, 1997), both of the pathways for nitrate reduction to ammonia are expressed only during anaerobic growth. Transcription of narGHJI and

nirBD is also activated by the NarX-NarL two-component regulatory system in response to moderate concentrations of nitrate; nirBD, and to a much lesser extent narGHJI, are also activated by the alternative two-component system, NarQ-NarP (Rabin & Stewart, 1993). Classical genetic approaches and more recent whole genome transcriptomic studies have indicated that the cytoplasmic pathway is physiologically more significant only in nitrate-rich environments that might occur in soil, in some highly contaminated sediments, and waste water treatment plants (Potter et al., 1999). In contrast, the transcription of genes for the periplasmic Nap-Nrf pathway Wnt inhibition is activated by NarQ-NarP in response to low concentrations of nitrate (< 100 μM) click here but are repressed by NarX-NarL when nitrate is abundant (Page et al., 1990).

This indicates that the periplasmic pathway confers a selective advantage for bacterial survival in the nitrate limited environment of the gastro-intestinal tract of humans and other warm blooded animals (Potter et al., 1999; Constantinidou et al., 2006). Based upon the accumulation of very small quantities of nitrous oxide during nitrite reduction, it was assumed that the rate of NO production was two to three orders of magnitude slower than the rate of nitrite reduction (Smith, 1983).

It was predicted that NO was a side product released during science nitrite reduction by either NirBD or NrfA. However, there is an extensive literature showing that the major source of nitrosative stress is NO generated by the interaction of the cytoplasmic nitrate reductase, NarG, with nitrite (reviewed in the accompanying paper by Vine et al., 2011). Realization that enteric bacteria can reduce nitrite to NO re-opened the question whether NO is generated by a single mechanism or by more than one pathway, depending on the conditions under which the bacteria are grown. Specifically, is more NO generated by the membrane-associated nitrate reductase, NarG, by one of the nitrite reductases, NirBD or NrfAB, or by other molybdoproteins that are active during anaerobic growth? The sensitive response of the transcription repressor, NsrR, to NO provides a method to detect the presence of NO in the bacterial cytoplasm (Hutchings et al., 2000; Corker & Poole, 2003; Bodenmiller & Spiro, 2006; Tucker et al., 2008). By coupling an NsrR-regulated E. coli promoter to lacZ expression during anaerobic growth in the presence of nitrite, it was shown that mutations in nirBD or nrfAB resulted in greater expression of lacZ, indicative of the increased accumulation of NO in the cytoplasm (Vine et al., 2011). Conversely, deletion of the narGHJI operon significantly decreased but did not eliminate lacZ expression, indicative of less accumulation of cytoplasmic NO.