Requirement for SLU7 in yeast pre-mRNA 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 cleavage of the 3' splice site and exon ligation in vitro. Using natural and 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 branchpoint. SLU7 is only required when the interval between the branchpoint 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 spliceosome 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 spliceosome disassembles, in a reaction that requires ATP.