E sub 1 BF is an essential RNA polymerase I transcription factor with an intrinsic protein kinase activity that can modulate rRNA gene transcription

The authors previously described the purification and characterization of E{sub 1}BF, a rat rRNA gene core promoter-binding factor that consists of two polypeptides of 89 and 79 kDa. When this factor was incubated in the absence of any exogenous protein kinase under conditions optimal for protein phosphorylation, the 79-kDa polypeptide of E{sub 1}BF was selectively phosphorylated. The labeled phosphate could be removed from the E{sub 1}BF polypeptide by treatment with calf intestinal alkaline phosphatase or potato acid phosphatase. Elution of the protein from the E{sub 1}BF-promoter complex formed in an electrophoretic mobility-shift assay followed by incubation of the concentrated eluent with ({gamma}-{sup 32}P)ATP resulted in the selective labeling o the 79-kDa band. The E{sub 1}BF-associated protein kinase did not phosphorylate casein or histone H1. These data demonstrate that (1) polymerase I promoter-binding factor E{sub 1}BF contains an intrinsic substrate-specific protein kinase and (2) E{sub 1}BF is an essential polymerase I transcription factor that can modulate rRNA gene transcription by protein phosphorylation. Further, these studies have provided a direct means to identify a protein kinase or any other enzyme that can interact with a specific DNA sequence.