Mitosis specific centromere association of phosphorylated RNA polymerase II in human cells
RNA polymerase II (polII) plays critical roles in both transcription and processing of messenger RNAs. It consists of multiple subunits, the largest of which (RPB1) contains a C-terminal domain (CTD) with tandem repeats (52 in humans) of a heptapeptide sequence YSPTSPS. This sequence contains two major targets for phosphorylation at Ser-2 and Ser-5. The CTD undergoes a cycle of extensive phosphorylation and dephosphorylation that is coordinated with the transcription cycle. Generally, the phosphorylation status shifts from Ser-5 to Ser-2 as transcripts undergo elongation. Numerous studies have demonstrated a cell cycle specific association of RPB1 with interphase chromatin, with a general exclusion of polII from chromatin during mitosis. We have re-examined this using a panel of antibodies that recognise either unphosphorylated, Ser-2, or Ser-5 phosphorylated forms of CTD. Rather than being completely removed from mitotic chromosomes, we find a specific enrichment of phosphorylated CTD at metaphase centromeres in human cells. The underlying centromeric DNA is neither sufficient nor necessary for this localisation as evidenced by CTD association with human neocentromeres lacking classical centromeric satellite sequences, and the lack of association with inactive centromeres on dicentric chromosomes. The localisation of phosphorylated CTD to mitotic centromeres in a DNA sequence independent manner, raises the intriguing possibility of a novel function for activated RNA polII in proper centromere functioning.