A serine residue in the N-terminal acidic region of rat RPB6, one of the common subunits of RNA polymerases, is exclusively phosphorylated by casein kinase II in vitro

RPB6 is one of the common subunits of all eukaryotic RNA polymerases and is indispensable for the enzyme function. Here, we isolated a rat cDNA encodi ng RPB6. It contained 127 amino acid (a.a.) residues. From alignment of RPB 6 homologues of various eukaryotes, we defined two conserved regions, i.e. an N-terminal acidic region and a C-terminal core. In this study, we invest igated in vitro phosphorylation of rat RPB6 by casein kinase II (CKII), a p leiotropic regulator of numerous cellular proteins. Three putative CKII-pho sphorylated a.a. within rat RPB6 were assigned. We found that serines were phosphorylated by CKII in vitro. Mutagenesis studies provided evidence that a serine at a.a. position 2 was exclusively phosphorylated. Finally, an RP B6-engaged in-gel kinase assay clarified that CKII was a prominent protein kinase in rat liver nuclear extract that phosphorylates RPB6. Therefore, RP B6 was implied to be phosphorylated by CKII in the nucleus. We postulate th at the N-terminal acidic region of the RPB6 subunit has some phosphorylatio n-coupled regulatory functions. (C) 1999 Elsevier Science B.V. All rights r eserved.