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J. Bacteriol. doi:10.1128/JB.01278-06
Copyright (c) 2006, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Role of Rel_Gsu in Stress Response and Fe(III) Reduction in Geobacter sulfurreducens

Department of Microbiology, University of Massachusetts, Amherst, MA 01003, The Institute for Genomic Research, Rockville, MD 20850, and Department of Biological Science, University of Central Lancashire, Preston, Lancashire, PR1 2HE, UK

^* To whom correspondence should be addressed. Email: lauried{at}microbio.umass.edu,

	Abstract

Geobacter species are key members of the microbial community in many subsurface environments in which dissimilatory metal reduction is an important process. The genome of Geobacter sulfurreducens contains a relA/spoT homolog, designated rel_Gsu, which is predicted to catalyze both the synthesis and degradation of guanosine 3',5' bispyrophosphate (ppGpp), a regulatory molecule that signals slow growth in response to nutrient limitation in bacteria and plants. To evaluate the physiological role of Rel_Gsu in G. sulfurreducens, a rel_Gsu mutant was constructed and characterized, and ppGpp levels were monitored under various conditions in both the wild type and rel_Gsu mutant strains. In the wild type strain, ppGpp and ppGp, were produced in response to acetate and nitrogen deprivation, whereas exposure to oxygen resulted in an accumulation of ppGpp alone. Neither ppGpp nor ppGp could be detected in the rel_Gsu mutant. The rel_Gsu mutant consistently grew to a higher cell density than the wild type in acetate:fumarate medium, and was less tolerant of oxidative stress than the wild type. The capacity for Fe(III) reduction was substantially diminished in the mutant. Microarray and quantitative RT-PCR analyses indicated that during stationary phase growth, protein synthesis genes were up-regulated in the rel_Gsu mutant and genes involved in stress responses and electron transport, including several implicated in Fe(III) reduction, were down-regulated in the mutant. The results are consistent with a role for Rel_Gsu in regulating growth, stress responses, and Fe(III) reduction in G. sulfurreducens under conditions likely to be prevalent in subsurface environments.

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