The transcription elongation factor Spt5 interacts with Pleiohomeotic to mediate Polycomb Group Repression. Barbara H. Jennings, Robert Harvey. UCL Cancer Institute, University College London, London, United Kingdom.

   The checkpoint regulated by Positive Transcription Elongation Factor b (P-TEFb) in the early phase of transcription elongation is a critical control point for the expression of many genes. Spt5 interacts directly with RNA polymerase II and has an essential role in establishing this checkpoint, and also for further transcript elongation. Previously, missense mutations have been mapped to the Spt5 genes in Drosophila and Zebrafish that give rise to specific developmental defects, indicating that Spt5 activity can be influenced by contextual factors. Here we demonstrate that Spt5 can physically interact with the Polycomb Group protein Pleiohomeotic in Drosophila, and that mutations in Spt5 and pho interact during the processes of adult wing maturation and Polycomb Group (PcG) repression in vivo. Our results indicate that Pho interacts with Spt5 to prevent RNAP II transcribing through the P-TEFb checkpoint to maintain PcG repression.