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Synthesis of the 45S rRNA by RNA polymerase I limits cell growth. Knowledge of the mechanism of its regulation is therefore key to understanding growth control. rRNA transcription is believed to be regulated solely at initiation/promoter release. However, we found that stimulation of endogenous 45S rRNA synthesis by epidermal growth factor (EGF) and serum failed to induce an increase in RNA polymerase I engagement on the rRNA genes, despite robust enhancement of 45S rRNA synthesis. Further, endogenous transcription elongation rates were measured and found to be directly proportional to 45S rRNA synthesis. Thus, elongation is a rate-limiting step for rRNA synthesis in vivo. ERK phosphorylation of the HMG boxes of UBF, an RNA polymerase I factor essential for transcription enhancement, was shown to directly regulate elongation by inducing the remodeling of ribosomal gene chromatin. The data suggest a mechanism for coordinating the cotranscriptional assembly of preribosomal particles.

Rat cells contain a DNA-binding polymerase I transcription factor, rUBF, with properties similar to UBF homologs that have been purified from both human (hUBF) and frog (xUBF) cells. In this note we report the affinity purification of rUBF to apparent homogeneity and show that UBFs from both rat and frog have identical footprinting characteristics on templates from either species. Furthermore, xUBF was able to stimulate transcription from rat RNA polymerase I promoters in a partially fractionated rat extract that was UBF dependent. These results strengthen the conclusion that all vertebrate cells contain a UBF homolog whose DNA-binding specificity and function have been strongly conserved.

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