Cloning and characterisation of the pknD gene encoding an eukaryotic-type protein kinase in the cyanobacterium Anabaena sp. PCC7120

Abstract

Protein phosphorylation catalysed by protein kinases is an important mechanism for signal transduction in both prokaryotes and eukaryotes. A novel gene, pknD, encoding a protein similar to eukaryotic-type protein kinases, was cloned from Anabaena sp. PCC7120. The N-terminal region of PknD is 60% identical to that of PknA, another putative Ser/Thr kinase from the same strain. Both PknA and PknD have C-terminal regions that are rich in Pro and Thr residues. Expression of pknD was undetectable by RNA/DNA hybridisation and was thus examined by RT-PCR. The pknD transcript was detected in filaments cultured in the presence of either nitrate or ammonium as a source of combined nitrogen, and also in filaments transferred from nitrate-sufficient to N₂-fixing conditions. pknD mutants were created, and their growth characteristics under different nitrogen regimes and their capacity for heterocyst development were investigated. The growth rates of the mutants were similar to those of the wild-type strain in the presence of either nitrate or ammonium, but were only 20% that of the wild type under N₂-fixing conditions. The rate of nitrogenase activity is normal in pknD mutant under aerobic conditions. Under nitrogen-fixing conditions, the inactivation of pknD led to enhanced modification of the P_II protein compared to the weak phosphorylation of P_II observed in the wild-type strain. This high level of P_II phosphorylation in the pknD mutant is reminiscent of the situation in nitrogen-starved Synechococcus PCC7942 cells. PknD might be involved in regulating nitrogen metabolism or nitrogen trafficking from heterocysts to vegetative cells.