SUMMARY: AM1 contains a novel -type cytochrome, called cytochrome -553, previously thought to be a precursor of the electron acceptor (cytochrome ) for methanol dehydrogenase. Its amino acid composition and serological characteristics show that it has no structural relationship to cytochrome . It usually comprises less than 5% of the total -type cytochromes. In a mutant, which contains neither methanol dehydrogenase nor cytochrome , it comprises 30% of the soluble cytochrome and it has been purified and characterized from that mutant. Cytochrome -553 is large ( 23000), acidic and monohaem, with a redox potential of 194 mV. It reacts rapidly and completely with CO but is not autoxidizable. It is not autoreducible, and it is not an electron acceptor from methanol dehydrogenase or methylamine dehydrogenase, nor an important electron donor to the oxidase. It is able to accept electrons from cytochrome and to donate electrons to cytochrome . It is present in the soluble fraction (presumably periplasmic) and membrane fraction of wild-type bacteria during growth on a wide range of growth substrates, but its function in these bacteria or in the mutant has not been determined.


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