Enzymatic disruption of quorum-sensing ( QS) pathways in pathogenic organisms is a promising anti-infection therapeutic strategy. AHL-lactonase, a potent tool for biocontrol, can hydrolyze QS signal molecule N-acyl-homoserine lactones ( AHLs) into inactive products, thereby blocking the QS systems. A marine bacterial isolate Y2, identified as a Bacillus cereus subsp., was found capable of inactivating AHLs. The aiiA gene encoding the AHL-degrading enzyme from bacterial strain Y2 was cloned and expressed in Escherichia coli. The 28-kDa recombinant Y2-AiiA protein was purified and showed strong AHL-degrading activity. Sequence comparisons of Y2-aiiA with known AHL-lactonases revealed high identities in the deduced amino-acid sequences. Functional determination of a potential catalytic residue Tyr-194 of AHL-lactonases was performed by site-directed mutagenesis. As judged by AHL-degrading bioassay, substitution of Tyr-194 with Ala resulted in a dramatic decrease of activity compared with wild-type ( WT) recombinant Y2-AiiA, although the expression level of the mutated Y2-AiiA protein was equivalent to that of WT Y2-AiiA. These results suggested that the conserved residue Tyr-194 is critical for catalytic function of the novel AHL-lactonase. Enzymatic disruption of quorum-sensing (QS) pathways in pathogenic organisms is a promising anti-infection therapeutic strategy. AHL-lactonase, a potent tool for biocontrol, can hydrolyze QS signal molecule N-acyl-homoserine lactones (AHLs) into inactive products, thereby blocking the QS systems. A marine bacterial isolate Y2, identified as a Bacillus cereus subsp., was found capable of inactivating AHLs. The aiiA gene encoding the AHL-degrading enzyme from bacterial strain Y2 was cloned and expressed in Escherichia coli. The 28-kDa recombinant Y2-AiiA protein was purified and showed strong AHL-degrading activity. Sequence comparisons of Y2-aiiA with known AHL-lactonases revealed high identities in the deduced amino-acid sequences. Functional determination of a potential catalytic residue Tyr-194 of AHL-lactonases was performed by site-directed mutagenesis. As judged by AHL-degrading bioassay, substitution of Tyr-194 with Ala resulted in a dramatic decrease of activity compared with wild-type (WT) recombinant Y2-AiiA, although the expression level of the mutated Y2-AiiA protein was equivalent to that of WT Y2-AiiA. These results suggested that the conserved residue Tyr-194 is critical for catalytic function of the novel AHL-lactonase.