Functional interactions between the transcription and mRNA 3′ end processing machineries mediated by Ssu72 and Sub1

Transcription and processing of pre-mRNA are coupled events. By using a combination of biochemical, molecular, and genetic methods, we have found that the phylogenetically conserved transcription factor Ssu72 is a component of the cleavage/polyadenylation factor (CPF) of Saccharomyces cerevisiae. Our results demonstrate that Ssu72 is required for 3' end cleavage of pre-mRNA but is dispensable for poly(A) addition and RNAP II termination. The in vitro cleavage defect caused by depletion of Ssu72 from cells can be rescued by addition of recombinant Ssu72. Ssu72 interacts physically and genetically with the Pta1 subunit of CPF. Overexpression of PTA1 causes synthetic lethality in an ssu72-3 mutant. Moreover, Sub1, which has been implicated in transcription initiation and termination, also interacts with Pta1, and overexpression of SUB1 suppresses the growth and processing defect of a pta1 mutation. Physical interactions of Ssu72 and Sub1 with Pta1 are mutually exclusive. Based on the interactions of Ssu72 and Sub1 with both the Ptal of CPF and the TFIIB component of the initiation complex, we present a model describing how these novel connections between the transcription and 3' end processing machineries might facilitate transitions in the RNAP II transcription cycle.