Respiration and energy conservation in bacteria.

The characteristics of the respiratory system of the aerobic, nitrogen-fixing bacterium A.vinelandii were very similar during growth on a medium containing combined nitrogen (urea) compared with growth on nitrogen-free medium (nitrogen-fixing conditions; see Yates and Jones,1974). The growth yield of the organism with respect to ATP (YATP, g cells. mol ATP equivalents-1) was much higher during growth on combined nitrogen medium than during growth on nitrogen-free medium, probably due to the energy requirements of nitrogen fixation. In spite of its outstanding adaptations for growth in aerobic, nitrogen-deficient environments, A.vinelandii shows a poor ability to modify its respiratory system for efficient growth on a source of combined nitrogen. 2. Measurements of ? H+/O quotients (Mitchell, 1966) on batch- grown B.megaterium strains D440 and M suggested the presence of two functional proton-translocating loops in the respiratory chains of these organisms. Substrate-loading experiments (Lawford and Haddock, 1973) demonstrated the presence of proton-translocating loops 1&2 in strain D440 and 2&3 in strain M. The involvement of loop 3 in strain D440 was probably precluded by the absence of cytochrome c from the respiratory chain of this organism. Measurements of the in situ respiratory activity of chemostat cultures of B.megaterium allowed calculation of YATP and M (maintenance coefficient, mol ATP equivalents. g cells-1. h-1) during growth under various culture conditions. YATP values of 13.1 and 10.4 g cells. mol ATP equivalents-1 were obtained for strains D440 and M respectively during glycerol-limited growth. Similar values were obtained during NH4Cl-limited and oxygen-limited growth. These values are considerably lower than the theoretical maximum value for YATP (Forrest and Walker, 1971). Possible reasons for this are discussed.