Non-coordinated accumulation and synthesis of 5S ribonucleic acid by ovaries of Xenopus laevis.

Abstract

ALTHOUGH the primary structure of 5S RNA from a number of organisms1–3 is known and although it is clear that the attachment of a 5S RNA molecule is necessary if ribosomes are to be effective in protein synthesis, the precise role of these molecules is unknown. Molecules of 5S RNA are non-covalently attached to the large ribosomal subunit4–7 and can be removed only by procedures which change the conformation of the sub-unit8 and, in bacterial systems at least, they can be reunited to give particles which are effective in protein synthesis9. Studies of 5S RNA in HeLa cells have shown that, in the cytoplasm, 5S RNA molecules are found only in association with ribosomes, but that as much as a fifth of all the 5S RNA in a cell is to be found in the nucleus, much of it in the nucleolus10 in association with particles which seem to be precursors of ribosomes7. In the circumstances, it would be natural to expect that the three RNA molecules found in ribosomes—5S RNA, 18S RNA and 28S RNA—would be synthesized in equal numbers, and there is indeed some evidence of coordinated synthesis from the development of Xenopus laevis6–11. During the early stages of oogenesis in the same organism, however, we have now found that 5S RNA molecules are formed in great excess.