Coordinated decreases in rRNA gene transcription factors and rRNA synthesis during muscle cell differentiation.
rRNA synthesis decreases significantly during the differentiation of rat L6 myoblasts to myotubes. Nuclear run-on assays demonstrated that the decrease was attributable to decreased rates of rRNA gene transcription. Immunoblot analysis indicated a marked reduction in amounts of the RNA polymerase I transcription factors UBF1 and UBF2 (upstream binding factors 1 and 2, respectively). The levels of these factors dropped in parallel with the down-shift in rRNA gene transcription. The amount of UBF does not fall due to a general decrease in cellular protein, as myosin heavy-chain protein accumulates markedly during this same time. RNA blots of total RNA isolated from myoblasts and differentiating myotubes showed a decrease in the mRNA for UBF, at the same time the mRNA for myogenin was accumulating. The down-shift in UBF mRNA levels preceded the decrease in the protein levels for UBF. There have been reports that the acute response of the rRNA gene transcription system to physiological signals in many systems involves an RNA polymerase I-associated factor. However, our results imply that the regulation of rRNA gene DNA transcription in response to physiological processes, such as differentiation, may involve multiple regulatory pathways.
Department of Molecular Biology and Genetics, University of Guelph, ON, Canada.
D E
DE
Larson
W
W
Xie
M
M
Glibetic
D
D
O'Mahony
B H
BH
Sells
L I
LI
Rothblum
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002352:Carrier Proteins / Q000096:biosynthesis; D002454:Cell Differentiation / Q000502:physiology*; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000302:isolation & purification / Q000378:metabolism*; D009124:Muscle Proteins / Q000096:biosynthesis; D009132:Muscles / Q000166:cytology / Q000378:metabolism*; D018008:Myogenin; D010761:Phosphorus Radioisotopes; D035463:Pol1 Transcription Initiation Complex Proteins; D012318:RNA Polymerase I / Q000378:metabolism; D012322:RNA Precursors / Q000096:biosynthesis*; D012333:RNA, Messenger / Q000096:biosynthesis*; D012335:RNA, Ribosomal / Q000096:biosynthesis*; D051381:Rats; D014157:Transcription Factors / Q000302:isolation & purification / Q000378:metabolism*; D014158:Transcription, Genetic; D014544:Uridine Triphosphate / Q000378:metabolism
D002352:Carrier Proteins; D004268:DNA-Binding Proteins; D009124:Muscle Proteins; C503803:Myog protein, rat; D018008:Myogenin; D010761:Phosphorus Radioisotopes; D035463:Pol1 Transcription Initiation Complex Proteins; D012322:RNA Precursors; D012333:RNA, Messenger; D012335:RNA, Ribosomal; D014157:Transcription Factors; C063844:transcription factor UBF; D000255:Adenosine Triphosphate; D012318:RNA Polymerase I; D014544:Uridine Triphosphate