Genetic interactions between the ESS1 prolyl-isomerase and the RSP5 ubiquitin ligase reveal opposing effects on RNA polymerase II function.

Transcription of protein-coding genes by RNA polymerase II (pol II) is a highly coordinated process that requires the stepwise association of distinct protein complexes with the C-terminal domain (CTD) of Rpbl, the largest subunit of RNA pol II. Interaction of these complexes with the CTD might be subject to regulation by proteins such as Ess1 and Rsp5. Ess1, a prolyl-isomerase, binds the CTD and is thought to play a positive role in pol II transcription by generating conformational isomers of the CTD. Rsp5, a ubiquitin ligase, binds the CTD and is thought to play a negative role in transcription by mediating Rpbl ubiquitination and degradation. In this paper, we demonstrate that ESS1 and RSP5 interact genetically and that these interactions occur via RPBI. We show that over-expression of RSP5 enhances the growth defect of ess1ts cells and this effect is reversed by introducing extra copies of RPB1. Over-expression of RSP5 also mimics the sensitivity of ess1ts mutant cells to the toxicity of plasmids carrying dominant-negative CTD mutations, whereas mutations in RSP5 suppress this effect. Using a modified two-hybrid assay, we also demonstrate that Essl and Rsp5 compete directly for binding to the CTD. The results suggest a model in which Essl and Rsp5 act opposingly on pol II function to control the level of pol II available for transcription.