EVIDENCE FOR A MEDIATOR CYCLE AT THE INITIATION OF TRANSCRIPTION

Free and elongating (DNA-bound) forms of RNA polymerase II were separa ted from yeast. Most cellular polymerase II was found in the elongatin g fraction, which contained all enzyme phosphorylated on the C-termina l domain and none of the 15-subunit mediator of transcriptional regula tion, These and other findings suggest that mediator enters and leaves initiation complexes during every round of transcription, in a proces s that may be coupled to C-terminal domain phosphorylation.