BRANCH-SITE SELECTION IN A GROUP-II INTRON MEDIATED BY ACTIVE RECOGNITION OF THE ADENINE AMINO GROUP AND STERIC EXCLUSION OF NON-ADENINE FUNCTIONALITIES

The 21-hydroxyl on a specific bulged adenosine is the nucleophile duri ng the first step of splicing by group II introns. To understand the m eans by which the ribozyme core recognizes this adenosine, it was muta genized and effects on catalytic activity were quantified. The results indicate that a low level of mutational variability is tolerated at t he branch-site of group II introns, with no apparent loss of fidelity. Analyses of mutant and modified nucleotides at the branch-site reveal that adenine is recognized primarily through the N6 amino group and b y steric exclusion of functionalities found on other bases. The mutati onal and single atom effects reported here contrast with those observe d during spliceosomal processing, suggesting that there are important differences in adenosine recognition by the two systems. (C) 1997 Acad emic Press Limited.