Biochemistry and transcriptional regulation of bacterial sulfur oxidation

Sulfur is one of the most important and versatile elements in biology and forms the basis of energy metabolism in many prokaryotes. Its extensive large-scale redox transformations drive the global biogeochemical sulfur cycle and profoundly influence environmental chemistry. In dissimilatory sulfur oxidizers, the oxidation of reduced sulfur compounds is directly coupled to energy conservation via photosynthetic or respiratory electron transport chains. Sulfur-oxidizing bacteria and archaea are remarkably diverse, reflecting both the wide range of habitats they inhabit and the variety of metabolic pathways through which sulfur compounds are exploited as electron donors. Over the past decades, these pathways have been examined in depth using a spectrum of molecular genetic, biochemical, and omics-based approaches, primarily in model organisms amenable to genetic manipulation. Particular emphasis has been placed on thiosulfate oxidation mediated by the Sox multienzyme system and thiosulfate dehydrogenases, as well as on cytoplasmic sulfane sulfur oxidation involving dissimilatory sulfite reductase and the sulfur-oxidizing heterodisulfide reductase-like sHdr complex. Recent advances have highlighted the central role of specific lipoate-binding proteins that are essential for efficient sHdr-dependent sulfur oxidation. This review provides an overview of current knowledge on prokaryotic sulfur oxidation pathways. In addition, emerging insights into the complex regulatory networks operating in facultative sulfur oxidizers are provided, with particular attention to how these organisms coordinate sulfur metabolism with changing environmental and energetic conditions.