Control of branch-site choice by a group II intron

The branch site of group II introns is typically a bulged adenosine near th e 3'-end of intron domain 6. The branch site is chosen with extraordinarily high fidelity, even when the adenosine is mutated to other bases or if the typically bulged adenosine is paired. Given these facts, it has been diffi cult to discern the mechanism by which the proper branch site is chosen. In order to dissect the determinants for branch-point recognition, new mutati ons were introduced in the vicinity of the branch site and surrounding doma ins. Single mutations did not alter the high fidelity for proper branch-sit e selection. However, several combinations of mutations moved the branch si te systematically to new positions along the domain 6 stem. Analysis of tho se mutants, together with a new alignment of domain 5 and domain 6 sequence s, reveals a set of structural determinants that appear to govern branch-si te selection by group II introns.