The Iron-Sulfur Flavoprotein DsrL as NAD(P)H:Acceptor Oxidoreductase in Oxidative and Reductive Dissimilatory Sulfur Metabolism

Abstract

DsrAB-type dissimilatory sulfite reductase is a key enzyme of microbial sulfurdependent 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 reductivetype 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.

Novel taxa of Acidobacteriota involved in seafloor sulfur cycling

“…Fig. 3A) [136]. The DsrL had conserved YRR-motifs in the NAD(P)H substrate-binding domains that are present in the DsrL-2 group, and absent in DsrL-1 of sulfur-oxidizing aerobes (Supp.…”

“…Sulfomarinibacter MAGs encoded DsrL, which was previously thought to be exclusively found in sulfur-oxidizing bacteria [88]. However, recent work showed DsrL can function in a reductive manner in biochemical assays [88] [136], and was highly expressed during reductive sulfur- and thiosulfate-respiration by Desulfurella amilsii [88, 116]. The DsrL of Ca .…”

“…The DsrL of Ca . Sulfomarinibacter contained putative NADP(H)-binding domain structures that may enable coupling of NADPH as electron donor to sulfite reduction [136], as well as phylogenetic relatedness with DsrL of Desulfurella amilsii . Together, this indicates the DsrL of Ca .…”

Novel taxa of Acidobacteriota implicated in seafloor sulfur cycling

“…The DsrL sequences were phylogenetically related to group "DsrL-2" from Desulfurella amilsiiI (38% amino acid identity), peatland Acidobacteriota and other subsurface bacteria, and were phylogenetically distinct from group "DsrL-1" of sulfur-oxidizing aerobes (Supplementary Fig. 3A) [94]. The DsrL had conserved YRR-motifs in the NAD(P)H substrate-binding domains that are present in the DsrL-2 group, and absent in DsrL-1 of sulfur-oxidizing aerobes (Supplementary Fig.…”

“…The DsrL had conserved YRR-motifs in the NAD(P)H substrate-binding domains that are present in the DsrL-2 group, and absent in DsrL-1 of sulfur-oxidizing aerobes (Supplementary Fig. 3B) [94].…”