Passage of ribosomal RNA from nucleus to cytoplasm in differentiating yolk sac erythroid cells

Abstract

Studies on the passage of RNA from nuclei to cytoplasm in eucaryotic cells have indicated that only a small part of the large nuclear precursor of ribosomal RNA (45 S) is transferred to cytoplasm to form 18-S and 28-S ribosomal RNA. In the present study the rate at which nuclear precursors of ribosomal RNA are processed to cytoplasmic ribosomal RNA is measured in differentiating yolk sac erythroid cells from fetal mice. Yolk sac derived erythroid cells proliferate and differentiate in the circulation from the 9th to the 13th day of fetal development; they actively synthesize ribosomes. The capacity for ribosome formation decreases with cell differentiation. Yolk sac erythroid cells at the 10th, 11th, 12th and 13th day of fetal development were incubated in the presence of [ 3H]uridine. Nuclear pre-ribosomal RNA was pulse labeled for 10 min and chased into the cytoplasm by a large excess of unlabeled uridine for 35 min and 80 min. The nuclear and cytoplasmic fractions of RNA from these cells were prepared. The passage of ribosomal RNA from nuclei to cytoplasm was studied as the percent of 45-S nuclear RNA transferred to cytoplasm as 18 S+28 S after 35 min and 80 min chase. In most immature erythroid cells (from 10-day fetuses) RNA synthesis is very active, but only 4.7 % of pre-ribosomal RNA is transferred to cytoplasm. The proportion of pre-ribosomal RNA which is processed increases with cell age, being 9.7 % at day 11, 25 % at day 12 and 44 % at day 13, when erythroid cells are most mature and synthesize very little RNA. 32P analysis of nuclear newly synthesized 45-S RNA demonstrates the same G + C content and ribosomal nature of this RNA at different stages of erythroid differentiation. Experiments aimed to measuring the rate of preribosomal RNA processing on whole cells and by actinomycin chase confirm that in younger erythroblasts ribosomal RNA is transferred to cytoplasm less efficiently than in mature erythroid cells. The data suggest that in this population of erythroid cells the expression of ribosomal RNA genes is controlled by factors which are active at the post-transcriptional level and are related to cell differentiation.