The Ssu72 Phosphatase Mediates the RNA Polymerase II Initiation-Elongation Transition [DNA and Chromosomes]

October 22nd, 2014 by Rosado-Lugo, J. D., Hampsey, M.

Transitions between the different stages of the RNAP II transcription cycle involve the recruitment and exchange of factors, including mRNA capping enzymes, elongation factors, splicing factors, the 3′-end processing complexes and termination factors. These transitions are coordinated by the dynamic phosphorylation of the C-terminal domain (CTD) of the largest subunit of RNAPII (Rpb1). The CTD is composed of reiterated heptapeptide repeats (Y1S2P3T4S5P6S7) that undergo phosphorylation and dephosphorylation as RNAPII transitions through the transcription cycle. An essential phosphatase in this process is Ssu72, which exhibits catalytic specificity for Ser5-P and Ser7-P. Ssu72 is unique in that it is specific for Ser5-P in one orientation of the CTD and for Ser7-P in the opposite orientation. Moreover, Ssu72 interacts with components of the initiation machinery and affects start site selection, yet it is an integral component of the CPF 3′-end processing complex. Here we provide a comprehensive view of the effects of Ssu72 with respect to its Ser5-P phosphatase activity. We demonstrate that Ssu72 dephosphorylates Ser5-P at the initiation-elongation transition. Furthermore, Ssu72 indirectly affects the levels of Ser2-P during the elongation stage of transcription, but does so independently of its catalytic activity.