Intranuclear maturation pathways of rat liver ribosomal ribonucleic acids

Abstract

The maturation of pre-rRNA (precursor to rRNA) in liver nuclei was studied by agar/urea-gel electrophoresis, kinetics of labeling in vivo with [14C]orotate and EM observation of secondary structure of RNA molecules. Processing starts from primary pre-rRNA molecules with average MW 4.6 .times. 106 (45 S) containing the segments of both 28 S and 18 S rRNA. These molecules form a heterogeneous peak on electrophoresis. The 28 S rRNA segment is homogeneous in its secondary structure. However, the large transcribed spacer segment (presumably at the 5''-end) is heterogeneous in size and secondary structure. A minor early labeled RNA component with MW about 5.8 .times. 106 is reproducibly found, but its role as a pre-rRNA species remains to be determined. The following intermediate pre-rRNA species are identified: 3.25 .times. 106 MW (41 S), a precursor common to both mature rRNA species; 2.60 .times. 106 (36 S) and 2.15 .times. 106 (32 S) precursors to 28 S rRNA; 1.05 .times. 106 (21 S) precursor to 18 S rRNA. The pre-rRNA molecules in rat liver are identical in size and secondary structure with those observed in other mammalian cells. These results suggest that the endonuclease-cleavage sites along the pre-rRNA chain are identical in all mammalian cells. Labeling kinetics and the simultaneous existence of both 36 S and 21 S pre-rRNA reveal that processing of primary pre-rRNA in adult rat liver occurs simultaneously by at least 2 major pathways: 45 S .fwdarw. 41 S .fwdarw. 32 S + 21 S.fwdarw. 28 S + 18 S rRNA and 45 S .fwdarw. 41 S .fwdarw. 36 S + 18 S .fwdarw. 32 S .fwdarw. 28 S rRNA. The 2 pathways differ by the temporal sequence of endonuclease attack along the 41 S pre-rRNA chain. A minor fraction (MW 2.9 .times. 106, 39 S) is identified as most likely originating by a direct split of 28 S rRNA from 45 S pre-rRNA. In liver considerable flexibility apparently exists in the order of cleavage of pre-rRNA molecules during processing.