Mutational and biochemical analysis of cytochrome c′, a nitric oxide-binding lipoprotein important for adaptation of Neisseria gonorrhoeae to oxygen-limited growth

Abstract

Neisseria gonorrhoeae is a prolific source of c-type cytochromes. Five of the constitutively expressed cytochromes are predicted, based on in silico analysis of the N. gonorrhoeae genome, to be components of the cytochrome bc1 complex, cytochrome c oxidase cbb3 or periplasmic cytochromes involved in electron transfer reactions typical of a bacterium with a microaerobic physiology. Cytochrome c peroxidase was previously shown to be a lipoprotein expressed only during oxygen-limited growth. The final c-type cytochrome, cytochrome c', similar to cytochrome c peroxidase, includes a lipobox required for targeting to the outer membrane. Maturation of cytochrome c' was partially inhibited by globomycin, an antibiotic that specifically inhibits signal peptidase II, resulting in the accumulation of the prolipoprotein in the cytoplasmic membrane. Disruption of the gonococcal cycP gene resulted in an extended lag phase during microaerobic growth in the presence but not in the absence of nitrite, suggesting that cytochrome c' protects the bacteria from NO generated by nitrite reduction during adaptation to oxygen-limited growth. The cytochrome c' gene was overexpressed in Escherichia coli and recombinant cytochrome c' was shown to be targeted to the outer membrane. Spectroscopic evidence is presented showing that gonococcal cytochrome c' is similar to previously characterized cytochrome c' proteins and that it binds NO in vitro. The demonstration that two of the seven gonococcal c-type cytochromes fulfil specialized functions and are outer membrane lipoproteins suggests that the localization of these lipoproteins close to the bacterial surface provides effective protection against external assaults from reactive oxygen and reactive nitrogen species.