Correlation of chromatin composition with metabolic changes in nuclei of primitive erythroid cells from chicken embryos

Abstract

Changes in levels of biosynthesis of DNA, RNA, and histones were compared with relative proportions of each histone class during primitive erythropoiesis in embryonic chicks. We confirmed that erythrocyte-specific histone 5 (H5) was substantial in the earliest accessible, erythroblast-enriched stage and that it doubled in relative amount between polychromatic and orthochromatic stages to about 1 mol per 2 mol of each nucleosomal histone, still considerably less than in adult definitive erythrocytes. No other histones changed during primitive erythropoiesis, but the molar proportion of histone 1 (H1) always exceeded that of H5 in these cells, unlike definitive erythrocytes. The increase in content of H5 was accompanied by continued incorporation of lysine into this histone in orthochromatic erythrocytes in contrast to the decline in synthesis of the other histones and DNA. The accumulation of H5 during development appears to occur in steps corresponding to the maturation of the primitive and definitive erythroid cell lines. Lysine-rich histones were more easily extracted from nuclei of the erythroblast-enriched stage than from later stages; this change accompanied an early decrease in RNA synthesis in whole cells and in isolated nuclei.