Physical and functional interaction of the yeast corepressor Tup1 with mRNA 5′-triphosphatase

The Tup1-Ssn6 complex is an important corepressor in Saccharomyces cerevisiae that inhibits transcription through interactions with the basal transcription machinery and by remodeling chromatin. In a two-hybrid screen for factors that interact with the Schizosaccharomyces pombe Tup1 ortholog, Tup11, we isolated the pct1⁺ cDNA. The pct1⁺ gene encodes an mRNA 5′-triphosphatase, which catalyzes the first step of mRNA capping reactions. Pct1 did not interact with the S. pombe Ssn6 ortholog. In vitro glutathione S-transferase pull-down experiments revealed that Pct1 binds to the WD repeat regions of Tup11 and the functionally redundant Tup12 protein. Similarly, the S. cerevisiae Tup1 protein associates with the mRNA 5′-triphosphatase encoded by the CET1 gene. The highly conserved C-terminal domain of Cet1 interacts with Tup1 in vitro, and Tup1-Ssn6 complexes co-purify with the Cet1 protein, indicating that in vivo interactions also occur between these proteins. Over-expression of CET1 compromised repression of an MFA2-lacZ reporter gene that is subject to Tup1-Ssn6 repression. These genetic and biochemical interactions between Tup1-Ssn6 and Cet1 indicate that the capping enzyme associated with RNA polymerase II is a target of the corepressor complex.