Unco-ordinate regulation of ribosomal RNA and ribosomal protein synthesis during L 6E 9 myoblast differentiation

Abstract

Using the rat myoblast L 6E 9 cell line, we have shown previously that a five- to tenfold reduction in ribosome synthesis occurs concomitant with the differentiation of myoblasts into myotubes (Krauter et al., 1979). Regulation of ribosome synthesis, which occurs under physiological conditions, has been shown to occur at the transcriptional level. This cell line provides an excellent model with which to study whether all ribosomal components (RNAs and protein) are co-ordinately or independently regulated. The rate of synthesis and degradation of individual ribosomal proteins has been studied by a combination of pulse labeling and two-dimensional gel electrophoresis. We have found that ribosomal protein synthesis is unco-ordinately regulated with respect to the rate of ribosomal RNA synthesis. In spite of the five- to tenfold decrease in rRNA synthesis in myotubes, ribosomal proteins are synthesized at the same rate in myoblasts and myotubes. The rate of ribosomal protein accumulation, however, reflects the rate of rRNA synthesis. Myotubes accumulated ribosomal proteins fivefold more slowly than myoblasts. The ribosomal proteins synthesized in excess of rRNA in myotubes are degraded with half-lives of one hour or less. On the contrary, the proteins which assemble onto mature ribosomes are stable for at least 24 hours. From these results, we conclude that in L 6E 9 cells the limiting step in ribosome assembly is the transcription of the 45 S precursor rRNA.