OXIDATIVE-METABOLISM OF INORGANIC SULFUR-COMPOUNDS BY BACTERIA

The history of the elucidation of the microbiology and biochemistry of the oxidation of inorganic sulfur compounds in chemolithotrophic bact eria is briefly reviewed, and the contribution of Martinus Beijerinck to the study of sulfur-oxidizing bacteria highlighted. Recent developm ents in the biochemistry, enzymology and molecular biology of su!fur o xidation in obligately and facultatively lithotrophic bacteria are sum marized, and the existence of at least two major pathways of thiosulfa te (sulfur and sulfide) oxidation confirmed. These are identified as t he 'Paracoccus sulfur oxidation' (or PSO) pathway and the 'S(4)interme diate' (or S4I) pathway respectively. The former occurs in organisms s uch as Paracoccus (Thiobacillus) versutus and P. denitrificans, and po ssibly in Thiobacillus novellus and Xanthobacter spp. The latter pathw ay is characteristic of the obligate chemolithotrophs (e.g. Thiobacill us tepidarius, T. neapolitanus, T. ferrooxidans, T. thiooxidans) and f acultative species such as T. acidophilus and T. aquaesulis, all of wh ich can produce or oxidize tetrathionate when grown on thiosulfate. Th e central problem, as yet incompletely resolved in all cases, is the e nzymology of the conversion of sulfane-sulfur (as in the outer [S-] at om of thiosulfate [S---SO3-]), or sulfur itself, to sulfate, and wheth er sulfite is involved as a free intermediate in this process in all, or only some, cases. The study of inorganic sulfur compound oxidation for energetic purposes in bacteria (i.e. chemolithotrophy and sulfur p hotolithotrophy) poses challenges for comparative biochemistry. It als o provides evidence of convergent evolution among diverse bacterial gr oups to achieve the end of energy-yielding sulfur compound oxidation ( to drive autotrophic growth on carbon dioxide) but using a variety of enzymological systems, which share some common features. Some new data are presented on the oxidation of S-35-thiosulfate, and on the effect of other anions (selenate, molybdate, tungstate, chromate, vanadate) on sulfur compound oxidation, including observations which relate to t he roles of polythionates and elemental sulfur as intermediates.