Abstract
Intracellular iron levels and the expression of superoxide dismutase (SOD) and hydroperoxidase (HP) are regulated in Gram-negative bacteria by the iron(II)-activated ferric uptake regula- tor (Fur). We have previously observed that the expression of SOD in exponential phase Escherichia coli is dependent upon the redox state of iron in media, consistent with the ferrous specificity of Fur regulation (Bertrand et al., Med. Hypotheses 78: 130 - 133, 2012). Through the non-denaturing electrophoretic technique we have determined the Escherichia coli expression profiles of SOD and HP in response to iron challenge throughout lag, logarithmic, and stationary phases of replication. Lag phase SOD presented an unusual expression profile such that SOD expression was unresponsive to iron challenge, analogous to observations of mutant strains lacking Fur and of E. coli incubated in iron-deplete media. Challenging Escherichia coli with iron during logarithmic phase revealed that length of exposure to oxidants is unlikely to be the cause of SOD unresponsiveness in lag phase. HP activity was up-regulated two- or three-fold throughout all growth phases in response to iron challenge, but did not present redox- or growth phase-specific outcomes in a manner analogous to SOD. We hypothesize that low Fur levels during lag phase are responsible for unresponsive SOD.
Highlights
Iron is an essential metal, required as a prosthetic group for many metallo-proteins
E. coli Growth Curves Untreated E. coli remained in lag phase for approximately two hours, following which cultures progressed into an exponential phase that ended after eight additional hours of incubation (Figure 1). 1.00 mM Sodium nitroprusside (SNP) was observed to uniquely hinder E. coli replication, evident by mid-exponential phase, resulting in a lower optical density at stationary plateau as compared to all other treatments (Figure 1)
These results suggest that SNP imparts a severe oxidative and/or metabolic threat to E. coli
Summary
Iron is an essential metal, required as a prosthetic group for many metallo-proteins. Iron potentiates oxidative stress through the catalytic formation of hydroxyl radicals from hydrogen peroxide through the Fenton reaction [1] (Eq.1): (1). Reactive oxygen species such as hydroxyl radical (OH·), superoxide O2. Escherichia coli, the model species of this study, copiously produces manganese- and iron-bearing forms (MnSOD, FeSOD), as well as a trace isozyme known as CuZnSOD [26,27]. These are encoded by the genes sodA, sodB, and sodC, respectively. MnSOD expression is regulated by the protein products of six global regulators: soxRS and soxQ, both of which activate sodA; and fur, arcA, fnr, and ihf, all of which sup-
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