Post-transcriptional regulation of 5S rRNA synthesis following myoblast differentiation

Terminal differentiation of proliferating rat L6 myoblasts to syncytial multi-nucleated myotubes results in a decline in the cellular levels of 5S rRNA. This decrease parallels the drop in ribosome number and the reduction in 45S rRNA precursor gene transcription previously documented [Jacobs et al. (1985) Eur. J. Biochem. 150, 255]. The activity of RNA polymerase III in 100,000 x g supernatants from myoblasts and myotubes is similar suggesting the level of this enzyme does not limit 5S rRNA synthesis. Nuclear and cell-free transcription assays were used to measure 5S rRNA synthesis in myoblasts and myotubes. No differences were observed in the rate of 5S rRNA gene transcription, thus demonstrating that 5S rRNA levels in myotubes are not transcriptionally controlled. To determine whether RNA stability plays a role, the half-life of newly synthesized 5S rRNA was measured in vivo. The half-life of nuclear 5S rRNA was calculated to be 370 min and 65 min in myoblasts and myotubes, respectively, demonstrating that the decrease in 5S rRNA levels following terminal differentiation is regulated post-transcriptionally.