Studies on the cytoplasmic protein tyrosine kinase activity of the Antarctic psychrotrophic bacterium Pseudomonas syringae

The Antarctic psychrotrophic bacterium Pseudomonas syringae contains a 66‐kDa cytoplasmic protein which was found to be phosphorylated on a tyrosine residue [Ray, M.K. et al. (1994) FEMS Microbiol. Lett. 122, pp. 49–54]. To investigate the nature of the cytoplasmic protein tyrosine kinase and its role in the bacterial physiology, we carried out some biochemical studies of the enzyme in vitro in the presence of exogenous peptide substrates and expression studies in vivo at low and high temperature during various phases of growth. The results suggest that the protein tyrosine kinase associated with the cytoplasmic fraction of the bacterium has certain similarities and dissimilarities with the known eukaryotic tyrosine kinases. The protein tyrosine kinase could phosphorylate exogenous substrate corresponding to the N‐terminal peptide of p34 cdc2 kinase but could not do so on poly(Glu:Tyr). The enzyme could not be inhibited by genistein, staurosporine and dimethyl aminopurine, but could be inhibited by piceatannol which is a known competitive inhibitor of the peptide binding site of mammalian protein tyrosine kinases. The enzyme activity in the cytoplasm is uniquely inhibited by sodium orthovanadate (IC 50 =20 μM) which is a known protein tyrosine phosphatase inhibitor. The expression studies show that the enzyme is produced more at a higher temperature (22°C) of growth than at lower temperature (4°C) and during the stationary phase of growth of P. syringae .