Inactivation of AHLs by Ochrobactrum sp. A44 depends on the activity of a novel class of AHL acylase

The soil isolate Ochrobactrum sp. A44 inactivates N-acyl homoserine lactone (AHL) quorum sensing signal molecules and is capable of quenching the AHL-dependent virulence of Pectobacterium carotovorum in planta. To characterize this AHL inactivating activity, Ochrobactrum cell extracts were prepared and their capacity to degrade a broad range of AHLs was determined. AHLs with acyl chains ranging from C4 to C14 with or without 3-oxo or 3-hydroxy substituents were all inactivated to varying extents; long chain AHLs were generally more susceptible than short chain compounds irrespective of the three position substituent. HPLC and LC-tandem mass spectrometry of the AHL degradation products revealed that the AHL inactivating activity present in the Ochrobactrum cell extract cleaved the AHL amide bond. To identify the gene(s) responsible for AHL degradation, Ochrobactrum sp. A44 was subjected to random transposon (Tn) mutagenesis and the resulting mutants screened for the loss of AHL acylase activity. The Tn insertion in mutant A6731 was mapped to a gene termed aiiO, the translated product of which belongs to the α/β hydrolase superfamily which constitutes a novel type of AHL acylase.