Multiple copies of a DNA sequence from Pseudomonas syringae pathovar phaseolicola abolish thermoregulation of phaseolotoxin production

Phaseolotoxin, a phytotoxin of Pseudomonas syringae pv. phaseolicola, is produced at 18°C but not at 28°C. Here we report that a fragment (24.4 kb) cloned from the wild-type strain, which does not harbour a gene(s) involved in phaseolotoxin biosynthesis, abolishes this thermoregulation in the wild type and suppresses a Tox^? mutant at both temperatures. A subclone harbouring a 465bp fragment contains motifs that are characteristic of DNA-binding sites. In mobility shift assays we have detected a protein(s) from the wild-type and the mutant strains, grown at appropriate temperatures, that specifically binds to the fragment containing the DNA-binding motifs. We propose that the binding protein is a repressor which is ‘titrated’ by this fragment when it is present in the cell on a multiple copy plasmid, thus allowing expression of phaseolotoxin genes.