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The sulfur oxidization (Sox) system is the central sulfur oxidization pathway of phototrophic and chemotrophic sulfur-oxidizing bacteria. Regulation and function of the Sox system in the chemotrophic Paracoccus pantotrophus has been elucidated; however, to date, no information is available on the regulation of this system in the chemolithotrophic Acidithiobacillus caldus, which is widely utilized in bioleaching. We described the novel tspSR- sox-like clusters in A. caldus and other chemolithotrophic sulfur-oxidizing bacteria containing Sox systems. The highly homologous σ-dependent two-component signaling system (TspS/R), upstream of the sox operons in these novel clusters, was identified by phylogenetic analyses. A typical σ-dependent promoter, P, was identified upstream of soxX-I in the sox-I cluster of A. caldus MTH-04. The transcriptional start site (G) and the −12/−24 regions (GC/GG) of P were determined by rapid amplification of cDNA ends (5′RACE), and the upstream activator sequences (UASs; TGTCCCAAATGGG ACA) were confirmed by electrophoretic mobility shift assays (EMSAs) in vitro and by UAS-probe-plasmids assays in vivo. Sequence analysis of promoter regions in tspSR- sox-like clusters revealed that there were similar σ-dependent promoters upstream of the soxX genes. Based on our results, we proposed a TspSR-mediated signal transduction and transcriptional regulation pathway for the Sox system in A. caldus. The regulation of σ-dependent two-component systems (TCSs) for Sox pathways were explained for the first time in A. caldus, A. thiooxidans, T. tepidarius, and T. denitrificans, indicating the significance of modulating the sulfur oxidization in these chemolithotrophic sulfur oxidizers. [ABSTRACT FROM AUTHOR]