Differential roles of phosphorylation in the formation of transcriptional active RNA polymerase I

Regulation of rDNA transcription depends on the formation and dissociation of a functional complex between RNA polymerase I (poi I) and transcription initiation factor Rrn3p. We analyzed whether phosphorylation is involved in this molecular switch. Rrn3p is a phosphoprotein that is predominantly pho sphorylated in vivo when it is not bound to poi I. In vitro, Rrn3p is able both to associate with poi I and to enter the transcription cycle in its no nphosphorylated form. By contrast, phosphorylation of poi I is required to form a stable poi I-Rrn3p complex for efficient transcription initiation. F urthermore, association of poi I with Rrn3p correlates with a change in the phosphorylation state of poi I in vivo. We suggest that phosphorylation at specific sites of poi I is a prerequisite for proper transcription initiat ion and that phosphorylation/dephosphorylation of poi I is one possibility to modulate cellular rDNA transcription activity.