Investigation of the sqr and fcc Genes Associated with the Sulfur Oxidation Activity of a Rhodobacter capsulatus Strain Isolated in Vietnam

Flavoproteins sulfide:quinone oxidoreductase (SQR) and flavocytochrome c sulfide dehydrogenase (FCC), two key enzymes involved in the sulfide oxidation pathways, are responsible for the hydrogen sulfide (H2S)-metabolizing capability of various bacterial strains employed in the biological treatments of the toxic gas H2S. Previous studies mainly focus on the isolation of sulfur-oxidizing bacteria (SOB) and the evaluation of the total sulfur oxidation activity. Thus, in this study, an in-depth investigation of the sqr, fcc genes, the encoded proteins, and their association with the sulfide oxidation activity in a SOB bacterial strain, Rhodobacter capsulatus, was carried out. Firstly, the specific primers were designed and used to amplify sqr and fcc genes of a Rhodobacter capsulatus strain isolated in Vietnam. In addition, the bacterial cells were fractionated to obtain periplasmic, membrane, and cytoplasmic fractions for denaturing SDS-PAGE analysis and activity assay. On the SDS-PAGE gel, the bands corresponding to the calculated molecular weights of SQR (approximately 47 kDa in the membrane fraction) and FCC (approximately 35 kDa in the periplasmic fraction) could be observed. Significantsulfide oxidation activity could be detected in the periplasmic and the membrane fractions compared to the cytoplasmic fraction (2 and 20 times higher, respectively). More importantly, sulfide oxidation activities in these fractions were inhibited by the specific flavoproteininhibitor KCN, indicating possible association of SQR and FCC with the recorded activity. The study has initially established the evidence supporting the link between the presence of sqr, fcc-encoded proteins and the sulfide oxidation activity in the studied R. capsulatusbacterial strain for further research utilizing SOB in environmental treatment applications.