Operon structure of putative heterodisulfide reductase complexes. Hdr cluster I is found in S. thermosulfidooxidans, AMDSBA1, AMDSBA2, and AMDSBA5. Hdr cluster IIa is found in S. thermosulfidooxidans AMDSBA1, AMDSBA2, AMDSBA4, and AMDSBA5. Hdr cluster IIb is found in S. acidophilus and AMDSBA3. Gene abbreviations are as follows: thdx, thioredoxin; gcsh, glycine cleavage system protein H; hdrABC, heterodisulfide reductases; hypo, hypothetical; dsrE, dsrE-like sulfur relay protein; tusA, sulfur transfer protein; lpl, lipoate protein ligase; etfAB, electron transfer flavoprotein. Gene colors correspond to protein orthology as defined in methods.