is widespread in bacteria. It is based on the uptake and integration of exogenous DNA into the recipient genome. This review examines whether induction of competence for genetic transformation is a general response to stress in gram-positive bacteria. It compares data obtained with bacteria chosen for their different lifestyles, the soil-dweller Bacillus subtilis and the major human pathogen S. pneumoniae. The review focuses on the relationship between competence and other global responses in B. subtilis, as well as on recent evidence for competence induction in response to DNA damage or antibiotics and for the ability of S. pneumoniae to use competence as a substitute for SOS. This comparison reveals that the two species use different fitness-enhancing strategies in response to stress conditions. Whereas B. subtilis combines competence and SOS induction, S. pneumoniae relies only on competence to generate genetic diversity through transformation.
205499369
DNA uptake during bacterial transformation
Naturally competent bacteria are able to take up exogenous DNA and undergo genetic transformation. The transport of DNA from the extracellular milieu into the cytoplasm is a complex process, and requires proteins that are related to those involved in the assembly of type IV pili and type II secretion systems, as well as a DNA translocase complex at the cytoplasmic membrane. Here, we will review the current knowledge of DNA transport during transformation.