SUMMARY: Protein kinase activities have been detected in cell-free extracts of the cyanobacterium PCC 7120. At least 12 polypeptides in the soluble fraction were phosphorylated in at the expense of [ P]ATP and the pattern of phosphorylation was shown to be regulated by intermediary metabolites and other effectors, at physiological concentrations. Glucose 6-phosphate exerted a regulatory effect on a phosphopolypeptide of 56000 (p56) by stimulating a protein phosphatase, whereas ribulose 5-phosphate inhibited the corresponding protein kinase. In addition, DTT and the calmodulin antagonist trifluoperazine influenced the phosphorylation state of several different polypeptides, indicative of control by redox conditions and a calmodulin-like mediator, respectively. Furthermore, it was established that the phosphorylation of p56 required Mg (> 100 μM) whereas that of a polypeptide of 16000 occurred in the absence of Mg and was inhibited by high concentrations (> 1 mM) of this cation. Several of the phosphopolypeptides detected corresponded in mobility on SDS-PAGE to species phosphorylated .


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