HISTONE-H1-MEDIATED INHIBITION OF TRANSCRIPTION INITIATION OF METHYLATED TEMPLATES IN-VITRO

The detailed mechanism underlying the inhibition of transcription by D NA methylation is still obscure. Chromatin structure has frequently be en proposed as a role player in this mechanism. Histone H1 is a known key element in the formation and stabilization of chromatin fibers. We describe here experiments designed to examine the effect of DNA methy lation on the binding of histone H1 to DNA and the consequent inhibito ry effect of the bound histone H1 on in vitro transcription. The resul ts of these experiments showed a clear preferential binding of histone H1 to methylated DNA as compared with unmethylated DNA. The in vitro transcription assay indicated that transcription of methylated templat es was inhibited at a lower histone H1/DNA ratio than of unmethylated templates, and that the extent of inhibition depends on the density of methyl groups in the promoter region. This inhibition of in vitro tra nscription was alleviated efficiently by methylated competitor DNA, wh ereas, under similar conditions, almost no effect was observed with un methylated competitor. Experiments designed to pinpoint the stage in t he transcription process that was suppressed by the preferred binding of histone H1 to methylated template revealed that inhibition occurred at the initiation and not at the elongation level.