N-Acylhomoserine lactones (AHLs) play an important role in regulating virulence factors in pathogenic bacteria. Recently, the enzymatic inactivation of AHLs, which can be used as antibacterial targets, has been identified in several soil bacteria. In this study, strain M664, identified as a Streptomyces sp., was found to secrete an AHL-degrading enzyme into a culture medium. The ahlM gene for AHL degradation from Streptomyces sp. strain M664 was cloned, expressed heterologously in Streptomyces lividans, and purified. The enzyme was found to be a heterodimeric protein with subunits of approximately 60 kDa and 23 kDa. A comparison of AhlM with known AHL-acylases, Ralstonia strain XJ12B AiiD and Pseudomonas aeruginosa PAO1 PvdQ, revealed 35% and 32% identities in the deduced amino acid sequences, respectively. However, AhlM was most similar to the cyclic lipopeptide acylase from Streptomyces sp. strain FERM BP-5809, exhibiting 93% identity. A mass spectrometry analysis demonstrated that AhlM hydrolyzed the amide bond of AHL, releasing homoserine lactone. AhlM exhibited a higher deacylation activity toward AHLs with long acyl chains rather than short acyl chains. Interestingly, AhlM was also found to be capable of degrading penicillin G by deacylation, showing that AhlM has a broad substrate specificity. The addition of AhlM to the growth medium reduced the accumulation of AHLs and decreased the production of virulence factors, including elastase, total protease, and LasA, in P. aeruginosa. Accordingly, these results suggest that AHL-acylase, AhlM could be effectively applied to the control of AHL-mediated pathogenicity.Many bacterial species employ complex communication systems that link cell density and gene expression to regulate a broad range of biological functions (11,27). Such cell-to-cell communication, termed quorum sensing (QS), depends on the production, diffusion, and recognition of small signal molecules. In gram-negative bacteria, the most intensively studied QS systems rely on the interaction of N-acylhomoserine lactones (AHLs), whose synthesis is directed by LuxI-type AHL synthases, with LuxR-type transcriptional regulators. AHLs share identical homoserine lactone rings yet vary in length and the substitution of an acyl side chain, which determines the signal specificities in different bacterial species. At low population densities, AHLs are present at a basal concentration level. As the population density increases, the accumulated AHLs reach concentrations that allow binding to regulators, and the complexes then activate or inactivate the expression of target genes. In particular, AHL-mediated QS systems play a key role in controlling a range of activities implicated in pathogen and host interactions, such as antibiotic production and the expression of virulence factors (4,5,28,30).Recently, AHLs have been found to be subject to biological inactivation, thereby attracting attention as QS signal interference. Two groups of AHL-degrading enzymes, classified according to the AHL cleavage site, are pro...