GENETIC AND PHYSIOLOGICAL EVIDENCE FOR THE PRODUCTION OF N-ACYL HOMOSERINE LACTONES BY PSEUDOMONAS-SYRINGAE PV. SYRINGAE AND OTHER FLUORESCENT PLANT-PATHOGENIC PSEUDOMONAS SPECIES

N-acyl homoserine lactones (AHLs) function as cell density (quorum) se nsing signals and regulate diverse metabolic processes in several gram negative bacteria, We report that strains of Pseudomonas syringae pvs . syringae (Pss), tabaci and tomato as well as P. corrugata and P. sav astanoi produce difussible AHLs that activate the lux operons of Vibri o fischeri or the tra::lacZ fusion of Agrobacterium tumefaciens. In Ps s strain B3A, AHL production occurs in cell density dependent manner. Nucleotide sequence and genetic complementation data revealed the pres ence of ahlI(Pss), a luxI homolog within the Ahl(+) DNA of Pss strain B3A. The ahlI(Pss)(+) DNA expresses in AHL-deficient strains of P. flu orescens and E. carotovora subsp. carotovora (Ecc), and restores extra cellular enzyme production and pathogenicity in the Ecc strain. The de rivatives of Pss strains B3A and 301D carrying chromosomal ahlI::lacZ do not produce AHL, but like their wild type parents, produce extracel lular protease and the phytotoxin syringomycin as well as elicit the h ypersensitive reaction in tobacco leaves. While these strains also pro duce a basal level of beta-galactosidase activity, the expression of a hlI::lacZ is substantially stimulated in the presence of multiple copi es of the ahlI(Pss)(+) DNA or by the addition of cell-free spent cultu res containing AHL. The activation of B-galactosidase production occur s with spent cultures of some, but not all Pseudomonas strains which p roduce AHL as indicated by the Lux and tra::lacZ assays. Pss strains d eficient in the global regulatory genes, gacA or lemA, produce very lo w levels of AHL. Since inactivation of ahlI(Pss) eliminates AHL produc tion and since Ahl(+) Pseudomonas strains carry the homolog of ahlI(Ps s), we conclude that ahlI(Pss) specifies a key step in AHL biosynthesi s and it has been conserved in many plant pathogenic pseudomonads.