203 74 74 4 4 G. N. de Groot A. H. Stouthamer Biological Laboratory Microbiology Department Free University Amsterdam The Netherlands Summary 1. The level of the specific activities of thiosulfate reductase and tetrathionate reductase in Proteus mirabilis was not influenced after anaerobic growth with or without thiosulfate, tetrathionate or azide. The formation of these reductases was repressed by both oxygen and nitrate. The repressive effect of nitrate, however, could be relieved by the presence of azide, which inhibits nitrate reductase (EC 184.108.40.206). 2. Thiosulfate reductase was rapidly inactivated by nitrate. A functional nitrate reductase system was necessary since inactivation was prevented by azide. Both thiosulfate and tetrathionate reductases were directly inactivated when cells were transferred from anaerobic to aerobic conditions. A shift of cells adapted to anaerobic growth with azide, however, did not result in inactivation. 3. Thiosulfate and tetrathionate reductases are coupled with hydrogenase (EC 220.127.116.11). In cells grown without azide the H 2 -thiosulfate reductase system was, in contrast to the H 2 -tetrathionate reductase system, inhibited by azide. However, both systems were insensitive to azide in cells grown with azide. 4. On addition of thiosulfate or tetrathionate to cells grown anaerobically with or without azide, two b-type cytochromes (a minor component absorbing at 563.5 nm and a b 561 component) were oxidized. The reductases were also coupled to cytochromes a 2 and a 1 in cells grown without azide, but not in cells grown with azide. 5. It is concluded that repression of the formation and inactivation of these reductases are separate processes. Inactivation can be explained by withdrawal of electrons via other components of the electron transport system. The formation seems to be repressed under conditions when electron transfer to the reductases cannot occur.
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