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Journal of Bacteriology, May 2000, p. 2831-2837, Vol. 182, No. 10
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Expression of Uptake Hydrogenase and Molybdenum Nitrogenase
in Rhodobacter capsulatus Is Coregulated by the
RegB-RegA Two-Component Regulatory System
Sylvie
Elsen,1,
Wanda
Dischert,2
Annette
Colbeau,2 and
Carl E.
Bauer1,*
Department of Biology, Indiana University,
Bloomington, Indiana 47405,1 and UMR
5092 CEA-CNRS-UJF, Laboratoire de Biochimie et Biophysique des
Systèmes Intégrés, Département de Biologie
Moléculaire et Structurale, CEA/Grenoble, 38054 Grenoble
cedex 9, France2
Received 2 December 1999/Accepted 16 February 2000
Purple photosynthetic bacteria are capable of generating cellular
energy from several sources, including photosynthesis, respiration, and
H2 oxidation. Under nutrient-limiting conditions, cellular energy can be used to assimilate carbon and nitrogen. This study provides the first evidence of a molecular link for the coregulation of
nitrogenase and hydrogenase biosynthesis in an anoxygenic
photosynthetic bacterium. We demonstrated that molybdenum nitrogenase
biosynthesis is under the control of the RegB-RegA two-component
regulatory system in Rhodobacter capsulatus. Footprint
analyses and in vivo transcription studies showed that RegA indirectly
activates nitrogenase synthesis by binding to and activating the
expression of nifA2, which encodes one of the two
functional copies of the nif-specific transcriptional
activator, NifA. Expression of nifA2 but not
nifA1 is reduced in the reg mutants up to
eightfold under derepressing conditions and is also reduced under
repressing conditions. Thus, although NtrC is absolutely required for
nifA2 expression, RegA acts as a coactivator of
nifA2. We also demonstrated that in reg mutants, [NiFe]hydrogenase synthesis and activity are increased up to
sixfold. RegA binds to the promoter of the hydrogenase gene operon and
therefore directly represses its expression. Thus, the RegB-RegA system
controls such diverse processes as energy-generating photosynthesis and
H2 oxidation, as well as the energy-demanding processes of
N2 fixation and CO2 assimilation.
*
Corresponding author. Mailing address: Department of
Biology, Indiana University, Jordan Hall, Bloomington, IN 47405. Phone: (812) 855-6595. Fax: (812) 855-6705. E-mail:
cbauer{at}bio.indiana.edu.

Present address: UMR 5092 CEA-CNRS-UJF, Laboratoire de Biochimie et
Biophysique des Systèmes Intégrés, Département
de
Biologie Moléculaire et Structurale, CEA/Grenoble, 38054 Grenoble
cedex 9,
France.
Journal of Bacteriology, May 2000, p. 2831-2837, Vol. 182, No. 10
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
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