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

When 3T6 cells are confluent, they withdraw from the cell cycle. Concomitan t with cell cycle arrest significant reduction in RNA polymerase I transcri ption (80% decrease at 100% confluence) is observed. In the present study, we examined mechanism(s) through which transcription of the ribosomal genes is coupled to cell cycle arrest induced by cell density. Interestingly wit h an increase in cell density (from 3-43% confluence), a significant accumu lation in the cellular content of hyperphosphorylated Rb was observed. As c ell density increased further, the hypophosphorylated form of Rb became pre dominant and accumulated in the nucleoli, Co-immunoprecipitation experiment s demonstrated there was also a significant rise in the amount of hypophosp horylated Rb associated with the rDNA transcription factor UBF, This increa sed interaction between Rb and UBF correlated with the reduced rate of rDNA transcription. Furthermore, overexpression of recombinant Rb inhibited UBF -dependent activation of transcription from a cotransfected rDNA reporter i n either confluent or exponential cells. The amounts or activities of the r DNA transcription components we examined did not significantly change with cell cycle arrest. Although the content of PAF53, a polymerase associated f actor was altered marginally (decreased 38%), the time course and magnitude of the decrease did not correlate with the reduced rate of rDNA transcript ion. The results presented support a model wherein regulation of the bindin g of UBF to Rb and, perhaps the cellular content of PAF53, are components o f the mechanism through which cell cycle and rDNA transcription are linked.