Transcription factors mediate rRNA synthesis during myogenesis

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Abstract
This study was designed to identify factors which control rRNA transcription during terminal differentiation of rat L6 myoblasts. A cell‐free system using 100000 ×g supernatants (S100 extracts) from rat L6 myoblasts and a genomic rRNA clone was established which accurately transcribes the rat rRNA gene. The myoblast S100 extract produced a high level of transcription with accurate initiation at the rRNA promoter. Myotube S100 extracts, on the other hand, displayed a reduced capacity to transcribe the rRNA gene. Thus constituents of the cell extract, rather than rearrangements of the DNA sequences, are implicated in regulating rRNA gene expression. The RNA polymerase content of the S100 extracts was analyzed using α‐amanitin to determine the activity of each polymerase species and found to be comparable in both myoblast and myotube S100 extracts. This observation suggests that factors other than RNA polymerase I are involved in regulating rRNA transcription. Two of the factors associated with accurate rRNA gene expression in vitro, namely transcription factors B and D, occurred at comparable levels in both myoblast and myotube extracts suggesting that other components of the transcription complex are responsible for limiting rRNA transcription in myotubes. The cell‐free system should facilitate identification and characterization of these factors which regulate expression of the rRNA gene.