THE INCREASE OF KCN-SENSITIVE ELECTRON FLOW IN THE BRANCHED RESPIRATORY-CHAIN OF MICROCOCCUS-LUTEUS GROWN SLOWLY IN CARBON-LIMITED CULTURE

The strictly aerobic bacterium Micrococcus luteus was grown in the presence of lactate as sole carbon source under conditions of excess substrates (in batch culture) or under strict lactate (C) or ammonium (N) limitation (in a chemostat, D = 0.02 h-1). KCN (2 mM) stimulated the respiration of batch-grown and N-limited cells, and inhibited the oxidase activity of C-limited cells by 40-60%. The content and composition of cytochromes and the KCN-dependences of the oxidase activities were found to be the same for the membranes of C-limited and batch-grown cells. The KCN sensitivity of the oxidase activities of isolated membranes decreased in the order NADH > malate > lactate. NAD-dependent lactate and malate dehydrogenase activities were observed in the cytoplasm of C-limited cells but not in that of batch-grown cells. The stimulation of cell respiration by lactate, malate, pyruvate or ethanol caused a marked increase of the steady-state level of NAD+ reduction only in C-limited cells. It is assumed that the slow growth of C-limited cells is accompanied by an increase in the formation of NADH, which is oxidized by the KCN-sensitive, tightly-coupled, main branch of the M. luteus respiratory chain. Under non-limited conditions of growth the respiration is due mainly to the direct oxidation of lactate via the weakly coupled alternative branch. The role of this switching of the electron flow from one pathway to the other during the adaptation of the bacteria to the slow C- and energy-limited growth is discussed.