coli is indeed NarG, but alternative enzymes also form NO from nitrite. These alternative sources might be more significant under some environmental conditions than others. Deletion of genes for the periplasmic nitrate reductase, NapAB, had no effect, suggesting that NapAB does not catalyze NO production from nitrite at a significant rate (J.A. Cole & C.E. Vine, unpublished data; D. Richardson & G. Rowley, pers.

commun.). The only enzyme that is currently known to function as a ‘specialized’ NO reductase in E. coli is NorVW, which consists of the reductase, NorV, (also known as flavorubredoxin), and NorW, a redox protein that CP 673451 reduces NorV (Gomes et al., 2002; Gardner et al., 2003). Synthesis of NorVW is induced by NO during both aerobic and anaerobic growth, suggesting that NorVW is a primary source of protection against Ibrutinib nitrosative stress. Expression of the norVW operon is regulated positively by the product of the divergently transcribed norR. NorR is a DNA-binding enhancer protein that in response to low concentrations of cytoplasmic NO activates norVW transcription by the σ54 version of RNA polymerase (Hutchings et al.,

2002; Gardner et al., 2003). The active site of NorR is a di-iron center that can be directly nitrosylated in the presence of very low concentrations of NO (D’Autreaux et al., 2005). The primary function of NrfA is to reduce nitrite entering bacteria from the environment to ammonia. It also has an extremely active NO reductase activity (Poock et al., 2002). Although the Km for NO is high, Van Wonderen et al. (2008) proposed that NrfA is likely to provide the first line of defense against external NO generated either by the host or by other neighboring bacteria. Any NO that escapes reduction by NrfA and enters the cytoplasm would then be mopped up by NorVW, which is also optimally induced under anaerobic ADAMTS5 conditions. Several critical questions arise from this proposal. First, is a NrfA mutant more sensitive than its parent to growth inhibition

by externally supplied NO? Is the rate of NO reduction by a NrfA mutant significantly lower than that of the isogenic parent? Is a nrfA norVW double mutant even more sensitive to NO? We are unaware of any direct biochemical evidence that the cytoplasmic nitrite reductase, NirBD, can also reduce NO to ammonia. Unlike NrfA, which is a relatively stable protein, the prosthetic groups of NirB are readily lost during purification, so very few studies of this protein have been reported (Jackson et al., 1981). However, we recently reassessed the relative roles of these four possible pathways for NO reduction by constructing mutants defective in one, two, three, or all four of the above-mentioned systems and grew the isogenic strains anaerobically in the presence of nitrate or nitrite.