Bacteria, when high in cell density, will produce chemical signal molecules, called atuoinducers (AI), to regulate gene expression. Autoinducers of high activity in the spent culture supernatant of Mesorhizobium tianshanense CCBAU060A were detected with the aid of AI bioassay strain JZA1. One autoinducerdeficient mutant was obtained by screening Mesorhizobium tianshanense CCBAU060A mutated with mariner transposon Analysis of the flanking region of transposon insertion found a LuxItype synthase gene, mrtI2, which wascloned into the broad host range expression vectors pYC12, and transformed into Escherichia coliDH5α . The putative AHL synthase gene was overexpressed in Escherichia coli, and four different autoinducers could be detected in the supernatant of the positive recombinant.