| Our interest in fatty acids as developmental signals has led us to examine the roles of phospholipases during development in the multicellular prokaryote, Myxococcus xanthus. In this study, we generated a phospholipase mutant (MD109) and used this mutant to examine the roles of both fatty acids and phospholipases during development. Characterization of MD109 led to the discovery of a tyrosine kinase that shares homology with eukaryotic tyrosine kinases, the first such identified in a prokaryote.; Based on the analysis of the developmental phenotype of MD109, we proposed that i-15:0, a branched-chain fatty acid, was acting as a developmental signal. This was supported by two observations; one, that there was a specific delay in the release of i-15:0, but not of other fatty acids during development in MD109; and two, exogenous addition of i-15:0 to developing MD109 cells rescued fruiting body formation, cell lysis and sporulation completely. These results were consistent with the hypothesis that branched-chain fatty acids may be part of a developmental signal called E-signal (76).; The complex phenotype of MD109 led us to propose the involvement of a kinase/phosphatase cascade during development. Characterization of vegetative phosphatase activity using phosphatase inhibitors, led to the surprising discovery of a protein tyrosine phosphatase and corresponding tyrosine phosphorylated substrates with apparent molecular masses of 83 and 40 kDa. Furthermore, the pattern of tyrosine phosphorylation changed during development, suggesting an involvement of tyrosine phosphorylation, a rare phosphorylation event in bacteria, in the developmental program of M. xanthus.; Finally, we identified a family of tyrosine kinase genes in M. xanthus by Southern Blot analysis using a probe corresponding to the consensus sequence of subdomain VIII of eukaryotic tyrosine kinases. Furthermore, the deduced amino acid sequence of a putative tyrosine kinase gene amplified from M. xanthus genomic DNA using primers corresponding to consensus sequences of eukaryotic tyrosine kinases, showed significant homology to eukaryotic tyrosine kinases. The presence of a tyrosine kinase in M. xanthus is consistent with the notion that the myxobacteria may have been among Nature's earliest experiments in multicellularity. |
