DsrL Mediates Electron Transfer between NADH and rDsrAB in Allochromatium vinosum.

Abstract

Dissimilatory sulfite reductase DsrAB occurs in sulfate/sulfite-reducing prokaryotes, in sulfur disproportionators and also in sulfur oxidizers, where it functions in reverse. Predictions of physiological traits in metagenomic studies relying on the presence of dsrAB, other dsr genes or combinations thereof suffer from the lack of information on crucial Dsr proteins. The iron-sulfur flavoprotein DsrL is an example of this group. It has a documented essential function during sulfur oxidation and was recently also found in some metagenomes of probable sulfate and sulfite reducers. Here, we show that DsrL and reverse acting rDsrAB can form a complex and are co-purified from the phototrophic sulfur oxidizer Allochromatium vinosum. Recombinant DsrL exhibits NAD(P)H:acceptor oxidoreductase activity with a strong preference for NADH over NADPH. In vitro, the rDsrABL complex effectively catalyzes NADH-dependent sulfite reduction, which is strongly enhanced by the sulfur-binding protein DsrC. Our work reveals NAD+ as suitable in vivo electron acceptor for sulfur oxidation in organisms operating the rDsr pathway, and points to reduced nicotinamide adenine dinucleotides as electron donors for sulfite reduction in sulfate/sulfite-reducing prokaryotes that contain DsrL. In addition, dsrL cannot be used as a marker distinguishing sulfate/sulfite reducers and sulfur oxidizers in metagenomic studies without further analysis. This article is protected by copyright. All rights reserved.