A MECHANISM TO ENHANCE MESSENGER-RNA SPLICING FIDELITY - THE RNA-DEPENDENT ATPASE PRP16 GOVERNS USAGE OF A DISCARD PATHWAY FOR ABERRANT LARIAT INTERMEDIATES

PRP16 encodes an RNA-dependent ATPase required for the second step of mRNA splicing in S. cerevisiae. We have isolated seven alleles of PRP1 6 that, like the original allele prp16-1, allow splicing of introns wi th a mutant branch site (UACUAAC to UACUACC), by forming lariat interm ediates at the mutant C nucleotide. Every suppressor mutation maps to the region of PRP16 common to RNA-dependent ATPases. We purified three of the mutant proteins and found that all exhibit reduced ATPase acti vity, as does Prp16-1. An in vivo analysis of the steady-state levels of the splicing intermediates and products provides evidence for a pat hway, under the genetic control of PRP16, to discard incorrectly branc hed substrates. We propose that decreasing the rate of ATP hydrolysis by Prp16 allows aberrantly formed lariat intermediates more time to pr oceed through the productive rather than the discard branch of this pa thway.