Sulfur transport and signal transduction for sulfur-dependent transcriptional regulation in Hyphomicrobium denitrificans

Many sulfur-oxidizing bacteria oxidize thiosulfate through a combination of initial periplasmic and downstream cytoplasmic reactions. In the case of facultative sulfur oxidizers, such as the Alphaproteobacterium Hyphomicrobium denitrificans, transcriptional regulation is necessary to respond to the availability of the sulfur compound. Although we have gathered a substantial amount of information on sulfur-oxidizing enzymes and two different sulfane sulfur-responsive transcriptional repressors in H. denitrificans [1,2], we lack knowledge about the transfer of sulfur from the periplasm into the cytoplasm and how the cytoplasmic regulator proteins, SoxR and sHdrR, detect the presence of oxidizable substrate outside of the cells.

In H. denitrificans, the genes for two potential sulfur compound transporters (SoxT1A and SoxT1B) are located in the same genetic island as those for sulfur oxidation (sox and shdr). Both transporters exhibit similarity to the structurally characterized YeeE/YedE-family thiosulfate transporter from Spirochaeta thermophila [3]. In this study, we examined the thiosulfate oxidation capabilities and transcription levels of the indicator genes soxXA and shdrA in H. denitrificans mutants with different combinations of transporter and regulator gene deletions. Although H. denitrificans strains lacking either SoxT1A or SoxT1B alone have the same negative phenotype for thiosulfate oxidation, we can now conclude that the two transporters fulfill fundamentally different roles. SoxT1B serves as the signal transducing unit for SoxR, as evidenced by the fact that soxXA and shdrA transcription remains very low in its absence unless the gene for the repressor is also removed. Accordingly, a ΔsoxR ΔsoxT1B double mutant is able to oxidize thiosulfate. SoxT1A functions as a transporter that allows sulfur oxidation in the cytoplasm. Mutants lacking SoxT1A are unable to oxidize thiosulfate, despite high levels of transcription of sox and shdr genes, particularly in regulator-deficient double mutants.

[1] Li et al 2022 BBA Bioenergetics 1864, 148932

[2] Li et al 2023 Antioxidants 12, 1620

[3] Tanaka et al 2020 Sci Adv 6, eaba7637