In Pseudomonas aeruginosa, like in other Gram-negative bacteria, quorum sensing regulates multiple phenotypic traits associated with virulence via the production of N-acyl homoserine lactones or other signal molecules such as the Pseudomonas quinolone signal (PQS). In our previous studies we observed that the Pseudomonas aeruginosa PA4204 gene, which encodes a protein with a signal peptide and with a COG2706 domain corresponding to a 3-carboxy-cis, cis-muconate lactonizing enzyme, could affect the production of quorum sensing signal molecules. The PA4203 gene, upstream of PA4204, encodes a LysR-type regulator similar to BenM and CatM in Acenitobacter spp. Cis,cis muconate is the effector for BenM and CatM, suggesting that the product of PA4204 could be the effector for PA4203 and a molecule similar to cis,cis muconate. Interestingly a deletion of PA4203 affected the level of expression of PA4204 and mexGHI-opmD, and the PA4203 was highly expressed in a PA4204 mutant. This suggests that the product of PA4204 is the effector of the LysR regulator which negatively regulates its own transcription. The PA4203 mutant grew like the wild-type, but did not produce the blue-green pigment pyocyanin on Pseudomonas P agar. Surprisingly, the same mutant was unable to produce the siderophore pyoverdine in CAA medium, but we could detect the same amount of pyochelin, the second siderophore, as in the wild-type using IEF-CAS assay. The mutant was also more sensitive to H2O2 compared to the wild-type. The above results suggest that the LysR regulator encoded by PA4203 is needed for the expression of PA4204 (which is a modulator of quorum sensing) and mexGHI-opmD (which are part of PQS regulon) and for the production of virulence factors such as pyoverdine.

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