"From the affinities and reaction rates it is clear that the enzyme will sense even a minute amount of sulfite and will neutralize even a large amount of sulfite very quickly. These properties suited the need of the ancient methanogens when oxygen appeared on earth," Mukhopadhyay said. "But, why did the organism have this enzyme in the first place?"
A clue comes from published works by Robert White, professor of biochemistry at Virginia Tech, who studies how metabolic systems evolved and collaborates with Mukhopadhyay. "It is possible that M. jannaschii had this enzyme for cofactor biosynthesis and having it in advance gave the organism a selective advantage when oxygen, and consequently sulfite, appeared," Mukhopadhyay said. "Since we now know that methanogens had a way to handle sulfite toxicity, we could hypothesize that the rest of the sulfate reduction pathway once existed in these organisms."
Johnson and Mukhopadhyay have already seen some remnants of this system in M. jannaschii. Thus, they say, it is possible that methanogenesis and sulfate reduction could have originated in the same organism after all, and, in the course of time, a loss of the sulfite reductase gene gave rise to a sulfite-sensitive methanogen. Similarly the loss of certain key genes gave rise to the archaea that reduces sulfate, but do not make methane. "But it is equally possible that the sulfite reduction system was developed in another organism and the methanogens acquired the sulfite reduction gene via horizontal transfer from that entity," Mukhopadhyay said.
Rolf Thauer, professor and head of the Department of Biochemistry at the Max Planck Institute for Terrestrial Microbiology in Marburg, Germany, and a noted authority on anaerobic microorganisms, commented: "The finding of a novel sulfite reductase in a methanogenic archaeon is an important discovery. It may prove to be directly relevant to the a