1. The chromatin of the oviducts of immature chicks undergoing estrogen-induced differentiation was studied. Both the species and quantitative level of the histones remained relatively unaltered throughout development. In contrast, the level of the acidic proteins displayed a biphasic pattern, increasing during the first few days of differentiation followed by a decrease during the final stages of maturation. The level of chromatin-associated RNA and the capacities of chromatin to serve as template in in vitro DNA-dependent RNA synthesis showed a similar biphasic pattern. 2. Fractionation of the acidic chromatin proteins into four groups followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis demonstrated a marked heterogeneity of the acidic chromatin proteins. The majority of protein bands were associated with two of the four groups (AP1 and AP2). Quantitative analysis revealed that variations primarily in the level of one fraction (AP2) were responsible for the changes observed in the total chromatin acidic proteins. Amino acid composition, sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunochemical analysis demonstrated that alterations in the molecular species of acidic proteins occurred during development. Using circular dichroism to probe the secondary structure of whole and dehistonized chromatin, a gradual conformational change was observed in both the DNA and protein components during oviduct development. 3. The observed changes in chromatin structure and composition are suspected to be involved in the changing pattern of gene expression during development of the chick oviduct.
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