Mechanism of Oxidation of Dimethyl Disulphide by Thiobacillus thioparus Strain E6

Summary: A recently isolated organism, capable of chemolithoautotrophic growth on dimethyl disulphide, was characterized as a strain of Thiobacillus thioparus. It had DNA with a base composition of 60.5 ± 1.0 mol% G + C, and ubiquinone-8 (UQ-8) as its only respiratory quinone. Its growth in chemostat culture (at a growth rate of 0.07-0.08 h−1) showed yields of 14.4, 11.8 and 2.45 g cell-carbon per mol of dimethyl disulphide (DMDS), dimethyl sulphide (DMS) and thiosulphate, respectively. This is consistent with energy generation from the oxidation of the methyl and the sulphide moieties of DMDS, with oxidation of sulphide to sulphate contributing a yield of about 2.8 g cell-carbon mol−1. From whole organism and cell-free extract studies, DMDS oxidation was shown to proceed by its (NADH-stimulated) reduction to methanethiol (MT), which was oxidized via sulphide, formaldehyde and formate to CO2 and sulphate by MT oxidase, formaldehyde and formate dehydrogenases, and an uncharacterized sulphide-oxidizing system. The MT oxidase had a K m of 9.7 μM and showed substrate inhibition with a K i of about 8 μM. The essential role of catalase during growth on DMDS was shown by the sensitivity of growth on DMDS (but not on thiosulphate) to 3-amino-1,2,4-triazole. Catalase is believed to destroy the peroxide produced by the MT oxidase reaction. DMDS-grown organisms oxidized sulphide, thiosulphate and tetrathionate (with the latter indicated to be an intermediate in thiosulphate oxidation), suggesting the pathway of sulphide oxidation to be similar to that in some other thiobacilli. Carbon assimilation was by the Calvin cycle, with ribulose bisphosphate carboxylase being present in cell-free extracts at a specific activity of 80 nmol CO2 fixed min−1 (mg protein)−1. Hydroxypyruvate reductase (HPR) was not detected at levels sufficient to indicate any role in primary carbon assimilation.