MAD1 and c-MYC regulate UBF and rDNA transcription during granulocyte differentiation

The regulation of cell mass (cell growth) is often tightly coupled to the cell division cycle (cell proliferation). Ribosome biogenesis and the control of rDNA transcription through RNA polymerase I are known to be critical determinants of cell growth. Here we show that granulocytic cells deficient in the c-MYC antagonist MAD1 display increased cell volume, rDNA transcription and protein synthesis. MAD1 repressed and c-MYC activated rDNA transcription in nuclear run-on assays. Repression of rDNA transcription by MAD1 was associated with its ability to interact directly with the promoter of upstream binding factor (UBF), an rDNA regulatory factor. Conversely, c-MYC activated transcription from the UBF promoter. Using siRNA, UBF was shown to be required for c-MYC-induced rDNA transcription. These data demonstrate that MAD1 and c-MYC reciprocally regulate rDNA transcription, providing a mechanism for coordination of ribosome biogenesis and cell growth under conditions of sustained growth inhibition such as granulocyte differentiation.