Summary auto-generated
This study investigates how Escherichia coli cells survive phosphate (Pi) starvation under aerobic conditions, which was previously thought to be lethal due to continuous reactive oxygen species (ROS) production from aerobic respiration. Using various bacterial strains and genetic approaches, the researchers found that Pi-starved E. coli maintain active aerobic metabolism for approximately 3 days, continuously degrading nutrients like arginine (converted to putrescine via polyamine synthesis) and glucose (converted to acetate). Despite high ROS levels, cells survive through protection mediated by RpoS and LexA regulons. The LexA regulon responds to DNA double-strand breaks generated by hydroxyl radicals, while the RpoS regulon upregulates protective enzymes like catalase (KatE, KatG) and DNA-binding protein Dps. Polyamine synthesis provides additional nucleic acid protection. A third global regulator, H-NS, coordinates metabolic and stress-response processes, maintaining aerobic enzyme activity while enhancing ROS defenses. The study demonstrates that Pi-starved cells accumulate suppressor mutations at higher frequency than glucose-starved cells, with Arg+ revertants gaining growth advantages in the Pi-limited environment.
Key findings
- E. coli maintains active aerobic metabolism during Pi starvation for 3 days, producing ROS that generates DNA double-strand breaks despite causing oxidative stress
- RpoS and LexA regulons provide essential ROS protection through induction of catalases, DNA-binding proteins, and DNA repair enzymes, particularly RecA and RuvAB
- H-NS coordinates both metabolic processes (favoring aerobic enzyme activity) and ROS defense mechanisms during Pi starvation
- Polyamine synthesis from arginine degradation provides additional protection for nucleic acids against oxidative damage
- Suppressor mutations accumulate at high frequency in Pi-starved cultures, with Arg+ revertants gaining growth advantages as culture medium composition changes
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Abstract
It has been suggested that Escherichia coli can resist aerobic, glucose-starvation conditions by switching rapidly from an aerobic to a fermentative metabolism, thereby preventing the production by the respiratory chain of reactive oxygen species (ROS) that can damage cellular constituents. In contrast, it has been reported that E. coli cannot resist aerobic, phosphate (P(i))-starvation conditions, probably because of the maintenance of an aerobic metabolism and the continuous production of ROS. This paper presents evidence that E. coli cells starved for P(i) under aerobic conditions indeed maintain an active aerobic metabolism for about 3 d, which allows the complete degradation of exogenous nutrients such as arginine (metabolized probably to putrescine via the SpeA-initiated pathway) and glucose (metabolized notably to acetate), but cell viability is not significantly affected because of the protection afforded against ROS through the expression of the RpoS and LexA regulons. The involvement of the LexA-controlled RuvAB and RecA proteins with the RecG and RecBCD proteins in metabolism and cell viability implies that DNA double-strand breaks (DSB), and thus hydroxyl radicals that normally generate this type of damage, are produced in P(i)-starved cells. It is shown that induction of the LexA regulon, which helps protect P(i)-starved cells, is totally prevented by introduction of a recB mutation, which indicates that DSB are actually the main DNA lesion generated in P(i)-starved cells. The requirement of RpoS for survival of cells starved for P(i) may thus be explained by the role played by various RpoS-controlled gene products such as KatE, KatG and Dps in the protection of DNA against ROS. In the same light, the degradation of arginine and threonine may be accounted for by the synthesis of polyamines (putrescine and spermidine) that protect nucleic acids from ROS. Besides LexA and RpoS, a third global regulator, the nucleoid-associated protein H-NS, is also shown to play a key role in P(i)-starved cells. Through a modulation of the metabolism during P(i) starvation, H-NS may perform two complementary tasks: it helps maintain a rapid metabolism of glucose and arginine, probably by favouring the activity of aerobic enzymes such as the NAD-dependent pyruvate dehydrogenase complex, and it may enhance the cellular defences against ROS which are then produced by increasing RpoS activity via the synthesis of acetate and presumably homoserine lactone.