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Journal of Bacteriology, August 2005, p. 5249-5258, Vol. 187, No. 15
0021-9193/05/$08.00+0     doi:10.1128/JB.187.15.5249-5258.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Identification of Genes Required for Recycling Reducing Power during Photosynthetic Growth

University of Wisconsin—Madison Department of Bacteriology, 420 Henry Mall, Madison, Wisconsin 53711

Received 2 February 2005/ Accepted 3 May 2005

Photosynthetic organisms have the unique ability to transform light energy into reducing power. We study the requirements for photosynthesis in the -proteobacterium Rhodobacter sphaeroides. Global gene expression analysis found that 50 uncharacterized genes were regulated by changes in light intensity and O₂ tension, similar to the expression of genes known to be required for photosynthetic growth of this bacterium. These uncharacterized genes included RSP4157 to -4159, which appeared to be cotranscribed and map to plasmid P004. A mutant containing a polar insertion in RSP4157, CT01, was able to grow via photosynthesis under autotrophic conditions using H₂ as an electron donor and CO₂ as a carbon source. However, CT01 was unable to grow photoheterotrophically in a succinate-based medium unless compounds that could be used to recycle reducing power (the external electron acceptor dimethyl sulfoxide (DMSO) or CO₂) were provided. This suggests that the insertion in RSP4157 caused a defect in recycling reducing power during photosynthetic growth when a fixed carbon source was present. CT01 had decreased levels of RNA for genes encoding putative glycolate degradation functions. We found that exogenous glycolate also rescued photoheterotrophic growth of CT01, leading us to propose that CO₂ produced from glycolate metabolism can be used by the Calvin cycle to recycle reducing power generated in the photosynthetic apparatus. The ability of glycolate, CO₂, or DMSO to support photoheterotrophic growth of CT01 suggests that one or more products of RSP4157 to -4159 serve a previously unknown role in recycling reducing power under photosynthetic conditions.

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