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Abstract
Sulfur is vital for the growth of all known organisms and is present in a wide variety of metabolites with different physiological functions. Consistent with their obligate anaerobic habitats, most methanogenic Archaea only assimilate sulfide and elemental sulfur as sulfur sources, whereas sulfate and other oxidized sulfur compounds are rarely utilized. The methanogenic lifestyle may have evolved from ~ 3.5 billion years ago, and contemporary methanogens may have preserved some of the metabolic relics which were common in the early Earth anaerobic lifestyles. Recent studies have revealed multiple novel traits of sulfur assimilation in methanogens. However, many aspects of the sulfur assimilation processes and their regulations remain to be investigated. Thereby, the study of the physiology and biochemistry of the sulfur metabolism in methanogens may provide new insights into the biology of ancient microbial life.Methanococcus maripaludis is unable to assimilate sulfate as a sulfur sole and does not produce sulfate when grew with sulfide as the sole sulfur source. Nevertheless, Methanococcus maripaludis possesses homologs of proteins involved in the sulfate assimilatory reduction pathway. None of these proteins was functional in the Escherichia coli mutant strains deficient in sulfate assimilation metabolism. These results indicated that the assimilatory sulfate reduction pathway is most unlikely to be present in Methanococcus maripaludis.When grown with elemental sulfur as the sole sulfur source, Methanococcus maripaludis produced sulfide at about 6 mmol per g cell dry weight per hour. Moreover, adenylyl-sulfate reductase (MMP1681), an enzyme that contains an iron-sulfur cluster, was found to be required for elemental sulfur assimilation. Furthermore, proteomics data indicated that the expression of this protein increased three-fold during growth with elemental sulfur in comparison to growth with sulfide. Together with bioinformatics analysis, a different physiological role of MMP1681 in elemental sulfur assimilation, in addition to its in vitro catalytic function as an adenylyl-sulfate reductase was demonstrated. Although the explicit mechanism by which MMP1681 participating in the elemental sulfur incorporation process remains to be elucidated, this evidence advances our understanding of how methanogen possessing MMP1681 assimilate elemental sulfur into key sulfur intermediates.