Kinetic enrichment of <sup>34</sup>S during proteobacterial thiosulfate oxidation and the conserved role of SoxB in S-S bond breaking
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During chemolithoautotrophic thiosulfate oxidation the phylogenetically-diverged proteobacteria Paracoccus pantotrophus, Tetrathiobacter kashmirensis and Thiomicrospira crunogena rendered steady enrichment of <sup>34</sup>S in the end product sulfate, with overall fractionation ranging between -4.6‰ and +5.8‰. Fractionation kinetics of T. crunogena was essentially similar to that of P. pantotrophus, albeit the former had a slightly higher magnitude and rate of <sup>34</sup>S enrichment. In case of T. kashmirensis, the only significant departure of its fractionation curve from that of P. pantotrophus was observed during the first 36 h of thiosulfate-dependent growth, over which tetrathionate-intermediate formation is completed and sulfate production starts. Almost identical <sup>34</sup>S enrichment rates observed during the peak sulfate-producing stage of all the three processes indicated the potential involvement of identical S-S bond-breaking enzymes. Concurrent proteomic analyses detected the hydrolase SoxB (which is known to cleave terminal sulfone groups from SoxYZ-bound cysteine S-thiosulfonates as well as cysteine S-sulfonates in P. pantotrophus) in the actively sulfate-producing cells of all three species. Inducible expression of soxB during tetrathionate oxidation as well as the second leg of thiosulfate oxidation by T. kashmirensis is significant because the current Sox pathway does not accommodate tetrathionate as one of its substrates. Notably however, no other Sox protein except SoxB could be detected upon MALDI-MS/MS analysis of all such T. kashmirensis proteins which appeared to be thiosulfate-inducible in 2D gel electrophoresis. Instead, several other redox proteins were found to be at-least twofold over-expressed during thiosulfate- or tetrathionate-dependent growth, thereby indicating that there is more to tetrathionate oxidation than SoxB alone.
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