Figure 2

Models for regulation of transcription by monoubiquitinated H2A. A. uH2A inhibits RNA polymerase II initiation in vitro through a crosstalk with H3K4 methylation. uH2A prevents H3K4 methylation by the MLL3 histone methyltransferase. The enzymatic removal of ubiquitin from H2A by USP21 can positively influence H3K4 methylation by the methyltransferase. This would allow H3K4 methylation, which is a prerequisite for initiation of gene transcription. B. At bivalent promoters uH2A and H3K4 Me coexist. At these promoters, transcription initiation occurs despite the presence of uH2A. C. uH2A might inhibit transcription elongation or the transition from initiation to elongation by preventing association of the FACT elongation factor. In addition, uH2A might regulate elongation by recruiting inhibitory factors and/or affecting higher order chromatin association.