THE SULFUR-OXIDIZING PROKARYOTESBiological oxidation of hydrogen sulfide to sulfate is one of the major reactions of the global sulfur cycle. Reduced inorganic sulfur compounds (referred to below as sulfur) are exclusively oxidized by prokaryotes, and sulfate is the major oxidation product. Sulfur oxidation in members of the Eukarya is mediated by lithoautotrophic bacterial endosymbionts (44).The sulfur-oxidizing prokaryotes are phylogenetically diverse ( Fig. 1). In the domain Archaea aerobic sulfur oxidation is restricted to members of the order Sulfolobales (21, 58), and in the domain Bacteria sulfur is oxidized by aerobic lithotrophs or by anaerobic phototrophs. The nonphototrophic obligate anaerobe Wolinella succinogenes oxidizes hydrogen sulfide to polysulfide during fumarate respiration (41). The ecology, physiology, and biochemistry of sulfur-oxidizing bacteria have been reviewed previously. The neutrophilic chemolithotrophic bacteria have been reviewed by Kelly et al. (31,32,35) and Takakuwa (63), the acidophilic sulfur-oxidizing bacteria have been reviewed by Harrison (23) and Pronk et al. (46), and the molecular genetics of Acidithiobacillus ferrooxidans has been reviewed by Rawlings and Kusano (49). The sulfur metabolism of phototrophic bacteria has been reviewed by Brune (9,10) and Trüper and Fischer (65). The physiology and genetics of both phototrophic and lithotrophic sulfur-oxidizing prokaryotes have been discussed recently (18).Prokaryotes oxidize hydrogen sulfide, sulfur, sulfite, thiosulfate, and various polythionates under alkaline (57), neutral, or acidic conditions (23). Aerobic sulfur-oxidizing prokaryotes belong to genera like

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The novel genes soxFGH were identified, completing the sox gene cluster of Paracoccus pantotrophus coding for enzymes involved in lithotrophic sulfur oxidation. The periplasmic SoxF, SoxG, and SoxH proteins were induced by thiosulfate and purified to homogeneity from the soluble fraction. soxF coded for a protein of 420 amino acids with a signal peptide containing a twin-arginine motif. SoxF was 37% identical to the flavoprotein reconstitute a system able to perform thiosulfate-, sulfite-, sulfur-, and hydrogen sulfide-dependent cytochrome c reduction, and this system is the first described for oxidizing different inorganic sulfur compounds. SoxF slightly inhibited the rate of hydrogen sulfide oxidation but not the rate of sulfite or thiosulfate oxidation. From use of a homogenote mutant with an in-frame deletion in soxF and complementation analysis, it was evident that the soxFGH gene products were not required for lithotrophic growth with thiosulfate.

Soluble NAD-reducing [NiFe]-hydrogenase (SH) from Ralstonia eutropha (formerly Alcaligenes eutrophus) has an infrared spectrum with one strong band at 1956 cm 31 and four weak bands at 2098, 2088, 2081 and 2071 cm 31 in the 2150^1850 cm 31 spectral region. Other [NiFe]-hydrogenases only show one strong and two weak bands in this region, attributable to the NiFe(CN) 2 (CO) active site. The position of these three bands is highly sensitive to redox changes of the active site. In contrast, reduction of the SH resulted in a shift to lower frequencies of the 2098 cm 31 band only. These and other properties prompted us to propose the presence of a Ni(CN)Fe(CN) 3 (CO) active site.z 2000 Federation of European Biochemical Societies.

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Comparative sequence analysis of in vitro-amplified 16s rRNA genes of Thiosphaera pantotropha GB17T (T = type strain) and Paracoccus denitrfficans LMG 42MT revealed identical 16s rRNA primary structures for the two organisms. The level of overall DNA similarity of Thiosphaera pantotropha GB17T and P. denitr$cans DSM 65T is 85%, as determined by quantitative DNA-DNA hybridization. Therefore, we propose the transfer of Thiosphaera pantotropha to P. denitrifians. The closest relative of Thiosphaera pantotrophu and P. denitr@cans is Thiobacillus versutus, as revealed by comparative 16s rRNA sequence analysis. These organisms are members of the alpha subclass of the Proteobacteria. Within this subclass, Thiosphaera pantotropha, P. denitrijicans, and Thiobacillus versutus form a phylogenetic group with Rhodobacter sphaeroides, Rhodobacter capsulatus, and "Erythrobacter longus."

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