CAP in LAC operon is an example of:(AIIMS Nov 2011, May 2011)
Ans. a. Positive regulator (Ref: Lippincott 4/e p418-420)CAP in LAC operon is an example of positive regulator."CAP-cAMP regulator is a positive regulator for Lac operonQ because its presence is required for gene expression."Operon is the segment of DN A strand, consisting of cluster of several genes involved in metabolic pathway.Operon = structural genes coding for enzymes of metabolic pathway + regulatory genes that determine their transcriptionStructural genes for Lac operonCoded proteinRegulator genes/ proteins and inducersLac Zb galactosidasePositive regulatorCRP = CAP + CAMPLac YPermeaseNegative regulatorLac 1 geneRepressor proteinLac AAcetylaseInducerLactoseIPTG (gratuitous inducer)Jacob and Monod's Operon Concept for Prokaryotic GeneIn prokaryotes, genes involved in a metabolic pathway are often present in a linear array called an operon.The cistron is the smallest unit of genetic expression coding for subunit of a protein molecule (One cistron-one subunit concept)Q.Polycistronie mRNA, predominantly found in prokaryotes is a m-RNA that encodes more than one separately translated proteins.Operon (transcription unit of prokaryotes) consists of structural (coding) genes and regulatory region consistingQ.Structural (coding) genesCarries the codon that code for proteins involved in a particular metabolic pathwayIn lac operon of E coli (responsible for lactose metabolism), 3 structural genes lac Z, lac Y, and lac A code respectively for b-galactosidase, (hydrolyze lactose or b- galactoside to galactose and glucose), permease (responsible for permeation of galactose into cell), and thiogalactosida transacetylaseQ.Inducible regulated genes: Their expression increases in response to an inducer or activator (a positive regulatory signal). In general, inducible genes have relatively low rates of transcriptionConstitutive (house keeping) genes: They are expressed at a reasonably constant rate and are not subjected to regulationQ. The product of these genes are required all the timeQ in a cell. A mutation resulting in constitutive expression of what was formerly a regulated gene is called constitutive mutationRegulatory region consistingPromoter (P) geneOperator (O) genesRegulator (I) genes* It is situated on the upstream (5'- side of transcription start site between operator and regulator gene.* It is the site, where DNA dependent- RNA polymerase (RNP) binds to form pre-initiation complex (PIC)Q, and commence transcription.* Catabolite gene activator protein (CAP) in conjunction with c-AMP (known as c-AMP regulatory protein = CRP) is required for efficient formation of PIC. So it is a positive regulator. Adenylate cyclase which forms c-AMP from ATP, is activated by absence of glucose. So the lac operon transcription only occurs in absence of glucoseQ (catabolite repression). And presence of glucose prevents it by depleting c-AMP.* It is a segment of ds DNA that regulates the activity of the structural genes.* In lac operon, it is 27 bp long segment situated between promoter site (at which RNAP binds) and lac Z (transcription initiation site) and show a twofold rotational symmetry and an inverted palindrome in 21 bp long region* When attached to operator (o) gene, the repressor (Lacl) acts as a negative regulator and prevents transcription of operator and structural (lac Z/Y/A) genesQ.* It is a constitutive gene, that forms subunits of lac repressor (MW 38,000), a lac repressor molecule (Lac I repressor protein) is formed by union of 4 subunits and has high affinity for operator locusQ.* At any time only 2 subunits of repressor binds to operator, mostly in its major groove without interrupting the base paired double helical nature of operator DNA.* Normally 20-40 repressor tetramers, occupy >95% of lac operator, thus ensuring a low lac operon transcription in absence of inducer.* Presence of lactose or gratuitous inducer such as isopropylthio galactoside (IPTG) on bacteria growing on poorly utilized carbon source (e.g. succinate) results induction of lac operon enzymes by de-repression.
Positive regulator
Negative regulator
Attenuation
Constitutive expression
Regulation of Gene Expression 