Coordinated decreases in rRNA gene transcription factors and rRNA synthesis during muscle cell differentiation.

Abstract

ABSTRACTrRNA synthesis decreases significantly during the differentiation ofrat 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 fail 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 downshift 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.

Involvement of in situ conformation of ribosomal genes and selective distribution of upstream binding factor in rRNA transcription.

“…We reported here that the amount of UBF is modulated during progression through interphase with a threefold increase in G2 compared with Gl. This increase in Sphase-G2 explains the progression of the amount of UBF found after 12 h of serum stimulation of starved cells (Cavanaugh et al, 1995), the time necessary for a majority of cells to reach S-phase-G2, whereas the low amount in differentiating cells (Larson et al, 1993) may reflect the amount of Go-G1 UBF.…”

“…It has been clearly established that in deprived and growing cells, regulation of rRNA synthesis is under the control, at least in part, of UBF activity (O'Mahony et al, 1992;Glibetic et al, 1995). This activity can depend on the level of UBF phosphorylation (O'Mahony et al, 1992;Voit et al, 1992Voit et al, , 1995 and/or the amount of UBF (Larson et al, 1993;Glibetic et al, 1995;Hannan et al, 1995). The effect of stimulation or repression of the cell cycle on the amount of UBF is confirmed by the observation that the promoter of the UBF gene is regulated by growth-related control elements (Nishimura et al, 1994) and also by the fact that the UBF gene is a primary response gene to serum stimulation of cell growth (Glibetic et al, 1995).…”

“…It has been demonstrated that UBF plays a role in regulating the efficiency of RNA pol I transcription in vitro as well as in vivo (for a review, Moss and Stefanovsky, 1995). The stimulation of rDNA transcription by UBF appears to depend on the amount of UBF (Larson et al, 1993;Glibetic et al, 1995;Hannan et al, 1995), the level of UBF phosphorylation (O'Mahony et al, 1992;Voit et al, 1992Voit et al, , 1995Hershey et al, 1995) and/or the ratio of the two splice variants UBF1 and UBF2 (Hisatake et al, 1991). We anticipate that another level of regulation could be UBF distribution because it has been proposed that UBF is a potential regulator of SLi complex binding (Moss and Stefanovsky, 1995), and it has been suggested to be an essential factor in vivo .…”

RNA polymerase I transcription in confluent cells: Rb downregulates rDNA transcription during confluence-induced cell cycle arrest

“…Our results demonstrate quite clearly that rDNA transcription is regulated dierently depending on the stimuli evoking cell cycle arrest. Previous studies reported regulation of rDNA transcription during cell cycle arrest in response to various conditions such as nutritional deprivation, and stimuli that induced dierentiation and/or apoptosis (Cavanaugh et al, 1984(Cavanaugh et al, , 1995Mahajan and Thompson, 1990;O'Mahony et al, 1992;Larson et al, 1993;Paule, 1994;Moss and Stefanovsky, 1995). Under these conditions altering TFIC activity was attributed to be the major regulatory mechanism.…”

“…UBF content and phosphorylation In many cell lines, the amount of the transcription factor UBF is proportional to the rate of rDNA transcription (Larson et al, 1993;Hannan et al, , 1996b. In addition, overexpression of UBF is sucient to increase rDNA transcription from a co-transfected rDNA reporter construct (Hannan et al, 1996a, Figure 3 Cell cycle arrest is associated with minimal changes in RNA polymerase I content, PAF53 content, and TFIC activity.…”