The dominant cytochrome in thiosulphate-grown Thiobacillus A2 was found to be of the c-type with a reduced α-band at 548 nm (c548). This c548 component did not constitute an integral part of the membrane carrier system. It did, however, appear to be part of a large complex not tightly bound to membranes. Reconstitution experiments showed that cytochromes of the membrane ‘abc’ system could be reduced by the c548 component and vice versa. The reduction of membrane cytochromes of either lithotrophic or organotrophic origin by thiosulphate electrons was achieved, but it required the presence of a soluble fraction containing cytochrome c548. Evidence tending to rule out a reductive cleavage as the first step of the thiosulphate oxidation pathway in Thiobacillus A2 was obtained by following the reduction of partially purified c548 particles under various conditions.
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