Abstract
Escherichia coli B was cultured continuously in succinate-minimal medium under conditions of oxygen limitation in the phauxostat. With decreasing oxygenation and consequent decreasing growth rates, the complement of terminal cytochrome oxidases changed as follows: high growth rates, cytochrome o; intermediate growth rates, cytochromes o and d; lowest growth rates, cytochromes o, d, and a1. Respiratory kinetics exhibited by nongrowing cell suspensions obtained from continuous cultures indicated that terminal oxidase activity was exhibited by cytochrome o (Km for O2 = 0.2 micron; Vmax = 1.1 to 1.5 mumol of O2 per nmol of cytochrome o per min) and cytochrome d (Km for O2 = 0.024 micron; Vmax = 0.7 mumol of O2 per nmol of cytochrome d per min). During oxygen-limited growth, the molar growth yield referred to respiration, and corrected for maintenance respiration [Yo(max)], was 12.6 g (dry weight) per g-atom of oxygen, not significantly different from the succinate-limited value of 12.0 g (dry weight) per g-atom of oxygen. The rate of maintenance respiration of the oxygen-limited culture was only 3.4 mg-atoms of O per g (dry weight) per h, some threefold less than that of the succinate-limited culture. Respiration-driven proton extrusion did not vary with the growth rate or with the complement of terminal oxidases (H+/O = 3.7; standard deviation, 0.07). We conclude that the content of terminal oxidases is without effect on the efficiency of respiratory energy conservation.