Phosphorylation of rat liver ribonucleic acid polymerase I by nuclear protein kinases

Phosphorylation of rat liver RNA polymerase I occurred when intact rat liver nuclei were incubated with [gamma32P]ATP and N6,O2' dibutyryl cyclic 3':5'-AMP. In addition, partially purified RNA polymerase I could be phosphorylated in vitro by an endogenous protein kinase. Phosphorylation by either method was followed by extensive purification of the enzyme. This revealed that 32P remained bound to the enzyme throughout purification. Analysis of the homogeneous labeled protein by polyacrylamide gel electrophoresis under nondenaturing conditions followed by autoradiography revealed that only one of the two forms of RNA polymerase I in rat liver nuclei was phosphorylated. RNA polymerase II was not phosphorylated in intact nuclei. Polyacrylamide gel electrophoresis of the phosphorylated RNA polymerase I in the presence of 0.1% sodium dodecyl sulfate followed by autoradiography demonstrated that the 32P was located primarily on enzyme subunits SA1, SA3, and SA5-SA6. High voltage paper electrophoresis of a partial acid hydrolysate of phosphorylated RNA polymerase I revealed that both serine and threonine residues were phosphroylated. N6,O2'-Dibutyryl cyclic 3':5'-AMP stimulated endogenous RNA polymerase I activity and endogenous nuclear protein phosphorylation in intact nuclei. These results suggest that phosphorylation of RNA polymerase I by nuclear protein kinases may play a role in the control of transcription in mammalian cells.