SoxR, active

SoxR, OxyR Fur LAC9 Acel —rather uniform results showing activation -de-repression of genes that are under the control of Fur —attenuated DNA-binding in vitro —inhibition of DNA-binding in yeast... 

NADH, reduced nicotinamide-adenine dinueleotide Q, coenzyme Q FNR, fumarate nitrate reduction SoxR, superoxide activated regulatory protein IRP, iron regulatory protein NIFU, nitrogen fixation gene U product. 

Relatively recently Fe/S proteins have been found to function in the regulation of biosynthesis. This can be by promoting deoxyribonucleic acid (DNA) transcription, e.g. the [2Fe-2S] containing Escherichia coli superoxide-activated (SoxR) transcription activator [10-12], or the presumably [4Fe-4S]-containing E. coli transcription factor fumarate nitrate reduction (FNR) [13,14], Alternatively, the Fe/S protein can act by interference with messenger ribonucleic acid (mRNA) translation, i.e., the iron regulatory proteins (IRPs) [15,16], These interactions are stoichiometric, therefore not catalytic. Presumably, they are also a form of sensoring, namely, of oxidants and/or iron [17],... 

SoxR in the reduced state in the cell. Detailed understanding of molecular mechanism of transcription activation by SoxR awaits structural refinement. 

Ding H, Demple B. Direct nitric oxide signal transduction via nitrosylation of iron-sulfur centers in the SoxR transcription activator. P. Natl. Acad. Sci. 2000 97 5146-5150. 

A model (Figure 11-1) has thus emerged in which SoxR exists in a dynamic equilibrium linked both to chemical oxidation and reduction, presumably enzymatic. This model can explain the rapid shifts from inactive to active SoxR, and back again [29]. It does not necessarily require specific oxidizing species, as both oxygen and superoxide might be capable of the activating reaction under aerobic conditions [30]. However, the soxRS system is also activated upon cellular exposure to NO, which is even more effective under anaerobic conditions where NO is not... 

An excellent example of the regulatory function of Fe-S clusters, can be found in SoxR, which is discussed by Demple in Chapter 11. This protein is a [2Fe-2S] cluster-containing transcriptional regulator, which is activated by oxidative stress through the oxidation of its iron-sulfur centers, thus acting thus as a regulatory redox switch. 

Hidalgo E, Ding H, DempleB (1997) Redox signal transduction via iron-sulfur clusters in the SoxR transcription activator. Trends Biochem Sci 22 207-210... 

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