Regulation of ribosome biogenesis in differentiated rat myotubes.

The rate of ribosome biogenesis is closely coupled with cell proliferation, representing a unique model system for studying gene regulation. Terminal differentiation of rat L6 myoblasts, an example of a rapidly proliferating population of cells being converted into a non-dividing syncytial population, results in an 80% decline in the rate of ribosome accumulation. Ribosome production during myogenesis is regulated by a down-shift in the rate of rRNA accumulation, controlled at the level of transcription by specific trans-acting factors. The synthesis of both ribosomal proteins and 5S rRNA remains unchanged in myotubes, however, resulting in an over-production of these precursors. The excess molecules are rapidly degraded, preventing the accumulation of a static pool of ribosome components.