Sulfide-quinone reductase from Rhodobacter capsulatus: Requirement for growth, periplasmic localization, and extension of gene sequence analysis

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 furthe r partial open reading frames has been determined. A correction of the prev iously published sqr gene sequence (M. Schutz, Y. Shahak, E. Padan, and G. Hauska, J. Biol. Chem. 272:9890-9894, 1997) which in the deduced primary st ructure of the sulfide-quinone reductase changes four positive into four ne gative charges and the number of amino acids from 425 to 427 was necessary, The correction has no Further bearing on the former sequence analysis. Del etion and interruption strains document that sulfide-quinone reductase is e ssential for photoautotrophic growth on sulfide. The sulfide-oxidizing enzy me is involved in energy conversion, not in detoxification. Studies with an alkaline phosphatase fusion protein reveal a periplasmic localization of t he enzyme. Exonuclease treatment of the fusion construct demonstrated that the C-terminal 38 amino acids of sulfide-quinone reductase were required fo r translocation. An N-terminal signal peptide for translocation was not fou nd in the primary structure of the enzyme. The possibility that the neighbo ring open reading frame, which contains a double arginine motif, may be inv olved in translocation has been excluded by gene deletion (rather, the prod uct of this gene functions in an ATP-binding cassette transporter system, t ogether with the product of one of the other open reading frames). The resu lts lead to the conclusion that the sulfide-quinone reductase of R. capsula tus functions at the periplasmic surface of the cytoplasmic membrane and th at this flavoprotein is translocated by a hitherto-unknown mechanism.