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Journal of Bacteriology, August 2005, p. 5460-5469, Vol. 187, No. 15
0021-9193/05/$08.00+0     doi:10.1128/JB.187.15.5460-5469.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Effective Symbiosis between Rhizobium etli and Phaseolus vulgaris Requires the Alarmone ppGpp

Martine Moris, Kristien Braeken, Eric Schoeters, Christel Verreth, Serge Beullens, Jos Vanderleyden, and Jan Michiels^*

Centre of Microbial and Plant Genetics, Katholieke Universiteit Leuven, 3001 Heverlee, Belgium

Received 20 January 2005/ Accepted 22 April 2005

The symbiotic interaction between Rhizobium etli and Phaseolus vulgaris, the common bean plant, ultimately results in the formation of nitrogen-fixing nodules. Many aspects of the intermediate and late stages of this interaction are still poorly understood. The R. etli relA gene was identified through a genome-wide screening for R. etli symbiotic mutants. RelA has a pivotal role in cellular physiology, as it catalyzes the synthesis of (p)ppGpp, which mediates the stringent response in bacteria. The synthesis of ppGpp was abolished in an R. etli relA mutant strain under conditions of amino acid starvation. Plants nodulated by an R. etli relA mutant had a strongly reduced nitrogen fixation activity (75% reduction). Also, at the microscopic level, bacteroid morphology was altered, with the size of relA mutant bacteroids being increased compared to that of wild-type bacteroids. The expression of the ^N-dependent nitrogen fixation genes rpoN2 and iscN was considerably reduced in the relA mutant. In addition, the expression of the relA gene was negatively regulated by RpoN2, the symbiosis-specific ^N copy of R. etli. Therefore, an autoregulatory loop controlling the expression of relA and rpoN2 seems operative in bacteroids. The production of long- and short-chain acyl-homoserine-lactones by the cinIR and raiIR systems was decreased in an R. etli relA mutant. Our results suggest that relA may play an important role in the regulation of gene expression in R. etli bacteroids and in the adaptation of bacteroid physiology.

These authors contributed equally to this article.