Sulfur oxidation is an essential component of the earth’s sulfur cycle. Acidithiobacillus spp. can oxidize various reduced inorganic sulfur compounds (RISCs) with high efficiency to obtain electrons for their autotrophic growth. Strains in this genus have been widely applied in bioleaching and biological desulfurization. Diverse sulfur-metabolic pathways and corresponding regulatory systems have been discovered in these acidophilic sulfur-oxidizing bacteria. The sulfur-metabolic enzymes in Acidithiobacillus spp. can be categorized as elemental sulfur oxidation enzymes (sulfur dioxygenase, sulfur oxygenase reductase, and Hdr-like complex), enzymes in thiosulfate oxidation pathways (tetrathionate intermediate thiosulfate oxidation (S4I) pathway, the sulfur oxidizing enzyme (Sox) system and thiosulfate dehydrogenase), sulfide oxidation enzymes (sulfide:quinone oxidoreductase) and sulfite oxidation pathways/enzymes. The two-component systems (TCSs) are the typical regulation elements for periplasmic thiosulfate metabolism in these autotrophic sulfur-oxidizing bacteria. Examples are RsrS/RsrR responsible for S4I pathway regulation and TspS/TspR for Sox system regulation. The proposal of sulfur metabolic and regulatory models provide new insights and overall understanding of the sulfur-metabolic processes in Acidithiobacillus spp. The future research directions and existing barriers in the bacterial sulfur metabolism are also emphasized here and the breakthroughs in these areas will accelerate the research on the sulfur oxidation in Acidithiobacillus spp. and other sulfur oxidizers.
The extremely acidophilic, chemolithoautotrophic Acidithiobacillus ferrooxidans is an important bioleaching bacterium of great value in the metallurgical industry and environmental protection. In this report, a mutagenesis system based on the homing endonuclease I-SceI was developed to produce targeted, unmarked gene deletions in the strain A. ferrooxidans ATCC 23270. A targeted phosphofructokinase (
Leptospirillum ferriphilum has been identified as the dominant, moderately thermophilic, bioleaching microorganism in bioleaching processes. It is an acidic and chemolithoautrophic bacterium that gains electrons from ferrous iron oxidation for energy production and cell growth. Genetic information about this microorganism has been limited until now, which has hindered its further exploration. In this study, the complete genome of L. ferripilum ML-04 is sequenced and annotated. The bacterium has a single circular chromosome of 2,406,157 bp containing 2,471 coding sequences (CDS), 2 rRNA operons, 48 tRNA genes, a large number of mobile genetic elements and 2 genomic islands. In silico analysis shows L. ferriphilum ML-04 fixes carbon through a reductive citric acid (rTCA) cycle, and obtains nitrogen through ammonium assimilation. The genes related to "cell envelope biogenesis, outer membrane" (6.9%) and "DNA replication, recombination and repair" (5.6%) are abundant, and a large number of genes related to heavy metal detoxification, oxidative and acidic stress defense, and signal transduction pathways were detected. The genomic plasticity, plentiful cell envelope components, inorganic element metabolic abilities and stress response mechanisms found the base for this organism's survival in the bioleaching niche.
Acidithiobacillus caldus (A. caldus) is a common bioleaching bacterium that possesses a sophisticated and highly efficient inorganic sulfur compound metabolism network. Thiosulfate, a central intermediate in the sulfur metabolism network of A. caldus and other sulfur-oxidizing microorganisms, can be metabolized via the tetrathionate intermediate (S4I) pathway catalyzed by thiosulfate:quinol oxidoreductase (Tqo or DoxDA) and tetrathionate hydrolase (TetH). In A. caldus, there is an additional two-component system called RsrS-RsrR. Since rsrS and rsrR are arranged as an operon with doxDA and tetH in the genome, we suggest that the regulation of the S4I pathway may occur via the RsrS-RsrR system. To examine the regulatory role of the two-component system RsrS-RsrR on the S4I pathway, ΔrsrR and ΔrsrS strains were constructed in A. caldus using a newly developed markerless gene knockout method. Transcriptional analysis of the tetH cluster in the wild type and mutant strains revealed positive regulation of the S4I pathway by the RsrS-RsrR system. A 19 bp inverted repeat sequence (IRS, AACACCTGTTACACCTGTT) located upstream of the tetH promoter was identified as the binding site for RsrR by using electrophoretic mobility shift assays (EMSAs) in vitro and promoter-probe vectors in vivo. In addition, ΔrsrR, and ΔrsrS strains cultivated in K2S4O6-medium exhibited significant growth differences when compared with the wild type. Transcriptional analysis indicated that the absence of rsrS or rsrR had different effects on the expression of genes involved in sulfur metabolism and signaling systems. Finally, a model of tetrathionate sensing by RsrS, signal transduction via RsrR, and transcriptional activation of tetH-doxDA was proposed to provide insights toward the understanding of sulfur metabolism in A. caldus. This study also provided a powerful genetic tool for studies in A. caldus.
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