Regulation of ribosome biogenesis during myogenesis of rat myoblasts

Jacobs, Frederik Andre (1985) Regulation of ribosome biogenesis during myogenesis of rat myoblasts. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (43MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

The ribosome is an organelle which translates the genetic information provided by the nucleus into proteins. Because of its central role in gene expression, the production of the numerous ribosomal components must be coordinated for the efficient assembly of the ribosome so that it may perform effectively. The rat-derived muscle cell line L6-5 provides an excellent system in which to study ribosome biogenesis since, after terminal differentiation, ribosome accumulation rates are much reduced. -- The synthesis rates of rRNA and ribosomal proteins (r-proteins) were investigated in proliferating L6-5 myoblasts and terminally differentiated myotubes. The rRNA and r-proteins were coordinately produced in myoblasts, with little detectable turnover of the individual components. In myotubes, however, the rate of rRNA synthesis was reduced by approximately 75% while the r-proteins continued to be synthesized at levels near those observed for myoblasts. The production of rRNA and r-proteins, which was coordinate in myoblasts, became uncoupled in myotubes. -- Various possible explanations of the uncoupling of rRNA and r-protein production in myotubes were explored. It was determined that the levels of the r-protein mRNA’s (rp-mRNA's) were similar in myoblasts and myotubes, suggesting that their translational efficiencies were unaltered by terminal differentiation. It therefore appeared that the rate of r-protein synthesis was determined by cellular rp-mRNA levels. -- To ascertain whether altered rp-mRNA metabolism accounted for the uncoupling of r-protein and rRNA synthesis in myotubes, the half lives of the rp-mRNA's were measured. The results reveal that the stability of rp-mRNA’s were similar in myoblasts and myotubes, with half lives of approximately 11 hours. The data indicate that the rate of r-protein synthesis was regulated at the level of r-protein gene transcription, which suggests that rRNA and r-protein production was uncoupled at the transcriptional level in myotubes. -- The uncoupling of rRNA and r-protein gene transcription in myotubes was demonstrated by measuring their transcription rates in vitro. It was determined that rRNA genes in myotubes were transcribed at approximately 30% of the rate found in myoblasts when compared to rp-mRNA genes. These results directly demonstrated the differential expression of these two gene families in L6-5 cells. The implications of these findings with respect to the regulation of ribosome biogenesis and myogenesis are discussed.

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