The H؉ -ATP synthase is a reversible engine of mitochondria that synthesizes or hydrolyzes ATP upon changes in cell physiology. ATP synthase dysfunction is involved in the onset and progression of diverse human pathologies. During ischemia, the ATP hydrolytic activity of the enzyme is inhibited by the ATPase inhibitory factor 1 (IF1 In oxidative phosphorylation, ATP is synthesized by the mitochondrial ATP synthase, a H ϩ -driven rotatory engine of the inner membrane that utilizes as driving force for ATP synthesis the H ϩ electrochemical gradient generated by the respiratory chain (1-4). The cellular expression level of ␤-F1-ATPase, 2 which is the catalytic subunit of the H ϩ -ATP synthase, is diminished in diverse human pathologies (5), which include cancer (6 -9), affording a relevant marker of disease progression (6, 7, 10 -12) and of the response to chemotherapy (7,(13)(14)(15). Moreover, the down-regulation of ␤-F1-ATPase in lung carcinomas (12) and colon cancer cells (15) also provides a mechanistic explanation to the increased glucose avidity of carcinomas, i.e. to the enhanced aerobic glycolysis of cancer cells (16,17). Interestingly, the quantitative determination of ␤-F1-ATPase relative to the content of glyceraldehyde-3-phosphate dehydrogenase in human tumors has revealed that cancer abolishes the tissue-specific differences in the cellular complement of the bioenergetic ␤-F1-ATPase protein (18).It is well established that when mitochondrial respiration is impaired, the H ϩ -ATP synthase can function in reverse acting as an ATP hydrolase for maintaining the proton motive force (1,19). This process is regulated by an inhibitor peptide called ATPase inhibitory factor 1 or IF1 (19 -21), a highly conserved nuclearly encoded protein. When matrix pH drops, IF1 becomes activated and binds ␤-F1-ATPase, blocking ATP hydrolysis and preventing a useless waste of energy (20). The substitution of histidine 49 in IF1 by a lysine residue renders a mutant form (H49K) that inhibits the ATP hydrolase activity in a pH-insensitive way (22). The structure and in vitro mechanism of action of IF1 has been studied in detail, and its role as an inhibitor of the hydrolase activity of the H ϩ -ATP synthase is well documented (19,20,23). However, the information on IF1 expression in human tissues and its putative contribution to the development of human pathology are unknown. In this study, we demonstrate that IF1 is overexpressed in human carcinomas. Moreover, we document that IF1 plays a regulatory role in controlling cellular energetic metabolism, strongly supporting its participation as an additional molecular switch used by cancer cells to trigger the induction of aerobic glycolysis, i.e. their Warburg phenotype. 2 The abbreviations used are: ␤-F1-ATPase, ␤ catalytic subunit of the H ϩ -ATP synthase; IF1, ATPase inhibitory factor 1; FCCP, carbonyl cyanide-p-trifluoromethoxyphenylhydrazone; siRNA, small interfering RNA; NRK, normal rat kidney. EXPERIMENTAL PROCEDURES

While persisters are a health threat due to their transient antibiotic tolerance, little is known about their phenotype and what actually causes persistence. Using a new method for persister generation and high‐throughput methods, we comprehensively mapped the molecular phenotype of Escherichia coli during the entry and in the state of persistence in nutrient‐rich conditions. The persister proteome is characterized by σS‐mediated stress response and a shift to catabolism, a proteome that starved cells tried to but could not reach due to absence of a carbon and energy source. Metabolism of persisters is geared toward energy production, with depleted metabolite pools. We developed and experimentally verified a model, in which persistence is established through a system‐level feedback: Strong perturbations of metabolic homeostasis cause metabolic fluxes to collapse, prohibiting adjustments toward restoring homeostasis. This vicious cycle is stabilized and modulated by high ppGpp levels, toxin/anti‐toxin systems, and the σS‐mediated stress response. Our system‐level model consistently integrates past findings with our new data, thereby providing an important basis for future research on persisters.