Leaving a mark: the many footprints of the elongating RNA polymerase II.

The elongation phase of transcription by RNA polymerase II involves a complex choreography of events besides the polymerization of RNA. In addition to coordinating the processing of the nascent transcript, elongating RNA polymerase II recruits histone methyltransferases to methylate lysines 4 and 36 of histone H3 in nucleosomes in the body of actively transcribed genes. Methylation at these sites is genetically implicated in marking actively transcribed genes. Recent studies link transcriptional elongation by RNA polymerase II to H3K9 methylation and the recruitment of the HP1 family protein HP1gamma. These findings expand the role for RNA polymerase II elongation in targeting chromatin modifications to include a histone methyl mark more commonly associated with gene silencing.