Response of the NAD(P)H-oxidising flavohaemoglobin (Hmp) to prolonged oxidative stress and implications for its physiological role in Escherichia coli.

The Escherichia coli flavohaemoglobin (Hmp) has a globin-like N-terminal domain and a ferredoxin-NADP-reductase-like C-terminal domain. We show here that purified Hmp oxidises both NADH and NADPH with Km values of 1.8 and 19.6 microM, respectively. Prolonged incubation of a hmp-lacZ fusion strain with the redox cycling agent paraquat resulted in a 28-fold induction of hmp gene expression, nearly 3-fold higher than after short periods of exposure. A strain overproducing Hmp was significantly more sensitive to paraquat than was the wild-type strain but, in vitro, purified Hmp was not an effective NADPH-paraquat diaphorase. Prolonged incubation of a wild-type strain with paraquat increased intracellular Hmp to spectrally detectable levels.