Ribosomal precursor particles were extracted from the yeast Saccharomyces carlsbergensis and analysed. After a brief labelling of yeast protoplasts with 3H-uridine, three basic ribonucleoprotein components were detected, sedimenting at approx. 90S, 66S and 43S in sucrose gradients containing magnesium. The 90S particles contained the 37S ribosomal precursor RNA as a major component and a small though variable amount of 29S ribosomal precursor RNA. The 66S and 43S particles contained 29S and 18S ribosomal precursor RNA, respectively. Kinetic data indicate a precursor-product relationship between the 90S particles and the two other ribonucleoprotein components, consistent with the conversion: 90S -> 66S + 43S. The 90S and 66S preribosomes appeared to be present exclusively in the nucleus, whereas the 43S particles were mainly present in the cytoplasmic fraction. Apparently, the final maturation step in the formation of the 40S ribosomal subunits takes place in the cytoplasm. The 90S and 66S precursor particles have a relatively higher ratio of protein to RNA than the mature large ribosomal subunits, as judged from their buoyant densities in CsCl gradients. This finding suggests that also in a primitive eukaryotic organism, like yeast, ribosome maturation involves, in addition to a decrease in the size of the RNA components, an even stronger decrease in the amount of associated protein. In contrast, the 43S particles appeared to have the same buoyant density as the 40S ribosomal subunits.

• PMID: 1122947
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Journal Article

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Trapman J, Retèl J, Planta RJ

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