The Quorum-Sensing System In A Plant Bacterium Mesorhizobium Huakuii Affects Growth Rate And Symbiotic Nodulation

Previous studies demonstrated that the detection of quorum sensing signals from Mesorhizobium huakuii CCBAU 21173 by KYC55 (pJZ372) (pJZ384) (pJZ410), which is a ultrasensitive AHL bioassay strain. To further investigate the properties of these molecules and the possible role of quorum sensing in the process of symbiosis, we have isolated and characterized two transposon insertion mutants (YJG2 and YJG4) that are defective in autoinducer production using transposon mutagenesis.To characterize the transposon insertions of two autoinducer-deficient strains, we performed an arbitrary PCR to identify transposon sites. Sequence analysis of arbitrary PCR products showed that the transposon insertion of mutant YJG2 disrupted a gene whose product is homologous to (72% identity) the peptidase M20D in Ralstonia metallidurans CH34. The transposon in the mutant YJG4 disrupted a gene encoding a protein similar to (38% identity) a hypothetic protein (ML19148) of Mesorhizobium loti MAFF303099. Interestingly, this gene is located in an operon that is involved in the peptide ABC transporter system.Although we detected AHL-like diffusible quorum signals from M. huakuii, sequence analysis of two mutants that were unable to produce autoinducers indicated that genes involved in autoinducer synthesis are not common luxI-family homologs, but were involved in peptide synthesis and transport. To further investigate the characteristics of M. huakuii autoinducers, we introduced a plasmid that constitutively expresses an AHL lactonase gene (attM) from A. tumefaciens into M. huakuii strains and assayed the supernatant for autoinducer activity. This lactonase is able to degrade AHLs by hydrolyzing the AHL lactone ring (10). Overexpression of the attM gene in M. huakuii did not reduce the autoinducer activity, while the same construct significantly reduced AHL production in A. tumefaciens. These data suggested that M. huakuii autoinducers we detected using the AHL bioassay strains were not AHLs. Considering that peptide synthesis and transport genes...