Sulfide-Quinone Reductase (SQR) of Rhodobacter Capsulatus: Expression, Induction and Inactivation

Inorganic reduced sulfur compounds serve as electron donors in many phototrophic and chemotrophic bacteria1’2. Sulfide-quinone reductase (SQR; E.C.1.8.5. “.) is one of the main enzymes involved in the oxidation of sulfide to sulfur. This sulfide induced membrane bound flavoprotein with an apparent MW of approximately 56-kDa was shown to be widely distributed among bacteria3. SQR was first purified from thylakoids of the cyanobacterium Oscillatoria limnetica4 and, more recently, from membranes of the ”non-sulfur“ purple bacterium Rb. capsulatus5. The sqr-gene of Rb. capsulatus had been sequenced and functionally expressed in E. coli5. Rb. capsulatus oxidizes sulfide only to the redox level of sulfur. However, it is not known, whether SQR is the only sulfide oxidizing enzymatic system in this bacterium. Here we demonstrate that Rb. capsulatus looses its capability of sulfide dependent growth after inactivation of SQR. In former investigations it was shown that expression of SQR in Rb. capsulatus depends on sulfide and is possibly modulated by oxygen6. This results were achieved using a mutant strain with the genes of the luciferase of Vibrio fischeri (luxAB) inserted behind the putative sqrpromotor. However, the activity of the luciferase itself depends on oxygen. For this reason a second reporter gene, the alkaline phosphatase of E. coli (PhoA), was used in this investigation.