Genetic studies of toxin synthesis and resistance in Pseudomonas syringae pv. phaseolicola

Abstract

The genetic basis of toxin production and resistance in the plant pathogenic bacterium Pseudomonas syringae pv. Phaseolicola has been investigated. The major findings are: 1. No correlation was found between the plasmid content of six P.s. phaseolicola isolates and the ability of these isolates to synthesise phaseolotoxin. Very large plasmids were not detected in these isolates. 2. Transposon mutagenesis using Tn5 indicated that at least 5 separate genes are essential for phaseolotoxin synthesis. These genes are encoded by chromosomal DNA and are not arranged as an operon. This is the first direct evidence to indicate the number and genetic locations of tox genes in P.s.phaseolicola. 3. Non-toxigenic mutants generated with transposon Tn5 were pathogenic on beans and retained resistance to phaseolotoxin. 4. The gene encoding ornithine carbamoyltransferase (OCTase) was isolated from a toxigenic, toxin-resistant P.s. phaseolicola strain. This gene does not encode toxin resistance in E.coli or in a non-toxigenic, toxin-sensitive P.s.phaseolicola strain. 5. The isolated OCTase gene shares no detectable DNA sequence homology with the OCTase gene of E.coli; limited homology exists with the P.aeruginosa OCTase gene. Substantial homology exists between the isolated gene and DNA in one restriction fragment from DNA of a non-toxigenic strain.