Dephosphorylation of RNA polymerase I by Fcp1p is required for efficient rRNA synthesis.

Differently phosphorylated forms of RNA polymerase (Pol) II are required to guide the enzyme through the transcription cycle. Here, we show that a phosphorylation/dephosphorylation cycle is also important for RNA polymerase I-dependent synthesis of rRNA precursors. A key component of the Pol II transcription system is Fcp1p, a phosphatase that dephosphorylates the C-terminal domain of the largest Pol II subunit. Fcp1p stimulates transcription elongation and is required for Pol II recycling after transcription termination. We found that Fcp1p is also part of the RNA Pol I transcription apparatus. Fcp1p is required for efficient rDNA transcription in vivo, and also, recombinant Fcp1p stimulates rRNA synthesis both in promoter-dependent and in nonspecific transcription assays in vitro. We demonstrate that Fcp1 activity is not involved in the formation of the initiation-active form of Pol I (the Pol I-Rrn3p complex) and propose that dephosphorylation of Pol I by Fcp1p facilitates chain elongation during rRNA synthesis.