Abstract
Binding modes of histones H1 and H5, and their competition for chromatin-binding sites in rat liver nuclei, were correlated with aberrant N-methylation of H1 histone lysine residues, induced by chicken erythrocyte histone H5, in order to gain more insight into the integration of lysine-rich histones in chromatin. Addition of approx. 2.5 molecules of histone H5 per nucleosome to rat liver nuclei increases the ratio of total basic residues in histones to DNA nucleotides ( BR NT ) in the nuclear chromatin from 1.0 to 1.5. At this concentration, approx. 0.7 molecule of histone H5 is bound per nucleosome, and there is no displacement of histone H1 from the nuclear chromatin. If S-adenosyl[ Me- 3H]methionine is present in the incubation mixture, the aberrant incorporation of labeled methyl groups into histone H1 reaches a maximum at this concentration of histone H5. The radioactivity present in histone H1 from nuclei incubated with labeled AdoMet at a total BR NT ratio of 1.5: (i) resides mainly in a histone H1 subfraction tentatively identified by Bio-Rex 70 chromatography and acrylamide gel electrophoresis as histone H1c; (ii) presents as a single spot upon peptide mapping of tryptic hydrolysates by means of two-dimensional thin-layer chromatography; and (iii) elutes in the position of mono- N-methyllysine upon ion-exchange chromatography of histone H1 hydrolysates. Upon further increase of the BR NT ratio, the following changes are produced: (i) a gradual decrease in radioactive methyl uptake into histone H1; (ii) a gradual displacement of histone H1 from the chromatin; (iii) increased binding of histone H5 in chromatin, up to a maximum of 3.4 residues per nucleosome; and (iv) a slowly increasing uptake of label into histone H5. The combined data from histone H1/H5 binding and histone H1 methylation studies suggest that upon addition of exogenous histone H5 to rat liver nuclei the binding of two lysine-rich histones per nucleosome plays a significant role in the induction of specific changes in chromatin structure, which in vivo may have important functional implications in terms of chromatin condensation and suppression of transcription.
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