Iron availability enhances the cellular energetics of aerobic Escherichia coli cultures by activating anaerobic respiratory chains

Aerobic Escherichia coli growth at restricted iron concentrations (= 1.75 ± 0.04 mM) is characterized by lower biomass yield, higher acetate accumulation, and higher activation of the siderophore iron-acquisition systems. Although iron homeostasis in E. coli has been studied intensively, these studies focused only on understanding the regulation of the iron import systems and the iron-requiring enzymes. In this study, the effect of iron availability on the energy metabolism of E. coli was investigated. It was established that aerobic cultures growing at limiting iron conditions showed lower ATP yield per glucose, lower growth rate, and lower TCA cycle activity and respiration, and at the same time increased glucose consumption, acetate and pyruvate accumulation, practically mimicking microaerobic growth. However, at excess iron, independently of oxygen availability, the cultures showed high cellular energetics (5.8 ATP/mol of glucose) by using pathways requiring iron-rich complex proteins found in the TCA cycle and respiration chain. At conditions of iron excess, some iron requiring terminal reductases of the respiratory chain, that were supposed to be anaerobic, were used by the E. coli, when in aerobic conditions, to keep high respiration activity. This high respiration activity allowed E. coli to produce more biomass and more reactive oxygen species that were controlled by the higher activity of the antioxidant defenses (SOD, peroxidase, and catalase) and the iron-sulfur cluster repair systems. Overall design: We performed gene expression profiling analysis using data obtained from RNA-seq of E. coli chemostat cultures grown in iron limiting and iron excess conditions. The samples were collected from the steady state cultures growing at three dissolved oxygen conditions i.e., 6.3, 21 and 63%.