1Department of Soil and Water Sciences, 2159 McCarty Hall A, Institute of Food and Agricultural Sciences (IFAS), University of Florida, Gainesville 32611-0290, U.S.A.; 2Australian Research Council Centre of Excellence for Integrative Legume Research, Genomic Interactions Group, Research School of Biological Sciences, GPO Box 475, Australian National University, Canberra ACT 2601, Australia; 3Department of Microbiology, Wing Hall 359, Cornell University, Ithaca, NY 14853, U.S.A.; 4Chemistry Department, SUNY College at Fredonia, Fredonia, NY 14063, U.S.A.; 5Biology Department, University of Dayton, Dayton, OH 45460, U.S.A.; 6Department of Plant Sciences, Mail Stop #1, Plant and Environmental Sciences Bldg., University of California, Davis 95616, U.S.A.
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Accepted 15 February 2007.
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 AiiA lactonase from Bacillus produced in S. meliloti was shown to enzymatically inactivate S. meliloti's N-acyl homoserine lactone (AHL) QS signals, thereby disrupting normal QS regulation. Sixty proteins were differentially accumulated in the AiiA-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 AiiA-producing strain, although the effects of AiiA on protein accumulation did not always correspond to effects on transcription. The QS signal-deficient strain was reduced significantly in nodule initiation during the first 12 h 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.
© 2007 The American Phytopathological Society