Abstract
DsrAB-type dissimilatory sulfite reductase is a key enzyme of microbial sulfur-dependent energy metabolism. Sulfur oxidizers also contain DsrL, which is essential for sulfur oxidation in Allochromatium vinosum. This NAD(P)H oxidoreductase acts as physiological partner of oxidative-type rDsrAB. Recent analyses uncovered that DsrL is not confined to sulfur oxidizers but also occurs in (probable) sulfate/sulfur-reducing bacteria. Here, phylogenetic analysis revealed a separation into two major branches, DsrL-1, with two subgroups, and DsrL-2. When present in organisms with reductive-type DsrAB, DsrL is of type 2. In the majority of cases oxidative-type rDsrAB occurs with DsrL-1 but combination with DsrL-2-type enzymes is also observed. Three model DsrL proteins, DsrL-1A and DsrL-1B from the sulfur oxidizers A. vinosum and Chlorobaculum tepidum, respectively, as well as DsrL-2 from thiosulfate- and sulfur-reducing Desulfurella amilsii were kinetically characterized. DaDsrL-2 is active with NADP(H) but not with NAD(H) which we relate to a conserved YRR-motif in the substrate-binding domains of all DsrL-2 enzymes. In contrast, AvDsrL-1A has a strong preference for NAD(H) and the CtDsrL-1B enzyme is completely inactive with NADP(H). Thus, NAD+ as well as NADP+ are suitable in vivo electron acceptors for rDsrABL-1-catalyzed sulfur oxidation, while NADPH is required as electron donor for sulfite reduction. This observation can be related to the lower redox potential of the NADPH/NADP+ than the NADH/NAD+ couple under physiological conditions. Organisms with a rdsrAB and dsrL-1 gene combination can be confidently identified as sulfur oxidizers while predictions for organisms with other combinations require much more caution and additional information sources.
Highlights
Sulfur is a highly reactive element in reduced form and has several stable oxidation states in the range from -2, in sulfide or reduced organic sulfur, up to + 6 in sulfate
We have repeatedly suggested that the dsrEFH genes are unique to sulfur oxidizers and absent from sulfate/sulfite reducers and sulfur disproportionating organisms and may be indicators for sulfur metabolism operated in the oxidative direction (Sander et al, 2006; Stockdreher et al, 2012; Venceslau et al, 2014)
We have shown previously that the iron-sulfur flavoprotein DsrL1A from A. vinosum acts as physiological reaction partner for oxidative-type sulfite reductase, rDsrAB from the same organism (Löffler et al, 2020)
Summary
Sulfur is a highly reactive element in reduced form and has several stable oxidation states in the range from -2, in sulfide or reduced organic sulfur, up to + 6 in sulfate. The biogeochemical cycle of sulfur on Earth is driven mainly by microbial activity, on one hand by microbial sulfate reduction, on the other by sulfur compound oxidation. DsrL in Dissimilatory Sulfur Metabolism an important process of sulfur-based energy conservation in the absence of oxygen (Finster, 2008). Our understanding of sulfur cycling processes and the biology of microorganisms that catalyze them has improved considerably during recent years (Wasmund et al, 2017; Anantharaman et al, 2018). Significant questions remain regarding the biology of microorganisms and factors that control the turnover of sulfur compounds
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