Mobilization of newly synthesized RNAs into polysomes inXenopus laevis embryos

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Abstract
The mobilization of newly synthesized 18S and 28S rRNAs, 4S RNA and poly(A)+ RNA into polysomes was studied in isolated cells ofXenopus laevis embryos between cleavage and neurula stages. Throughout these stages, 4S RNA and poly(A)+ RNA were mobilized immediately following their appearance in the cytoplasm. 18S rRNA however, stayed in the ribosomal subunit fraction for about 30 min until the 28S rRNA appeared, when the two rRNAs were mobilized together at an equimolar ratio. This mobilization, at a 1:1 molar ratio, appeared to be realized at initiation monome formation. Thus, the efficiency of the mobilization of two newly synthesized rRNAs, shortly after their arrival at the cytoplasm, differed considerably but difference disappeared once steady state was reached. The contribution of newly synthesized 18S and 28S rRNAs to polysomes remains small throughout early development. around 3% of newly synthesized 4S RNA is polysomal which is the same distribution observed for unlabeled 4S RNA. Less than 10% of the newly synthesized cytoplasmic poly(A)+ RNA was mobilized into polysomes during cleavage, but in later stages the proportion increased to around 20%–25%. These results show that newly synthesized RNAs are utilized for protein synthesis at characteristic rates soon after they are synthesized during early embryonic development. On the basis of the data presented here and elsewhere we discuss quantitative aspects of the utilization of newly synthesized and maternal RNAs during early embryogenesis.