Cable bacteria are multicellular filaments found in aquatic sediments that perform a unique form of energy conservation. The cells in the anaerobic zone oxidize reduced sulfur compounds, and the electrons released by this oxidation are transported by fibers in the periplasm to cells in the aerobic zone, where oxygen acts as a terminal electron acceptor. Cable bacteria, like almost all members of the Desulfobulbaceae family, possess genes for the rDsr pathway of sulfur oxidation, including reverse-acting dissimilatory sulfite reductase rDsrAB, DsrC and the sulfur transferase TusA [1]. However, the sulfur relay system in cable bacteria has not been fully elucidated. It shuttles sulfane sulfur to rDsrAB, where it is oxidized to sulfite. Cable bacteria have never been shown to contain genes for the DsrEFH sulfurtransferase, despite serving as a long-standing marker for sulfur oxidizers.

The aim of this study was to clarify the enzymes, transfer reactions, and potential terminal acceptors involved in sulfur trafficking in cable bacteria. We identified a candidate sulfurtransferase, DsrEFH, by comparative proteomics and transcriptomics, despite its low amino acid sequence similarity. We heterologously produced the candidate sulfurtransferase, as well as the TusA and DsrC proteins from the cable bacterium Ca. Electronema sp. GS in E. coli. The recombinant proteins were analyzed using polyacrylamide gel electrophoresis and size exclusion chromatography. A study of sulfur transfer reactions between the proteins was conducted in vitro using MALDI-TOF mass spectrometry.

Whether it carries a StrepTag on DsrE or DsrH, the novel DsrEFH from Ca. Electronema sp. GS forms heterotrimers and heterohexamers in solution, similar to canonical DsrEFH from Allochromatium vinosum [2]. In vitro, cable bacterial DsrEFH serves as a sulfur acceptor for TusA and a sulfur donor for DsrC. In conclusion, the novel dsrEFH-like genes encode a previously missing component in the cytoplasmic sulfur relay system of cable bacteria. These genes should be included in the canon of marker genes for sulfur oxidizers.

[1] Kjeldsen et al 2019 PNAS 116, 19116

[2] Dahl et al 2008 J Mol Biol 284, 1287