Reciprocity between O-GlcNAc and O-phosphate on the carboxyl terminal domain of RNA polymerase II

The carboxyl terminal domain of RNA polymerase II has multiple essential ro les in transcription initiation, promoter clearance, transcript elongation, and the recruitment of the RNA processing machinery. Specific phosporylati on events are associated with the spatial and temporal coordination of thes e different activities. The CTD is also modified by beta -O-linked GlcNAc o n a subset of RNA Pol II molecules. Using synthetic CTD substrates, we show here that O-GlcNAc and phosphate modification of the CTD are mutually excl usive at the level of the enzymes responsible for their addition. In additi on, we show that O-GlcNAc transferase and CTD kinase have different CTD rep eat requirements for enzymatic activity. The K-m values of the two enzymes for CTD substrates are in a similar range, indicating that neither enzyme h as a distinct kinetic advantage. Thus, the in vivo regulation of O-GlcNAc a nd phosphate modification of the CTD may involve the differential associati on of these two enzymes with the CTD at specific stages during the transcri ption cycle. Furthermore, direct competition between OGT and CTD kinase in vivo could generate multiple functionally distinct isoforms of RNA Pol II. Taken together, these results suggest that O-GlcNAc may give rise to additi onal functional states of RNA Pol II and may coordinate with phosphorylatio n to regulate class II gene transcription.