Runx1 binds positive transcription elongation factor b and represses transcriptional elongation by RNA polymerase II: possible mechanism of CD4 silencing.

Runx1 binds the silencer and represses CD4 transcription in immature thymocytes. In this study, we found that Runx1 inhibits P-TEFb, which contains CycT1, CycT2, or CycK and Cdk9 and stimulates transcriptional elongation by RNA polymerase II (RNAPII) in eukaryotic cells. Indeed, its inhibitory domain, spanning positions 371 to 411, not only bound CycT1 but was required for silencing CD4 transcription in vivo. Our chromatin immunoprecipitation assays revealed that Runx1 inhibits the elongation but not initiation of transcription and that RNAPII is engaged at the CD4 promoter but is unable to elongate in CD4(-) CD8(+) thymoma cells. These results suggest that active repression by Runx1 occurs by blocking the elongation by RNAPII, which may contribute to CD4 silencing during T-cell development.