Fcp1 directly recognizes the C-terminal domain (CTD) and interacts with a site on RNA polymerase II distinct from the CTD

Fcp1 is an essential protein phosphatase that hydrolyzes phosphoserines within the C-terminal domain (CTD) of the largest subunit of RNA polymerase II (Pol II). Fcp1 plays a major role in the regulation of CTD phosphorylation and, hence, critically influences the function of Pol II throughout the transcription cycle. The basic understanding of Fcp1-CTD interaction has remained ambiguous because two different modes have been proposed: the "dockingsite" model versus the "distributive" mechanism. Here we demonstrate biochemically that Fcp1 recognizes and dephosphorylates the CTD directly, independent of the globular non-CTD part of the Pol II structure. We point out that the recognition of CTD by the phosphatase is based on random access and is not driven by Pol II conformation. Results from three different types of experiments reveal that the overall interaction between Fcp1 and Pol II is not stable but dynamic. In addition, we show that Fcp1 also interacts with a region on the polymerase distinct from the CTD. We emphasize that this non-CTD site is functionally distinct from the docking site invoked previously as essential for the CTD phosphatase activity of Fcp1. We speculate that Fcp1 interaction with the non-CTD site may mediate its stimulatory effect on transcription elongation reported previously.