DNA-DEPENDENT RNA polymerase activities in eukaryotic cells change during developmental1,2 and physiological transitions3-8. Some changes occur rapidly and in the absence of protein synthesis. They presumably involve modulation of the activity of pre-existing RNA polymerase molecules rather than alterations of the rate of synthesis or degradation of these molecules. This modulation of polymerase activity could be a consequence of covalent modification of enzyme subunits, the rapid turnover of a protein factor required for catalysis, alteration of the template properties of the chromatin or some combination of these. The first possibility is attractive. The activity of many enzymes is controlled in this manner9. Cycles of phosphorylation and dephosphorylation regulate the activity of the enzymes involved in glycogen metabolism (reviewed in refs 9 and 10). Lee et al. 11 observed an increase in the specific activity of Rous sarcoma virus reverse transcriptase on incubation with protein kinase and ATP. Phosphoryla-of β and β′ subunits of E. coli RNA polymerase on infection by bacteriophage T7 has been implicated in the control of early transcription of T7 DNA12. Several groups5,13,14 have proposed and presented preliminary evidence that phosphorylation of RNA polymerase is involved in regulation of polymerase activity in eukaryotes. Jungmann et al.5 and Martelo and Hirsch13 observed enhanced RNA polymerase activity when partially purified protein kinase, RNA polymerase and ATP were mixed. They observed the phosphorylation of proteins in their preparations and attributed the increase in polymerase activity to phosphorylation of the enzyme. They did not demonstrate phosphorylation of RNA polymerase polypeptides, and since the observed increase in RNA polymerase activity could also be a consequence of phosphorylation of contaminating acidic nuclear proteins which can stimulate polymerase activity15-17, the cause of the enhanced polymerase activity is obscure. We report here the isolation of phosphorylated RNA polymerase I from yeast cells and the identification of the enzyme polypeptides phosphorylated in vivo. The same pattern of phosphorylation was obtained when highly purified RNA polymerase I was incubated with a yeast protein kinase preparation.
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