THE EFFECT OF HISTONE H1 AND DNA METHYLATION ON TRANSCRIPTION

We have previously shown that DNA methylation acts as a focus for the formation of inactive chromatin in vivo We have investigated the mecha nism further by in vitro transcription of a template containing two tR NA genes and an extensive (G+C)-rich sequence characteristic of a CpG island. The extent of transcription from the unmethylated or fully met hylated template was assayed in the presence of varied levels of histo ne H1. The transcriptional activity of both templates was inhibited by increasing amounts of histone H1, although inhibition with the methyl ated template occurs at a lower H1:DNA ratio. The H1c variant shows th e greatest preferential inhibition of the methylated template. We demo nstrate that histone H1 complexed to DNA is one of the factors that in hibits transcription by preventing the formation of initiation complex es, particularly on methylated template, rather than the formation of disordered H1 DNA aggregates.