Analysis of Mechanisms for Electron Uptake by 6Ac During Direct Interspecies Electron Transfer.

Direct interspecies electron transfer (DIET) is a syntrophic metabolism wherein free electrons are directly transferred between microorganisms without the mediation of intermediates such as molecular hydrogen or formate. Previous research has demonstrated that 6Ac is capable of reducing carbon dioxide through DIET. However, the mechanisms underlying electron uptake in 6Ac during DIET remain poorly understood. This study aims to elucidate the electron and proton flux in 6Ac during DIET and to propose a model for electron uptake in this organism, primarily based on the analysis of gene transcript levels, genomic characteristics of 6Ac, and the pathways generating fully reduced ferridoxin (Fd), reduced coenzyme F (FH), coenzyme M (CoM-SH), and coenzyme B (CoB-SH) during DIET. The findings suggest that membrane-bound heterodisulfide reductase (HdrED), FH-dehydrogenase lacking subunit F (Fpo), and cytoplasmic heterodisulfide reductase (HdrABC)-subunit B of F-reducing hydrogenase (FrhB) complex play critical roles in electron uptake in 6Ac during DIET. Specifically, Fpo is responsible for generating Fd with reduced methanophenazine (MPH), driven by a proton motive force, while HdrED facilitates the reduction of heterodisulfide of coenzyme M and coenzyme B (CoM-S-S-CoB) to CoM-SH and CoB-SH using MPH. Additionally, cytoplasmic heterodisulfide reductase HdrABC and subunit B of coenzyme F-hydrogenase complex (HdrABC-FrhB complex) catalyzes the reduction of oxidized coenzyme F (F) to FH, utilizing CoM-SH, CoB-SH, and Fd. This study represents the first genetics-based functional characterization of electron and proton flux in 6Ac during DIET, providing a model for further investigation of electron uptake in species. Furthermore, it deepens our understanding of the mechanisms underlying electron uptake in methanogens during DIET.