Model illustrating the effect of the Med8-Med18-Med20 submodule on heterochromatin . Med8-Med18-Med20 may block recruitment of RNA Pol II to the centromeric chromatin by interacting with Rpb4/Rpb7. Additionally, the submodule may stimulate the activity of RNAi and thus influence the methylation level of H3K9 in centromeric chromatin. Further, Med8-Med18-Med20 in concert with Rpb1/Rpb2 may decide the fate of non-coding transcripts by directing them towards the RNAi machinery or to other downstream processes. See text for details.