Sulfur Assimilation and Trafficking in Methanogens

Abstract

Methanogens are ancient obligate anaerobes, originating in an early Earth period before oxygen accumulated in the atmosphere and oceans. Unlike aerobic cells, all methanogens that persist in highly sulfidic, anaerobic niches are able to grow with sulfide as the sole sulfur source. These organisms have retained a unique apparatus for the assimilation and distribution of sulfur from the ambient environment. Recent work has revealed the presence of a unique set of at least six highly conserved genes likely responsible for sulfide uptake and synthesis of persulfide groups, cysteine, homocysteine, coenzyme M, coenzyme B, and cysteinyl-tRNACys. Phylogenetic studies show that these ancient sulfur assimilatory proteins share an evolutionary history with methanogenesis enzymes, suggesting that they played a key role in supporting the development of this metabolism. Little is yet known about most methanogen pathways that mobilize sulfur to form thiolated tRNA, iron-sulfur clusters, molybdopterin, and other important cofactors. However, for at least some of these pathways, it appears that the sulfur can be assimilated directly as sulfide, which is present at high intracellular levels without causing toxic effects. The many unique aspects of sulfur assimilation and trafficking in contemporary methanogens appear to be relics of ancient metabolic processes that were prevalent on the early anaerobic Earth.