Genetic interactions with C-terminal domain (CTD) kinases and the CTD of RNA Pol II suggest a role for ESS1 in transcription initiation and elongation in Saccharomyces cerevisiae.

Ess1 is an essential prolyl isomerase that binds the C-terminal domain (CTD) of Rpb1, the large subunit of RNA polymerase II. Ess1 is proposed to control transcription by isomerizing phospho-Ser-Pro peptide bonds within the CTD repeat. To determine which step(s) in the transcription cycle might require Ess1, we examined genetic interactions between ESS1 and genes encoding the known CTD kinases (KIN28, CTK1, BUR1, and SRB10). Although genetic interactions were identified between ESS1 and all four kinases, the clearest interactions were with CTK1 and SRB10. Reduced dosage of CTK1 rescued the growth defect of ess1(ts) mutants, while overexpression of CTK1 enhanced the growth defects of ess1(ts) mutants. Deletion of SRB10 suppressed ess1(ts) and ess1Delta mutants. The interactions suggest that Ess1 opposes the functions of these kinases, which are thought to function in preinitiation and elongation. Using a series of CTD substitution alleles, we also identified Ser5-Pro6 as a potential target for Ess1 isomerization within the first "half" of the CTD repeats. On the basis of the results, we suggest a model in which Ess1-directed conformational changes promote dephosphorylation of Ser5 to stimulate preinitiation complex formation and, later, to inhibit elongation.