Sulfide-Quinone Reductase from Rhodobacter capsulatus : Requirement for Growth, Periplasmic Localization, and Extension of Gene Sequence Analysis

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Abstract
The entire sequence of the 3.5-kb fragment of genomic DNA from Rhodobacter capsulatus which contains the sqr gene and a second complete and two further partial open reading frames has been determined. A correction of the previously published sqr gene sequence (M. Schütz, Y. Shahak, E. Padan, and G. Hauska, J. Biol. Chem. 272:9890–9894, 1997) which in the deduced primary structure of the sulfide-quinone reductase changes four positive into four negative charges and the number of amino acids from 425 to 427 was necessary. The correction has no further bearing on the former sequence analysis. Deletion and interruption strains document that sulfide-quinone reductase is essential for photoautotrophic growth on sulfide. The sulfide-oxidizing enzyme is involved in energy conversion, not in detoxification. Studies with an alkaline phosphatase fusion protein reveal a periplasmic localization of the enzyme. Exonuclease treatment of the fusion construct demonstrated that the C-terminal 38 amino acids of sulfide-quinone reductase were required for translocation. An N-terminal signal peptide for translocation was not found in the primary structure of the enzyme. The possibility that the neighboring open reading frame, which contains a double arginine motif, may be involved in translocation has been excluded by gene deletion (rather, the product of this gene functions in an ATP-binding cassette transporter system, together with the product of one of the other open reading frames). The results lead to the conclusion that the sulfide-quinone reductase of R. capsulatus functions at the periplasmic surface of the cytoplasmic membrane and that this flavoprotein is translocated by a hitherto-unknown mechanism.