Phosphorylation of synthetic acidic peptides by casein kinase II: evidence for competition with phosphorylation of proteins involved in transcription.

Phosphorylation of several synthetic acidic peptides by biochemically isolated casein kinase II (CKII) and by cellular and nuclear extracts containing CKII-like activity has been investigated. Especially the synthetic peptide pyroGlu-Asp-Asp-Ser-Asp-Glu-Glu-Asn comprising the carboxy-terminal acidic hepta-peptide of the largest subunit of RNA polymerase II was found to serve as an excellent substrate for purified CKII. Moreover, this peptide reduces the rate of 'in vitro' ATP-dependent stimulation of DNA transcription induced by the proteins in the extracts. Since the peptide itself is also significantly phosphorylated in such assays, it is supposed that it serves as a competitive substrate for the phosphorylation of proteins in the extracts whose phosphorylation seems to be a prerequisite for their activity in the transcription process. This points to the involvement of CKII and substrate(s) of CKII in the process of transcription.