Pseudomonas aeruginosa is a wide-host-range gram negative opportunistic pathogen. P. aeruginosa produces many virulence factors the production of which is under control of quorum sensing. Several genes were found in P. aeruginosa which can affect quorum sensing. Unlike P. aeruginosa, P. syringae is a plant pathogen with around fifty pathovars and the quorum sensing circuitry is not well known in this species. The PA4204 gene encodes a periplasmic protein with a COG2706 domain corresponding to a 3-carboxy-cis, cis-muconate lactonizing enzyme (CMLE). Homologues are present in the genomes of all fluorescent pseudomonads, including in P. syringae where it is the predicted product of Psyr_1712. A PA4204 mutant grows slowly in LB and CAA (Casamino acid) media after a long lag-phase (6h) and formed small colonies on plates. Pseudo-revertants appeared frequently, as larger colonies. The PA4204 mutant could not swarm or twitch and it has reduced swimming capacity while the revertants could swim, twitch and swarm almost like the wild-type. Virulence factors such as pyoverdine and elastase were produced in lower amount by the mutant and there was almost no production of pyocyanin. Analysis of signal molecules revealed that the production of C4-HSL was greatly reduced in the PA4204 mutant while the production of 3-oxo-C12-HSL was reduced, but to a lesser extent. Production of PQS was however not affected. Complementation with the wild-type gene restored pyocyanin production to the PA4204 mutant while a mutant with a histidine to alanine substitution at position 181 (predicted CMLE active site) could not. Introduction in trans of the Psyr_1712 gene also restored pyocyanin production, confirming that the products of the two genes are homologues. Interestingly, the PA4204 mutant could elicit rapid hypersensitive reaction on tobacco which was faster and stronger than the wild-type. In the Psyr_1712 mutant pyoverdine production and swarming motility were higher comparing to the wild-type. Production of 3-oxo-C6-HSL was also strongly reduced in the Psyr_1712 mutant. Quantitative real time PCR showed that the expression of Psyr_1712 is under control of quorum sensing. The P. syringae Psyr_1712 mutant showed intermediate virulence in bean pods, intermediary between the wild-type and a ahlI negative mutant (quorum sensing-negative), which produces larger lesions. The HR reaction on tobacco in the case of Psyr_1712 mutant was similar to the wild-type while the ahlI mutant could elicit stronger and faster HR. Since PA4204 and Psyr_1712 could modulate fitness and quorum sensing in these bacteria we suggest the name of fqsM (fitness and quorum sensing modulator).

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