Psi regulates dmyc transcription via modulation of RNA Polymerase II. Nicola J. Cranna¹, Amanda Lee¹, Naomi Mitchell¹, Ross Hannan², Leonie Quinn¹. 1) Anatomy and Neuroscience, University of Melbourne, Melbourne, VIC, Australia; 2) Peter MacCallum Cancer Centre, Melbourne, VIC, Australia.

   Two single stranded DNA binding proteins have been implicated in gene specific control of RNA Polymerase II (Pol II) pausing at the c-MYC oncogene transcriptional start site through in vitro mammalian studies; FBP1 and FIR. These studies suggest that FBP1 may be required for the activation of c-myc transcription and show that FIR acts antagonistically as a repressor. The Drosophila FIR homolog, Hfp binds to the Drosophila myc (dmyc) promoter and is required for repression of transcription. In mammals there are 3 FBP family members which bind overlapping targets, leading to difficulty in dissecting the role of FBP1. Psi is the sole Drosophila ortholog of the mammalian FBP family. In support of these in vitro studies we have evidence that Psi is required for regulation of dmyc transcription, through the control of RNA Polymerase II. qPCR on Psi RNAi knockdown animals demonstrates a significant reduction in dmyc mRNA levels suggesting Psi is required for the activation of transcription. In line with a transcriptional role, ChIP experiments indicate Psi is directly bound to the dmyc promoter, suggesting an important role in the control of dmyc transcription. To determine how Psi may regulate dmyc transcription, ChIP revealed enrichment of initiated RNA pol II (Ser 5) around the dmyc transcription start site and the Psi RNAi results in a significant reduction in RNA pol II Ser 5 accumulation. Combined data suggests Psi is normally required to stimulate the activation of RNA pol II and for regulation dmyc transcription in vivo.These data demonstrate that Psi, the ortholog of FBP1, is required for controlling myc transcription. Psi is required for the activation of RNA pol II and is necessary for controlling the level of enrichment of the dmyc repressor Hfp, suggesting Psi might modulate Pol II activity to control dmyc transcription via Hfp. Our future studies are aimed towards elucidating the mechanism by which Psi controls Hfp enrichment and/or Pol II activity.