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J. Biol. Chem., Vol. 283, Issue 23, 15762-15770, June 6, 2008

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Identification of an Atypical Membrane Protein Involved in the Formation of Protein Disulfide Bonds in Oxygenic Photosynthetic Organisms^*

From the Department of Biology, Washington University, St. Louis, Missouri 63130

The evolution of oxygenic photosynthesis in cyanobacteria nearly three billion years ago provided abundant reducing power and facilitated the elaboration of numerous oxygen-dependent reactions in our biosphere. Cyanobacteria contain an internal thylakoid membrane system, the site of photosynthesis, and a typical Gram-negative envelope membrane system. Like other organisms, the extracytoplasmic space in cyanobacteria houses numerous cysteine-containing proteins. However, the existence of a biochemical system for disulfide bond formation in cyanobacteria remains to be determined. Extracytoplasmic disulfide bond formation in non-photosynthetic organisms is catalyzed by coordinated interaction between two proteins, a disulfide carrier and a disulfide generator. Here we describe a novel gene, SyndsbAB, required for disulfide bond formation in the extracytoplasmic space of cyanobacteria. The SynDsbAB orthologs are present in most cyanobacteria and chloroplasts of higher plants with fully sequenced genomes. The SynDsbAB protein contains two distinct catalytic domains that display significant similarity to proteins involved in disulfide bond formation in Escherichia coli and eukaryotes. Importantly, SyndsbAB complements E. coli strains defective in disulfide bond formation. In addition, the activity of E. coli alkaline phosphatase localized to the periplasm of Synechocystis 6803 is dependent on the function of SynDsbAB. Deletion of SyndsbAB in Synechocystis 6803 causes significant growth impairment under photoautotrophic conditions and results in hyper-sensitivity to dithiothreitol, a reductant, whereas diamide, an oxidant had no effect on the growth of the mutant strains. We conclude that SynDsbAB is a critical protein for disulfide bond formation in oxygenic photosynthetic organisms and required for their optimal photoautotrophic growth.

Received for publication, February 6, 2008 , and in revised form, March 24, 2008.

^* This work was supported by National Science Foundation-Frontiers in Integrative Biological Research program Grant EF0425749. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1–S2 and Tables S1–S2.

¹ To whom correspondence should be addressed: Campus Box, 1137, Washington University, One Brookings Dr., St. Louis, MO 63130. Tel.: 314-935-6853; Fax: 314-935-6803; E-mail: Pakrasi{at}wustl.edu.

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