Phaseolotoxin synthesis is regulated by the GacS/GacA two-component system

Pseudomonas syringae pv. phaseolicola is the causal agent of halo blight disease of beans (Phaseolus vulgaris L.), which is characterized by water-soaked lesions surrounded by a chlorotic halo resulting from the action of a non-host-specific toxin known as phaseolotoxin, that inhibits the enzyme ornithine carbamoyltransferase involved in the arginine biosynthesis pathway. It was previously reported that genes within the Pht cluster were involved in the regulation and synthesis of phaseolotoxin. The GacS/GacA two component signal transduction system controls important pathogenicity and virulence mechanisms in several Gram-negative bacteria. In the present study we hybridized a genomic microarray of P. syringae pv. phaseolicola NPS3121 to compare transcriptional profiles from wild type strain and a gacA- null mutant with a Tox- 11 phenotype. Results show that GacA controls expression of genes within the Pht cluster as well as another group of clustered genes located in a 13 different region in the bacterial chromosome that contains at least one gene unambiguously shown to be directly involved in phaseolotoxin biosynthesis. Results suggest that this cluster is a new pathogenicity island containing genes whose regulation is also under GacA regulatory cascade and it will require further investigation to determine gene functions and their relationship to virulence mechanisms Transcriptional profiles of a P. syringae pv. phaseolicola NPS3121 null gacA mutant were compared to those of the wild-type strain. Cultures were grown in minimal medium at 18°C (which is the temperature at which phaseolotoxin is produced and the bacteria achieves full virulence) and harvested during the late log phase of the exponential growth. RNA from three biological replicates and each with two technical replicates were hybridize to control variation that might affect data interpretation.