Electron‐Transport Chains of Escherichia coli

1 The properties of a 5‐aminolaevulinic acid synthetase‐deficient mutant of Escherichia coli K12Ymel grown aerobically or anaerobically in the presence or absence of 5‐aminolaevulinic acid are described. In the absence of 5‐aminolaevulinic acid no detectable cytochromes are produced, respiration rates are lowered, and the concentration of menaquinone in the cell is increased. The absence of 5‐aminolaevulinic acid from the growth medium has no apparent effect on the Mg 2+ dependent ATPase activity or on the concentrations of ubiquinone and flavoprotein(s) in the cell. 2 Two methods for the reconstitution of respiration by cells grown aerobically or anaerobically in the absence of 5‐aminolaevulinic acid are described. With intact cells, reconstitution is achieved in 3 h at 37 °C and is dependent on the presence of 5‐aminolaevulinic acid and a fermentable carbon source. With cell‐free extracts, respiration can be reconstituted by the addition of haematin in 15–20 min at 37 °C provided that ATP is also present. 3 It is concluded that the reconstitution of respiration by both methods is achieved in the absence of protein synthesis, indicating that the apoproteins of cytochromes, involved in electron transport with oxygen as terminal electron acceptor, are synthesized and incorporated into the membrane in the absence of haem synthesis. 4 From an examination of the cytochromes produced during the reconstitution experiments, it is suggested that two membrane‐bound electron transport systems using oxygen as terminal electron acceptor can co‐exist in E. coli. The first involving cytochrome b 556 with cytochrome o as terminal oxidase is the system normally present under aerobic growth conditions, and the second is induced under conditions when the concentrations of NADH and ADP in the cell are likely to be high and involves cytochrome b 558 with cytochrome a 2 as the terminal oxidase.