1. Nucleated avian erythrocytes have been fractionated on the basis of buoyant density in linear bovine serum albumin density gradients. An inverse relationship has been observed between cell buoyant density, on the one side, and average RNA content, RNA- and protein-synthesizing activity of chicken and duck erythrocytes, on the other. From the known relationship between these biochemical parameters and the erythrocyte maturation process, and from the autoradiographic analysis of the distribution of protein-synthesizing activity in the erythrocyte population, it is concluded that buoyant-density centrifugation in bovine serum albumin gradients separates bird erythrocytes of increasing degree of maturity from the top to the bottom of the gradient. 2. The synthesis of the two chicken hemoglobins appears to occur at high rate in the cells at relatively early stages of maturation present in the blood from anemic animals, and declines progressively in the late stages. On the contrary, the ratio of the rates of synthesis of the two hemoglobin components remains constant in all developmental stages. This suggests that the factors involved in the regulation of synthesis of the two proteins operate in a coordinate manner throughout cell development. 3. Actinomycin D has no significant influence on the formation of either hemoglobin by erythrocytes from anemic chickens during a 24-h incubation in vitro.
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