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Journal of Bacteriology, September 2003, p. 5602-5610, Vol. 185, No. 18
0021-9193/03/$08.00+0     DOI: 10.1128/JB.185.18.5602-5610.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Mutations in rpoBC Suppress the Defects of a Sinorhizobium meliloti relA Mutant

Derek H. Wells and Sharon R. Long^*

Department of Biological Sciences, Stanford University, Stanford, California 94305

Received 21 March 2003/ Accepted 3 July 2003

The nitrogen-fixing symbiosis between Sinorhizobium meliloti and Medicago sativa requires complex physiological adaptation by both partners. One method by which bacteria coordinately control physiological adaptation is the stringent response, which is triggered by the presence of the nucleotide guanosine tetraphosphate (ppGpp). ppGpp, produced by the RelA enzyme, is thought to bind to and alter the ability of RNA polymerase (RNAP) to initiate and elongate transcription and affect the affinity of the core enzyme for various sigma factors. An S. meliloti relA mutant which cannot produce ppGpp was previously shown to be defective in the ability to form nodules. This mutant also overproduces a symbiotically necessary exopolysaccharide called succinoglycan (38). The work presented here encompasses the analysis of suppressor mutants, isolated from host plants, that suppress the symbiotic defects of the relA mutant. All suppressor mutations are extragenic and map to either rpoB or rpoC, which encode the ß and ß' subunits of RNAP. Phenotypic, structural, and gene expression analyses reveal that suppressor mutants can be divided into two classes; one is specific in its effect on stringent response-regulated genes and shares striking similarity with suppressor mutants of Escherichia coli strains that lack ppGpp, and another reduces transcription of all genes tested in comparison to that in the relA parent strain. Our findings indicate that the ability to successfully establish symbiosis is tightly coupled with the bacteria's ability to undergo global physiological adjustment via the stringent response.

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* Corresponding author. Mailing address: Department of Biological Sciences, Stanford University, 371 Serra Mall, Stanford, CA 94305-5020. Phone: (650) 723-3232. Fax: (650) 725-8309. E-mail: srl{at}stanford.edu.

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Journal of Bacteriology, September 2003, p. 5602-5610, Vol. 185, No. 18
0021-9193/03/$08.00+0     DOI: 10.1128/JB.185.18.5602-5610.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

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