Biochemical characterization of RNA and protein synthesis in erythrocyte development.

Abstract

Newts (Triturus cristatus) made anemic with acetylphenylhydrazine (APH) fail to regenerate erythrocytes (RBC) immediately and exhibit a latent period of 1.5-2 wk during which animals lack RBC and are aplastic. With the establishment of erythroid regeneration at 10-14 days, relatively homogeneous populations of successive erythropoietic stages occur in the blood. Previous studies have described a primitive cell population referred to as erythroid precursor cells (EPC) which precedes the appearance of definitive erythroid elements. EPC and early erythroid cells are apparently engaged mainly in ribosomal production, including synthesis of rRNA and ribosomal proteins. EPC and early erythroid cells also synthesize tRNA and a presumed Hb-mRNA which was identified by its sedimentation rate at 9-12 s and its content of polyadenylic acid. In intermediate stages, there occurs a 4-fold decrease in the level of RNA synthesis, and while rRNA continues to be formed there is a disproportionate accumulation of the 2 major cytoplasmic rRNA species in favor of the large ribosomal subunit RNA. In late developmental stages, the level of RNA synthesis is markedly diminished with little or no evidence of formation of defined RNA classes. Correlated radioautographic and biochemical studies with radioactive .delta.-aminolevulinic acid and leucine indicate that EPC and other early erythroid elements synthesize Hb, ferritin and ribosomal proteins. Erythroid RNA synthesis apparently is most pronounced in the early developmental stages, being manifested predominantly by rRNA production but including tRNA and Hb-mRNA; intermediate developmental stages show ribosomal wastage and decreased growth rate, marking a pivotal point between the transcriptional activities of early stages and translational activities of late stages. EPC seem to represent a cell population already committed to RBC formation and are excluded from a role as the pluripotential stem cell.