We have cloned a positive regulatory locus, designated lemA, that is required for two diverse aspects of plant disease: lesion formation by Pseudomonas syringae pv. syringae on beam and the production of the phytotoxins tabtoxin and sytringomycin by several Pseudomonas syringae strains. In P. syringae pv. syringae, a functional lemA gene is required for lesion formation and for the production of syringomycin and protease. The lemA analog within P. syringae pv. coronafaciens is required for tabtoxin production but not lesion formation by this pathovar. Analysis of the sequence of this gene reveals extensive amino acid similarity of the predicted LemA protein to a family of transmembrane transcription regulators. In addition, northern blot analysis has shown that a transcript from the tblA gene within the tabtoxin biosynthetic region is absent in derivatives of P. syringae pv. coronafaciens containing a lemA mutation. It appears, therefore, that the lemA gene functions as a positive transcriptional regulator of genes involved in lesion formation and toxin production in P. syringae. It is the goal of this proposal to purify the enzyme encoded by the lemA gene to test whether, as predicted by sequence comparisons, it is a plasma membrane proteins with an extracellular receptor domain and a cytoplasmic protein-phosphylating domain. We will use DNA fragments from the promoter region of the tblA gene to test the potential for DNA binding by the LemA protein. Using random Tn5lac insertions into a P. syringae pv. syringae mutant containing a temperature-sensitive lemA mutation, we will identify lemA- regulated genes in P. syringae pv. syringae. We have identified an additional Tn5 mutation of P. syringae pv. syringae that does not map to the lemA locus but which also eliminates lesion formation. We will characterize this gene by DNA sequence analysis and examine its potential regulatory linkage to lemA. %%% The plant pathogen Pseudomonas syringae has become a model system for understanding how bacteria cause plant disease. These studies focus on a particular genetics locus, lemA, which is required for two aspects of the disease process-lesion formation and toxin production. The mechanism by which a single protein controls two disease processes will be probed.
