Regulation of tabtoxin production by the lemA gene in Pseudomonas syringae

Pseudomonas syringae pv. coronafaciens, a pathogen of oats, was mutagenized with Tn5 to generate mutants defective in tabtoxin production. From a screen of 3,400 kanamycin-resistant transconjugants, seven independent mutants that do not produce tabtoxin (Tox-) were isolated. Although the Tn5 insertions within these seven mutants were linked, they were not located in the previously described tabtoxin biosynthetic region of P. syringae. Instead, all of the insertions were within the P. syringae pv. coronafaciens lemA gene. The lemA gene is required by strains of P. syringae pv. syringae for pathogenicity on bean plants (Phaseolus vulgaris). In contrast to the phenotype of a P. syringae pv. syringae lemA mutant, the Tox- mutants of P. syringae pv. coronafaciens were still able to produce necrotic lesions on oat plants (Avena sativa), although without the chlorosis associated with tabtoxin production. Northern (RNA) hybridization experiments indicated that a functional lemA gene was required for the detection of a transcript produced from the tblA locus located in the tabtoxin biosynthetic region. Marker exchange mutagenesis of the tblA locus resulted in loss of tabtoxin production. Therefore, both the tblA and lemA genes are required for tabtoxin biosynthesis, and the regulation of tabtoxin production by lemA probably occurs at the transcriptional level.