Effects of AiiA-mediated quorum quenching in Sinorhizobium meliloti on quorum-sensing signals, proteome patterns, and symbiotic interactions

Many behaviors in bacteria, including behaviors important to pathogenic and symbiotic interactions with eukaryotic hosts, are regulated by a mechanism called quorum sensing (QS). A quorum-quenching approach was used here to identify QS-regulated behaviors in the N-fixing bacterial symbiont Sinorhizobium meliloti. The AHA lactonase from Bacilluv produced in S. meliloti was shown to enzymatically inactivate S. meffloti's N-acyl homoserine lactone (AHL) QS signals, thereby disrupting normal QS regulation. Sixty proteins were differentially accumulated in the AHA-producing strain versus the control in early log or early stationary phase cultures. Fifty-two of these QS-regulated proteins, with putative functions that include cell division, protein processing and translation, metabolite transport, oxidative stress, and amino acid metabolism, were identified by peptide mass fingerprinting. Transcription of representative genes was reduced significantly in the AHA-producing strain, although the effects of AHA on protein accumulation did not always correspond to effects on transcription. The QS signal-dericient strain was reduced significantly in nodule initiation during the first 12 It after inoculation onto Medicago truncatula host plants. The AiiA lactonase also was found to substantially inactivate two of the AHL mimic compounds secreted by M. truncatula. This suggests some structural similarity between bacterial AHLs and these mimic compounds. It also indicates that quorum quenching could be useful in identifying Sinorhizobium genes that are affected by such host QS mimics in planta.