Effect of Salinity on Hydroxylamine Oxidation in a Marine Ammonia-Oxidizing Gammaproteobacterium, Nitrosococcus oceani strain NS58: Molecular and Catalytic Properties of Tetraheme Cytochrome c-554

Tetraheme cytochrome c-554 is a physiological electron acceptor of hydroxylamine oxidoreductase (HAO), a core enzyme of ammonia oxidation in chemoautotrophic nitrifiers. Here we report the purification of cytochrome c-554 from Nitrosococcus oceani strain NS58, a marine gammaproteobacterial ammonia-oxidizing bacterium. The NS58 cytochrome is a 25 kDa-protein having four hemes c. The absorption spectrum of the cytochrome showed peaks at 420 nm, 523 nm, and 554 nm, with shoulders at around 430 nm and 580 nm in the reduced state. In contrast to the highly basic counterpart from the betaproteobacterium Nitrosomonas europaea, the NS58 cytochrome c-554 was an acidic protein whose isoelectric point was 4.6. HAO was also purified, and the reaction with the NS58 cytochrome was found to be salt-tolerant. Compared with the activity observed in a non-salt solution, 60% of the activity remained in a saline concentration comparable to that of seawater.