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

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Abstract
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.