Ipa1 Is an RNA Polymerase II Elongation Factor that Facilitates Termination by Maintaining Levels of the Poly(A) Site Endonuclease Ysh1.

The yeast protein Ipa1 was recently discovered to interact with the Ysh1 endonuclease of the pre-mRNA cleavage and polyadenylation (C/P) machinery, and Ipa1 mutation impairs 3'end processing. We report that Ipa1 globally promotes proper transcription termination and poly(A) site selection, but with variable effects on genes depending upon the specific configurations of polyadenylation signals. Our findings suggest that the role of Ipa1 in termination is mediated through interaction with Ysh1, since Ipa1 mutation leads to decrease in Ysh1 and poor recruitment of the C/P complex to a transcribed gene. The Ipa1 association with transcriptionally active chromatin resembles that of elongation factors, and the mutant shows defective Pol II elongation kinetics in vivo. Ysh1 overexpression in the Ipa1 mutant rescues the termination defect, but not the mutant's sensitivity to 6-azauracil, an indicator of defective elongation. Our findings support a model in which an Ipa1/Ysh1 complex helps coordinate transcription elongation and 3' end processing.