Recruitment of P-TEFb (Cdk9-Pch1) to chromatin by the cap-methyl transferase Pcm1 in fission yeast

Capping of nascent pre‐mRNAs is thought to be a prerequisite for productive elongation and associated serine 2 phosphorylation of the C‐terminal domain (CTD) of RNA polymerase II (PolII). The mechanism mediating this link is unknown, but is likely to include the capping machinery and P‐TEPb. We report that the fission yeast P‐TEFb (Cdk9‐Pch1) forms a complex with the cap‐methyltransferase Pcm1 and these proteins colocalise on chromatin. Ablation of Cdk9 function through chemical genetics causes growth arrest and abolishes serine 2 phosphorylation on the PolII CTD. Strikingly, depletion of Pcm1 also leads to a dramatic decrease of phospho‐serine 2. Chromatin immunoprecipitations show a severe decrease of chromatin‐bound Cdk9‐Pch1 when Pcm1 is depleted. On the contrary, Cdk9 is not required for association of Pcm1 with chromatin. Furthermore, compromising Cdk9 activity leads to a promoter‐proximal PolII stalling and sensitivity to 6‐azauracil, reflecting elongation defects. The in vivo data presented here strongly support the existence of a molecular mechanism where the cap‐methyltransferase recruits P‐TEFb to chromatin, thereby ensuring that only properly capped transcripts are elongated.