Summary: In purple sulphur bacteria of the family sulphite oxidation via intermediary formation of adenylylsulphate is an enzymologically well characterized process. In contrast, the role of an alternative direct oxidation pathway via the enzyme sulphite: acceptor oxidoreductase has not been resolved. This paper reports the cloning of the genes encoding the adenylylsulphate-forming enzyme adenosine-5′-phosphosulphate (APS) reductase from strain D (DSM 180), a representative of the purple sulphur bacteria, and the construction of mutations in these genes by insertion of a kanamycin ω cartridge. The mutated genes were transferred to on suicide vectors of the pSUP series by conjugation and delivered to the chromosome by double homologous recombination. Southern hybridization and PCR analyses of the recombinants obtained verified the first insertional gene inactivation in purple sulphur bacteria. Enzymological studies demonstrated the absence of APS reductase from the mutants. Further phenotypic characterization showed no significant effect of APS reductase deficiency on the sulphite-oxidizing ability of the cells under photolithoautotrophic growth conditions. In the wild-type as well as in mutant strains, tungstate, the specific antagonist of molybdate, led to the intermediary accumulation of sulphite in the medium during sulphide oxidation and strongly inhibited growth with sulphite as photosynthetic electron donor; this indicates that a molybdoenzyme, probably sulphite: acceptor oxidoreductase, is the main sulphite-oxidizing enzyme in Specific inactivation of selected genes as developed for in this study provides a powerful genetic tool for further analysis of sulphur metabolism and other metabolic pathways in phototrophic sulphur bacteria.


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