Synthesis of Ribonucleic Acid in Rabbit Blood Cells In Vivo*

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Abstract
Rabbits with a phenylhydrazine-induced reticulocytosis were given P32, and a study was made of the time course of incorporation of this precursor into various components of the RNA isolated from circulating blood cells. No radioactivity was associated with reticulocyte ribosomal RNA before 2 to 3 hours after injection of the isotope. The maximal level of specific activity of reticulocyte RNA was achieved at 40 to 42 hours after P32 administration. In contrast to reticulocytes, nucleated leukocytes contained a heterogeneously sedimenting labeled RNA component within 30 minutes after being introduced to the isotope. These data suggest that reticulocyte RNA is synthesized predominantly before the orthochromic normoblast stage of erythroid cell maturation. The RNA in reticulocytes that directs protein synthesis is probably associated in a relatively stable form with the polyribosomes.