This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Sulfur-reducing bacteria of the phylum Desulfobacterota play a crucial role in the global biogeochemical sulfur cycle. Species such as Geobacter sulfurreducens use elemental sulfur as a terminal electron acceptor during anaerobic respiration, producing sulfide that participates in the global sulfur cycle.
Crucially, G. sulfurreducens uses elemental sulfur but cannot use oxidized sulfur compounds such as sulfate, thiosulfate or sulfite as terminal electron acceptors, making it an important model organism for understanding elemental sulfur respiration. Little is known about the enzymes involved in the sulfur reduction process. Multiheme cytochrome c (MCC) proteins are a class of iron-containing proteins involved in many aspects of respiration and energy generation.
"The G. sulfurreducens genome encodes 111 c-type cytochromes, whereas Desulfuromonas acetoxidans—the first sulfur-reducing bacterium to be discovered—contains 47 putative MCC proteins," notes Hisaaki Mihara, the lead author from the College of Life Sciences at Ritsumeikan University in Japan. Mihara's research group had previously identified an unusual MCC in G. sulfurreducens that was predicted to contain selenocysteine, the 21st amino acid, and later named it MccSep. Selenocysteine resembles cysteine but contains selenium instead of sulfur and is present in many redox enzymes across diverse organisms.
Mihara hypothesized that MccSep may be involved in sulfur reduction. His team investigated its catalytic activity and physiological role, while his colleague Hiroyoshi Matsumura from Ritsumeikan University led the structural analysis. Together, these studies revealed how MccSep contributes to elemental sulfur respiration.
Their findings were published in Science Advances on Aug. 26, 2026. Unlike the 20 standard amino acids that have clearly defined three-letter codons in DNA and RNA, selenocysteine is encoded when UGA, normally a stop codon, is recoded. The research team found this recoded codon in the genetic sequence for MccSep.
They also found that MccSep contains five iron-containing heme groups. Crystallographic analysis revealed an unusual histidine–cysteine ligation at the active-site heme (heme 2), with selenocysteine positioned nearby. Could this unusual structure around heme 2 play a role in sulfur reduction?
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