D101 is critical for the function of AttJ, a repressor of quorum quenching system in Agrobacterium tumefaciens.

The quorum quenching system of Agrobacterium tumefaciens is specifically activated upon entering the stationary phase. Evidence has shown that this system includes two key components: the IclR-type transcriptional factor AttJ (also named as BlcR) and the AHL-lactonase AttM (also named as BlcC). At exponential phase, AttJ binds to the promoter region of attM and thus suppresses the expression of attM. At stationary phase, however, the small molecule SSA directly binds to AttJ and relieves its inhibition of AttJ and thereby triggers the expression of attM. While the regulation of AttM has been extensively investigated, little is known about the regulation of AttJ. In this study, we demonstrated the D101 amino acid of AttJ is essential for the AttJ function. In vitro, the variant protein of AttJD101H appeared to be readily aggregated. In vivo, the D101H mutation in AttJ entirely abolished the inhibitory activity of AttJ and overexpressed attM in A. tumefaciens A6. In addition, D101H mutation led to an overexpression of attJ, indicating an auto-regulatory mechanism for the attJ regulation. Put together, these findings demonstrate that D101 is an important amino acid for the transcription activity of AttJ and the transcription of attJ is regulated by a negative feedback loop. These results expand previous biochemical characterization of AttJ and provide new mechanistic insights into the regulation of quorum quenching in A. tumefaciens.