In Pseudomonas syringae pv. syringae B728a, the Psyr_1712 locus ID encodes a putative protein with a signal peptide and a COG2706 domain of the type present in 3-carboxy-cis,cis-muconate lactonizing enzymes. An amino acid sequence alignment of the P. aeruginosa PpgL with other genome sequenced fluorescent pseudomonads such as P. syringae Psyr_1712 showed that they have the same enzymatic active site residue comprising one histidine, one glutamic acid and two arginines. Based on the similarity of the Psyr_1712 locus ID and PpgL of P. aeruginosa, it was designated as PspL (P seudomonas s yringae PpgL- like) protein. Deletion of the pspL gene caused a delay in lag phase growth of bacterium. Mutants lacking pspL were defective in N-acylhomoserine lactones production. The PspL with signal peptide was expressed in a ppgL mutant of P. aeruginosa and restored the defects. The presence of a lux-like box sequence in upstream of pspL along with decreased expression level of the pspL gene in an ahlI negative mutant indicated that the pspL gene is under control of quorum sensing. Furthermore, two acylhomoserinelactone regulated phenotypes, swarming motility and susceptibility to hydrogen peroxide were enhanced in ΔpspL mutant. Together, this work reveals the important role of the new PpgL-like protein PspL in quorum sensing of P. syringae pv. syringae B728a.