RNA splicing is the removal of an intron and the simultaneous ligation of its flanking exons in the generation of[unreadable] mature cellular RNA molecules. Splicing provides a critical level of genetic control. Through alternative splicing, the[unreadable] proteome of a higher eukaryote is substantially more complex than the number of genes in its genome. The[unreadable] importance of RNA splicing to human health is manifest by the observation that at least 15% of point mutations[unreadable] leading to heritable human diseases cause defects in pre-mRNA splicing. While most introns are removed by the[unreadable] spliceosome, some introns are able to catalyze their own removal from the primary transcript. The discovery of the[unreadable] group I class of introns provided the first indication that not all enzymes are proteins. Their existence proves that[unreadable] RNA can select 5' and 3' splice sites and catalyze the two transesterification reactions of intron removal. Our[unreadable] understanding of the structural basis of RNA splicing is still in its infancy, but in 2004, almost 25 years after its[unreadable] initial discovery, the first crystal structure of a complete group I intron in complex with both its 5' and 3' exons was[unreadable] finally determined. This result will now be exploited to achieve several important objectives relevant to the[unreadable] structural basis of RNA splicing and the dynamic conformational rearrangements that occur during the splicing[unreadable] process. The overriding goals of these studies are: (i) to determine the X-ray crystal structure of each step in an RNA[unreadable] splicing pathway, (ii) to explain how RNA tertiary structure is formed and active sites created in the absence of[unreadable] proteins, (iii) to reveal how metal ions contribute to RNA catalysis and how alteration of ligands affects metal ion[unreadable] specificity, (iv) to visualize the nature of the transition state of the phosphoryl transfer reaction promoted during the[unreadable] 5'-exon cleavage and exon ligation reactions, and (v) to explore how group I intron splicing is facilitated by protein[unreadable] cofactors. Many ribonucleoprotein complexes are expected to undergo complex conformational changes during their[unreadable] function, so understanding how an RNA is reconfigured during a multi-step splicing reaction will provide a valuable[unreadable] precedent for considering these complex dynamic processes. RNA enzymes, or ribozymes, are the molecules most[unreadable] likely to be the progenitors of modern biological catalysts, and understanding how they promote their reactions will[unreadable] provide critical insight into enzymological function. This series of structural snapshots will reveal principles of[unreadable] RNA folding, structural dynamics, and metal mediated catalysis, principles that are certain to have parallels in most[unreadable] cellular machines that include RNA.