Characterization of PknC, a Ser/Thr kinase with broad substrate specificity from the cyanobacterium Anabaena sp strain PCC 7120

Eukaryotic-like protein Ser/Thr and Tyr kinases have only recently been dis covered in prokaryotes. In most cases, their biochemical properties have be en poorly characterized. The nitrogen-fixing and heterocyst-forming cyanoba cterium Anabaena sp. strain PCC 7120 houses a family of eukaryotic-like Ser /Thr kinases. Some of these enzymes are required for cell growth or develop ment under certain conditions. None of them, however, has been shown experi mentally to possess Ser/Thr kinase activity. A gene, pknC, encoding a novel putative Ser/Thr kinase was isolated from Anabaena sp. PCC 7120. The recom binant PknC was shown to be phosphorylated on a Thr residue. This phosphory lation was probably due to the autophosphorylation activity of PknC itself because mutation of two amino acid residues within the subdomain II of its catalytic domain eliminated the phosphorylation of PknC. PknC displayed als o a Ser kinase activity towards several nonspecific substrates, and the two residues needed for PknC autophosphorylation was equally required for the phosphorylation of other substrates. PknC is thus a Ser/Thr kinase with bro ad substrate specificity. The activity of PknC is likely to be regulated in vivo in order to limit the spectrum of its substrate specificity.