Ribosomal RNA: kinetic analysis of incorporation of labelled precursors

Abstract

According to a current hypothesis, the assembly of ribosomes in mammalian cells involves several species of RNA differing in sedimentation values. We have subjected recently published data on the incorporation of labelled precursors, to kinetic analysis. The turnover rates of 45 s and 32 s RNA are consistent with a precursor-product relationship. An analysis of incorporation data for 32 s, 28 s (nuclear) and 28 s (cytoplasm) suggests that (i) only 10% of the radioactivity lost from 32 s RNA can be accounted for by the observed gain in 28 s nuclear RNA and, (ii) the turnover rate of 28 s nuclear RNA is inadequate to account for the radioactivity in the cytoplasmic 28 s fractions. A close resemblance between the turnover rates of cytoplasmic 28 s and 18 s RNA obtained from tracer experiments and the exponential rate constant estimated from the doubling period of cells in culture, is consistent with a high metabolic stability of ribosomes.