Synchronous appearance of short-term labelled ribosomal ribonucleic acids in the two ribosomal subparticles from rat liver polysomes

Abstract

1. 1. The in vivo incorporation of [ 14C]orotic acid into the RNA of different rat liver cytoplasmic structures was investigated. The bulk of cytoplasmic high-molecular-weight RNA (about 95 per cent) is represented by the rRNA of the two ribosomal subparticles in free and membrane-bound polysomes. 2. 2. When isolated free of non-ribosomal rapidly labelled RNA, the two rRNA's from polysomes display an equal specific activity at all labelling periods from 20 to 120 minutes. 3. 3. The 18 S RNA from the fraction of free postmicrosomal RNP particles is more highly labelled than the 28 S RNA of this fraction. 4. 4. The amount of the nascent smaller RNP particles in the postmicrosomal fraction is higher than of the larger RNP particles. The reverse correlation is found in nuclei. 5. 5. The results suggest that the two ribosomal subparticles in liver turn over synchronously. The maturation rate of the larger subparticle in the nucleus is identical to the maturation rate of the smaller subparticle in the cytoplasm.