COORDINATED DECREASES IN RIBOSOMAL-RNA GENE-TRANSCRIPTION FACTORS ANDRIBOSOMAL-RNA SYNTHESIS DURING MUSCLE-CELL DIFFERENTIATION

rRNA synthesis decreases significantly during the differentiation of r at L6 myoblasts to myotubes. Nuclear run-on assays demonstrated that t he decrease was attributable to decreased rates of rRNA gene transcrip tion. Immunoblot analysis indicated a marked reduction in amounts of t he RNA polymerase I transcription factors UBF1 and UBF2 (upstream bind ing factors 1 and 2, respectively). The levels of these factors droppe d in parallel with the down-shift in rRNA gene transcription. The amou nt of UBF does not fall due to a general decrease in cellular protein, as myosin heavy-chain protein accumulates markedly during this same t ime. RNA blots of total RNA isolated from myoblasts and differentiatin g myotubes showed a decrease in the mRNA for UBF, at the same time the mRNA for myogenin was accumulating. The downshift in UBF mRNA levels preceded the decrease in the protein levels for UBF. There have been r eports that the acute response of the rRNA gene transcription system t o physiological signals in many systems involves an RNA polymerase I-a ssociated factor. However, our results imply that the regulation of rR NA gene DNA transcription in response to physiological processes, such as differentiation, may involve multiple regulatory path ways.