Summary: Three classes of cytochrome a deficient mutants of have been isolated. These classes were defined by concentration ratios of cytochromes a, b and c relative to the parent strain, and by responses to non-fermentable substrates. The cytochrome deficiencies were not due to blocks in haem biosynthesis or to failure of the control mechanisms which alter the respiratory chain during growth of . Mutational loss of cytochrome a did not reduce proportionately the rate of oxygen consumption by late exponential phase cells, although exponential growth rates were generally lower than the parent strain. The cytochrome system of wild-type appears to contain cytochromes a (or a+a), b, c, c, and o. A previously unreported absorption maximum at 617 nm was discovered, which was elevated in some of the cytochrome a deficient mutants. In these mutants, a new maximum also appeared at 627 nm.


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