Metabolism of 5S RNA in the absence of ribosome production

The results presented in this report show that during early development of Xenopus laevis the synthesis of 5S RNA occurs in blastula embryos, whereas the synthesis of 18S and 28S RNA cannot be detected until gastrulation. Thus the initiation of synthesis of the three ribosomal RNAs is not coordinate during early development. Blastula embryos are similar to anucleolate mutants of Xenopus laevis, in that they both synthesize 5S RNA, but are unable to assemble new ribosomes because they do not synthesize 18S and 28S RNA or ribosomal proteins. The blastula and anucleolate embryos thus provide a unique opportunity to determine if newly synthesized soluble 5S RNA can exchange with the 5S RNA present in existing ribosomes. The results show that newly synthesized 5S RNA is not incorporated into the ribosomes of blastula or anucleolate embryos. Furthermore, the 5S RNA synthesized by anucleolate mutants has a shorter half-life than the 5S RNA made by normal embryos. The synthesis of excess 5S RNA and its subsequent degradation in the absence of ribosome production appears to be another example of the phenomenon of wastage of newly synthesized ribosomal RNA.