The Yeast Paf1-RNA Polymerase II Complex Is Required for Full Expression of a Subset of Cell Cycle-Regulated Genes

Abstract

We have previously described an alternative form of RNA polymerase II in yeast lacking the Srb and Med proteins but including Paf1, Cdc73, Hpr1, and Ccr4. The Paf1-RNA polymerase II complex (Paf1 complex) acts in the same pathway as the Pkc1-mitogen-activated protein kinase cascade and is required for full expression of many cell wall biosynthetic genes. The expression of several of these cell integrity genes, as well as many other Paf1-requiring genes identified by differential display and microarray analyses, is regulated during the cell cycle. To determine whether the Paf1 complex is required for basal or cyclic expression of these genes, we assayed transcript abundance throughout the cell cycle. We found that transcript abundance for a subset of cell cycle-regulated genes, including CLN1 , HO , RNR1 , and FAR1 , is reduced from 2- to 13-fold in a paf1 Δ strain, but that this reduction is not promoter dependent. Despite the decreased expression levels, cyclic expression is still observed. We also examined the possibility that the Paf1 complex acts in the same pathway as either SBF (Swi4/Swi6) or MBF (Mbp1/Swi6), the partially redundant cell cycle transcription factors. Consistent with the possibility that they have overlapping essential functions, we found that loss of Paf1 is lethal in combination with loss of Swi4 or Swi6. In addition, overexpression of either Swi4 or Mbp1 suppresses some paf1 Δ phenotypes. These data establish that the Paf1 complex plays an important role in the essential regulatory pathway controlled by SBF and MBF.