Ssu72 Phosphatase-dependent Erasure of Phospho-Ser7 Marks on the RNA Polymerase II C-terminal Domain Is Essential for Viability and Transcription Termination

The C-terminal domain (CTD) of the largest subunit of RNA polymerase II (Pol II) serves an important role in coordinating stage-specific recruitment and release of cellular machines during transcription. Dynamic placement and removal of phosphorylation marks on different residues of a repeating heptapeptide (YSPTSPS) of the CTD underlies the engagement of relevant cellular machinery. Whereas sequential placement of phosphorylation marks is well explored, genome-wide engagement of phosphatases that remove these CTD marks is poorly understood. In particular, identifying the enzyme that erases phospho-Ser7 (Ser7-P) marks is especially important, because we find that substituting this residue with a glutamate, a phospho-mimic, is lethal. Our observations implicate Ssu72 as a Ser7-P phosphatase. We report that removal of all phospho-CTD marks during transcription termination is mechanistically coupled. An inability to remove these marks prevents Pol II from terminating efficiently and will likely impede subsequent assembly into the pre-initiation complex. Reversible phosphorylation of the RNA Polymerase II CTD coordinates co-transcriptional recruitment of factors. Ssu72 is required for erasure of phospho-serine7, and it facilitates Fcp1-mediated phospho-serine2 removal. Removal of phospho-Ser7 mark plays a key role in the transcription cycle. Persistent negative charge at position 7 of the CTD renders cells non-viable, and Ssu72 plays a prominent role in removing phospho-Ser7.