POSTSYNTHETIC METHYLATION OF CORE HISTONES IN K562 CELLS IS ASSOCIATED WITH BULK ACETYLATION BUT NOT WITH TRANSCRIPTIONAL ACTIVITY

The relationship between the postsynthetic modification of core histones by methylation and transcriptionally active chromatin was investigated in K562 erythroleukemia cells. Cells were incubated with L-[methyl-H-3]methionine in the presence of cycloheximide. Under these conditions, only histones H-3 and H-4 were detectably methylated. Chromatin was fractionated by several methods, including low-salt elution, mononucleosome gel mobility shift using HPLC-purified HMG 17, and cesium chloride-guanidine equilibrium gradient centrifugation of formaldehyde-fixed chromatin. By these latter two methods, chromatin highly enriched for transcriptionally engaged or competent genes was isolated. A significant correlation was noted between postsynthetic modification of histones by methylation and by the slow-turnover form of acetylation. However, there was no enrichment of methylated histones in the transcriptionally competent fraction of chromatin isolated by HMG 17 binding. Moreover, only minor enrichment of methylated histone H-3.1 and no enrichment of methylated histones H-3.2 and H-4 was detected in transcriptionally engaged chromatin isolated by gradient centrifugation. Chromatin soluble in low-salt buffer was found to be significantly enriched in methylated histones, but not in active genes. We conclude that histone methylation is associated with both transcriptionally active and transcriptionally inactive chromatin. The function of this modification, like that of bulk histone acetylation, remains to be determined.