Chemolithotrophic oxidation of reduced sulfur compounds was studied in the betaproteobacterium Tetrathiobacter kashmirensis in correlation with its transposon (Tn5-mob)-inserted mutants impaired in sulfur oxidation (Sox(-)) and found to be carried out via the tetrathionate intermediate (S(4)I) pathway. The group of physiologically identical Sox(-) mutant strains presently examined could fully oxidize thiosulfate supplied in the media to equivalent amounts of tetrathionate but could only convert 5-10% of the latter to equivalent amounts of sulfite (equivalences in terms of mug atoms of S ml(-1)). These mutants were found to possess intact thiosulfate dehydrogenase, but defunct sulfite dehydrogenase, activities. Single copies of Tn5-mob in the genomes of the Sox(-) mutants were found inserted within the moeA gene, responsible for molybdopterin cofactor biosynthesis. This explained the inactivity of sulfite dehydrogenase. Chemolithotrophic oxidation of tetrathionate and sulfite by T. kashmirensis was found to be inhibited by 12 mM tungstate, whose effect could however be reversed by further addition of 15 mM molybdate. In mixotrophic medium, the mutants showed uninterrupted utilization of maltose but inhibition of tetrathionate utilization due to accumulation of sulfite. When sulfite was added to wild type cultures growing on tetrathionate-containing chemolithoautotrophic medium, it was found to render concentration-dependent inhibition of oxidation of tetrathionate. Our findings indicate that sulfite molecules negatively regulate their own synthesis by plausible inhibitory interaction(s) with enzyme(s) responsible for the oxidation of tetrathionate to sulfite; thereby clearly suggesting that one of the control mechanisms of chemolithotrophic sulfur oxidation could be at the level of sulfite.