Many sulfur-oxidizing bacteria and archaea pursue a dissimilatory sulfur oxidation pathway involving the sHdr system. The core proteins of this widespread system are encoded in a conserved shdrC1B1AHC2B2 cluster in Pseudomonadota and Aquificae and resemble the subunits of the heterodisulfide reductase from methanogenic archaea. The sHdr proteins are indispensable for the use of thiosulfate as an accessory electron donor in the Alphaproteobacterium Hyphomicrobium denitrificans [1]. In the hyperthermophilic bacterium Aquifex aeolicus, the sHdr proteins have been shown to be membrane-associated [2]. On the other hand, the sHdrA subunit from H. denitrificans is soluble [1,3]. Our goal is to characterize the sulfur-oxidizing sHdr system from the obligately chemolithotrophic Gammaproteobacterium Thioalkalivibrio thiocyanoxidans. To achieve this, we conducted anion exchange chromatography followed by immunodetection. We discovered that all sHdrAB1B2C1 and -C2 proteins are soluble in this organism. Additionally, all five proteins co-purify in the same fractions upon anion exchange chromatography, indicating the formation of a heterocomplex.

The sHdrH protein is always encoded among the core shdr genes. It has no homologs outside of the sHdr system, indicating a special function in the context of dissimilatory sulfur oxidation. In the presence of thiosulfate, the transcription of shdrH is upregulated in H. denitrificans, along with that of the other shdr genes. The capacity for thiosulfate oxidation is significantly reduced in an H. denitrificans in frame deletion mutant lacking sHdrH, highlighting the importance of sHdrH in the overall process. In this study, we present the high-resolution crystal structure of the soluble dimeric sHdrH from T. thiocyanoxidans.

[1] Koch & Dahl 2018 ISME J 10, 2479

[2] Boughanemi et al. 2016 FEMS Microbiol Lett 363, fnw156

[3] Ernst et al. 2021 FEBS J 288, 1664