November 2009 news and views

Research highlights

PSI-SGKB [doi:10.1038/nchembio.230-psi1]

Bringing biochemical perspective

X-ray crystallography has provided key structural insights into ribozyme catalysis but also has led to new mechanistic questions. Numerous crystallographic studies of the group I intron, one of the first catalytic RNAs to be discovered, have resulted in different structural models of the ribozyme active site and conflicting views of some of the molecular details of catalysis. For instance, it is known that the 2´-hydroxyl of a highly conserved adenosine residue (A261) of the Tetrahymena ribozyme is essential for catalysis. However, crystallographic studies of two group I introns have suggested two conflicting models for how the 2´-hydroxyl of A261 binds the key guanosine cofactor in the splicing reaction. Forconi et al. now apply functional group mutagenesis and mutant cycle analysis to reconcile these structural models of the group I ribozyme. The authors prepared A261 ribozyme variants in which the hydrogen bonding ability of the 2´-hydroxyl of A261 was blocked, as well as a series of compensatory base functional group mutations in the splicing substrates. After measuring rate constants for the mutant ribozyme-substrate pairs and applying double-mutant cycle analysis, the authors provided data consistent with the structural model in which the 2´-hydroxyl of A261 helps position the guanosine nucleophile at the active site through a hydrogen bond to the guanosine exocyclic amino group. In addition to clarifying key interactions in the group I ribozyme active site, the study reminds us of the important role for chemical biology probes to test and extend structural data. ( Angew. Chem. Int. Ed., published online 25 August 2009, doi:10.1002/anie.200903006) TLS