Fig. 3
Structure of the sox cluster in T. tepidarius and other chemolithoautotrophic Proteobacteria. A reference sox operon encoding the Kelly-Friedrich pathway of thiosulfate oxidation from Paracoccus denitrificans ATCC 17741T (Alphaproteobacteria) is given, showing soxXYZABCDEF genes and intergenic spacers, against gene clusters from T. tepidarius DSM 3134T, A. thiooxidans ATCC 19377T and A. caldus ATCC 51756T (the only sulfur-oxidising Acidithiobacillus spp.) of the Acidithiobacillia; Thiohalorhabdus denitrificans DSM 15699T (Gammaproteobacteria) and Thiobacillus thioparus DSM 505T (Betaproteobacteria). The DUF302-family hypothetical protein gene is indicated where present. It is worth noting that P. denitrificans and T. thioparus do not grow on trithionate and that thiosulfate oxidation in Thermithiobacillus and Thiobacillus has been unequivocally shown not to proceed via the periplasmic Kelly-Friedrich oxidation pathway and instead occurs via tetrathionate as an intermediate, which is then oxidized to sulfate in the cytoplasm (the Kelly-Trudinger pathway [32, 33]). The highly conserved soxXYZAB cluster occurs in all of the genomes examined and the DUF302 gene appears highly conserved in the Acidithiobacillia. The function is as-yet unknown, as is that of the sox genes in these Kelly-Trudinger pathway organisms. Analysis of conserved domains indicates that DUF302 may form a homodimer