Figure 1. Development of dissimilatory sulfite reductase-like protein (Dsr-LP), F₄₂₀H₂-dependent sulfite reductase (Fsr) and dissimilatory sulfite reductase (Dsr).

Fsr-N and Fsr-C: N-terminal and C-terminal halves of Fsr, respectively. FGltS(I)-α and FGltS(I)-β: F₄₂₀H₂ dehydrogenase and glutamate synthase subunit of a putative F₄₂₀H₂-dependent glutamate synthase of methanogens; FpoF/FqoF: F₄₂₀H₂ dehydrogenase subunit of a membrane-bound proton pumping F₄₂₀H₂ dehydrogenase complexes of late evolving euryarchaea [36]; aFsr-α and aFsr-β: aSir and F₄₂₀H₂ dehydrogenase subunits of a putative F₄₂₀H₂-dependent assimilatory type siroheme sulfite reductase found in haloarchaea. * and ** are peripheral and additional iron sulfur cluster [Fe₄-S₄], respectively. Filled and unfilled boxed arrows show the path for the development of Fsr-N and Fsr-C, respectively. Dashed oval or circle, unidentified protein. X and Y_1–3, unknown electron acceptors and donor, respectively.