The Spt4p Subunit of Yeast DSIF Stimulates Association of the Paf1 Complex with Elongating RNA Polymerase II

Abstract

The Paf1 complex (Paf1C) interacts with RNA polymerase II (Pol II) and promotes histone methylation of transcribed coding sequences, but the mechanism of Paf1C recruitment is unknown. We show that Paf1C is not recruited directly by the activator Gcn4p but is dependent on preinitiation complex assembly and Ser5 carboxy-terminal domain phosphorylation for optimal association with ARG1 coding sequences. Importantly, Spt4p is required for Paf1C occupancy at ARG1 (and other genes) and for Paf1C association with Ser5-phosphorylated Pol II in cell extracts, whereas Spt4p-Pol II association is independent of Paf1C. Since spt4Δ does not reduce levels of Pol II at ARG1, Ser5 phosphorylation, or Paf1C expression, it appears that Spt4p (or its partner in DSIF, Spt5p) provides a platform on Pol II for recruiting Paf1C following Ser5 phosphorylation and promoter clearance. spt4Δ reduces trimethylation of Lys4 on histone H3, demonstrating a new role for yeast DSIF in promoting a Paf1C-dependent function in elongation.

We thank Grant Hartzog for anti-Spt4p and anti-Spt5p antibodies and Karen Arndt for anti-Rtf1p antibodies. We thank Steve Hahn for the kin28-as strain, Kevan Shokat for NA-PP1, Mark Solomon for the kin28-ts16 plasmids, and Fred Winston for SPT4 plasmids. We also thank Chhabi Govind and Dan Ginsburg for critical reading of the manuscript.

This research was supported by the Intramural Research Program of the NICHD, NIH.