Functions as the terminal component of an NADH peroxidase (NADH:H(2)O(2) oxidoreductase) when using NADH:rubredoxin oxidoreductase (NROR) as the electron transport intermediary from NADH to revRbr 2. Plays an important role in the oxidative stress defense system in C.acetobutylicum, an obligate anaerobic bacterium. Also exhibits NADH oxidase (NADH:O(2) oxidoreductase) activity in vitro, which is 100-fold lesser than that of FprA1/2 using the same electron transfer components. Therefore, its predominant function is most likely as a scavenger of its preferred substrate, H(2)O(2). H(+) + H2O2 + NADH = 2 H2O + NAD(+) Binds 3 Fe(3+) ions per subunit. Rubredoxin (Rd) increases the NADH consumption rate by serving as an intermediary electron-transfer shuttle between NROR and revRbr. Homotetramer. Up-regulated by heat and oxidative stress (exposure to air, O(2) and H(2)O(2)) (at mRNA and protein levels). Various other environmental stress conditions such as an increase of the pH of the growth medium from 4.5 to 6.2, addition of the salt NaCl or of the solvent butanol, and lowering the incubation temperature also result in transiently increased transcript levels. Is also expressed under non-stressful conditions. Repressed by PerR. This protein has been named 'reverse' rubrerythrin (revRbr) because the order of the two functional domains is reversed compared to 'classical' rubrerythrins: the rubredoxin-like FeS4 domain is located at the N-terminus and the ferritin-like diiron domain at the C-terminus. A revRbr-overexpressing strain of C.acetobutylicum shows greatly increased tolerance to H(2)O(2) and O(2) exposure.