REQUIREMENT FOR SLU7 IN YEAST PRE-MESSENGER-RNA SPLICING IS DICTATED BY THE DISTANCE BETWEEN THE BRANCHPOINT AND THE 3' SPLICE-SITE

Yeast pre-mRNA splicing factors SLU7 and PRP16 are required for cleava ge of the 3' splice site and exon ligation in vitro. Using natural a n d model precursor RNAs, we found that SLU7 is dispensable for splicing of RNAs in which the 3' splice site is in close proximity to the bran chpoint. SLU7 is only required when the interval between the branchpoi nt and the 3' splice site is greater than 7 nt. In contrast, PRP16 is essential for splicing of all pre-mRNAs tested. Immunoprecipitation of the products of step 1 by anti-SLU7 antibodies demonstrates that SLU7 is a component of the spliceosome. Recruitment of SLU7 to the spliceo some is greatly enhanced by prior addition of PRP16 PRP16 is liberated from the spliceosome after completion of step 2, whereas SLU7 remains bound to the excised intron and spliced mature RNA until the spliceos ome disassembles, in a reaction that requires ATP.