Non-heme iron proteins from anaerobic bacteria involved in nitrosative stress protection

Acetogenic bacteria grow anaerobically and use CO 2 and an electron donor such as H 2 to produce acetate. Some acetogens, such as Moorella thermoacetica , can alternatively use nitrate instead of CO 2 as terminal electron acceptor. In its natural habitat, M. thermoacetica is likely to encounter the toxic gas, nitric oxide (NO), generated either endogenously from nitrate reduction or exogenously, as a response to colonization or infection. Reported here is the characterization of a non-heme iron flavoprotein, FprA, from M. thermoacetica, which acts as a non-respiratory nitric oxide reductase, catalyzing reduction of NO to N 2O by NADH in vivo and in vitro. Also characterized is a flavo-iron reductase from M. thermoacetica, Hrb, which functions as an efficient NADH:FprA oxidoreductase. FprAs mechanism of NO reduction and its role in counteracting nitrosative stress are addressed with the use of structural, kinetic, spectral, biochemical and molecular biological methods. Evidence that FprAs from sulfate-reducing bacteria function as non-respiratory nitric oxide reductases is also presented.