Histones and DNA methylation in mammalian chromatin. Differential inhibition by histone H1

Histones (from calf thymus or from human placenta), if renatured in the presence of EDTA, caused a severe inhibition of in vitro methylation of double-stranded DNA (from Micrococcus luteus) by human placenta DNA methyltransferase. The absence of EDTA during the histone renaturation procedure abolished - at least in the 'physiological' range of the histones/DNA ratio - the inhibition. The H-1 component was responsible for this inhibition, no effect being exerted by the other histones. H-1 preparations were more effective if renatured in the presence of EDTA - 90% inhibition being reached at a 0.3:1 (w/w) H-1/DNA ratio. It seems likely that the requirement for the presence of EDTA during the renaturation process is correlated to its ability to induce a fairly stable ordered conformation of the histones, although this effect could also be shown with the 'inactive' H-2a, H-2b and H-3 components, and was instead less evident with histone H-1. The restriction to histone H-1 of the ability to inhibit enzymic DNA methylation may account for the lower methylation levels present in the internucleosomal DNA of mammalian chromatin.