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.