Histones and DNA methylation in mammalian chromatin. II. Presence of non-inhibitory tightly-bound histones

After removal, by high-salt extraction, of the loosely-bound components present in human placenta chromatin, tightly-bound cationic proteins could be solubilized, by acid extraction, from the 'stripped' chromatin, as well as from the 'stripped' loops or from the 'digested matrix'. These acid-soluble tightly-bound proteins are, in terms of apparent molecular mass and immunoreactivity, quite similar to the 'typical', loosely-bound histones, and, similarly to their 'loosely-bound' counterparts, they can be subdivided in distinct H-1-, H-2A-, H-2B-, H-3- and H-4-like components, the 'digested matrix' being however characterized by the absence of tightly-bound H-1. These tightly-bound histones, at variance from the 'typical' ones, readily find a right-handed helical conformation upon renaturation by progressive dialyses. The H-1 components strongly differ also in their effects on enzymic DNA methylation: while 'typical' H-1 has a strong inhibitory effect, its tightly-bound counterpart exerts a slight but definite stimulation.