Maturation of ribosomes in yeast. II. Position of the low molecular weight rRNA species in the maturation process.

Yeast protoplasts were pulse labelled with [5-3H] uridine and the labelling kinetics of the low molecular weight rRNA species were determined in order to gain more insight into the position of those small rRNAs in the process of ribosome maturation. 7-S RNA, the immediate precursor of 5.8-S rRNA, is found to be present only in the nucleus, indicating that the conversion of 7-S to 5.8-S RNA is a nuclear event. 5.8-S rRNA is observed in the cytoplasm almost immediately after its formation. This as well as the presence of only a small amount of 5.8-S RNA in the nucleus, shows that the ribosomal precursor particles of the large ribosomal subunit are very rapidly transported into the cytoplasm once 5.8-S rRNA is formed. Most of the newly synthesized 5-S RNA is found in the nucleus. This nuclear 5-S rRNA is mainly present in the ribosomal precursor particles. However, a small pool of free 5-S rRNA is probably also present.