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Journal of Bacteriology, February 2003, p. 809-822, Vol. 185, No. 3
0021-9193/03/$08.00+0     DOI: 10.1128/JB.185.3.809-822.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Quorum Sensing in Rhizobium sp. Strain NGR234 Regulates Conjugal Transfer (tra) Gene Expression and Influences Growth Rate

Xuesong He, William Chang,{dagger} Deanne L. Pierce, Laura Ort Seib,{ddagger} Jennifer Wagner, and Clay Fuqua*

Department of Biology, Indiana University, Bloomington, Indiana 47405

Received 26 July 2002/ Accepted 31 October 2002

Rhizobium sp. strain NGR234 forms symbiotic, nitrogen-fixing nodules on a wide range of legumes via functions largely encoded by the plasmid pNGR234a. The pNGR234a sequence revealed a region encoding plasmid replication (rep) and conjugal transfer (tra) functions similar to those encoded by the rep and tra genes from the tumor-inducing (Ti) plasmids of Agrobacterium tumefaciens, including homologues of the Ti plasmid quorum-sensing regulators TraI, TraR, and TraM. In A. tumefaciens, TraI, a LuxI-type protein, catalyzes synthesis of the acylated homoserine lactone (acyl-HSL) N-3-oxo-octanoyl-L-homoserine lactone (3-oxo-C8-HSL). TraR binds 3-oxo-C8-HSL and activates expression of Ti plasmid tra and rep genes, increasing conjugation and copy number at high population densities. TraM prevents this activation under noninducing conditions. Although the pNGR234a TraR, TraI, and TraM appear to function similarly to their A. tumefaciens counterparts, the TraR and TraM orthologues are not cross-functional, and the quorum-sensing systems have differences. NGR234 TraI synthesizes an acyl-HSL likely to be 3-oxo-C8-HSL, but traI mutants and a pNGR234a-cured derivative produce low levels of a similar acyl-HSL and another, more hydrophobic signal molecule. TraR activates expression of several pNGR234a tra operons in response to 3-oxo-C8-HSL and is inhibited by TraM. However, one of the pNGR234a tra operons is not activated by TraR, and conjugal efficiency is not affected by TraR and 3-oxo-C8-HSL. The growth rate of NGR234 is significantly decreased by TraR and 3-oxo-C8-HSL through functions encoded elsewhere in the NGR234 genome.

* Corresponding author. Mailing address: Department of Biology, 1001 E. 3rd St., Jordan Hall 142, Bloomington, IN 47405. Phone: (812) 856-6005. Fax: (812) 855-6705. E-mail: cfuqua{at}bio.indiana.edu.

{dagger} Present address: Department of Microbiology, Cornell University, Ithaca, NY 14853.

{ddagger} Present address: U.S. Plant Soil Nutrition Lab, USDA-Agricultural Research Station, Cornell University, Ithaca, NY 14853.

Journal of Bacteriology, February 2003, p. 809-822, Vol. 185, No. 3
0021-9193/03/$08.00+0     DOI: 10.1128/JB.185.3.809-822.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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