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J. Bacteriol. doi:10.1128/JB.01154-08
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Functional analysis of three sulfide:quinone oxidoreductase homologs in Chlorobaculum tepidum

College of Marine and Earth Studies and Delaware Biotechnology Institute, University of Delaware, 15 Innovation Way, DE 19711, USA

^* To whom correspondence should be addressed. Email: tehanson{at}udel.edu.

	Abstract

Sulfide:quinone oxidoreductase catalyzes sulfide oxidation during sulfide-dependent chemo- and phototrophic growth in bacteria. The green sulfur bacterium Chlorobaculum tepidum (formerly Chlorobium tepidum) can grow on sulfide as the sole electron donor and sulfur source. C. tepidum contains genes encoding three SQR homologs, CT0117, CT0876, and CT1087. This study examined which, if any, of the SQR homologs possess sulfide-dependent ubiquinone reduction activity and are required for growth on sulfide. Unlike CT0117 and CT0876, transcripts of CT1087 were only detected when cells actively oxidized sulfide. Mutation of CT0117 or CT1087 in C. tepidum decreased SQR activity in membrane fractions, and the CT1087 mutant could not grow with sulfide 6 mM. Mutation of both of CT0117 and CT1087 in C. tepidum completely abolished SQR activity and the double mutant failed to grow with sulfide 4 mM. A C-terminal His₆-tagged CT1087 protein was membrane-localized as was SQR activity. Epitope tagged CT1087 was detected only when sulfide was actively consumed by cells. Recombinantly produced CT1087 and CT0117 proteins had SQR activity, while CT0876 did not. In summary, under the conditions tested, we conclude that both CT0117 and CT1087 function as SQR proteins in C. tepidum. CT0876 may support the growth of C. tepidum at low sulfide concentrations, but no evidence was found for SQR activity associated with this protein.