Maturation of ribosomes in yeast. I Kinetic analysis by labelling of high molecular weight rRNA species.

To study the maturation of ribosomes in Saccharomyces carlsbergensis, protoplasts were pulse labeled with [5-3H]uridine at 15 degrees C. Investigation of the cellular location of pulse-labelled ribosomal RNA precursor and mature ribosomal RNA shows that both the 37-S precursor RNA, common to both 17-S and 26-S rRNA, as well as the 29-S RNA, the direct precursor of 26-S rRNA, are located in the nucleus. Most of the 18-S RNA, the direct precursor of 17-S rRNA, is found in the cytoplasmic fraction. Apart from 37-S and 29-S RNA the nucleus also contains an appreciable amount of 26-S rRNA as well as a small quantity of 18-S RNA. These data indicate that processing of 29-S to 26-S RNA occurs in the nucleus, whereas the conversion of 18-S RNA to 17-S rRNA takes place in the cytoplasm. The kinetics of appearance of pulse-labelled 26-S and 17-S rRNA in the various cytoplasmic ribosomal particles indicate, that newly formed 40-S ribosomal particles are almost immediately incorporated into 80-S ribosomes and polysomes. On the other hand, there appears to exist a fairly large cytoplasmic pool of newly synthesized ribosomal particles containing 26-S rRNA and sedimenting at about 60 S. The kinetics of appearance of newly formed 26-S and 17-S rRNA in mature ribosomes show that the maturation of the large ribosomal subunit takes about twice as much time as that of the small subunit.