coli (Vine & Cole,

2011). It is currently unclear whether this ‘activity’ is a previously unreported NO reductase or a combination of a primary chemical reaction (e.g. metal-catalyzed nitrosylation of iron-sulfur centers) followed by a reductive repair pathway. NO reacts directly with metal ions to form nitrosyl complexes. Thus, nitrosylation of iron atoms, especially iron-sulfur clusters, in enzymes such as aconitase and fumarase or in the transcription factors FNR, Fur, SoxR, OxyR, and NsrR inactivates their functions. Under oxidizing conditions, metal nitrosyl complexes can then transfer the NO moiety to –SH groups of proteins, peptides, and cysteine, or to nitrogen atoms of secondary amines. As NarG is a major catalyst for the formation of NO EPZ015666 in the cytoplasm, protection mechanisms become essential when NarL and FNR are both active, which is during anaerobic growth in the presence of a high concentration of nitrate. Consequently, protection against damage by NO must also be activated by NarL, FNR or both. But this poses a dilemma: how can the bacteria survive when NO-induced damage is so severe that FNR can

no longer function? Enteric bacteria appear to have solved this problem by evolving multiple repair pathways, some that function when FNR (and Fur, etc.) are active, and others that deal with the greater threat when damage is so severe that FNR is itself inactivated. Mechanisms to Sirolimus manufacturer minimize damage caused when FNR is active include nitrite reduction by the cytoplasmic nitrite reductase, NirBD, nitrite expulsion by the nitrate and nitrite transporter, NarK (Jia et al., 2009), and possibly repair by the hybrid cluster protein, Hcp, and its reductase. Expression of the genes for all of these proteins is dependent upon FNR activation. However,

contrary to our earlier report (Filenko et al., 2007), hcp expression is not activated by NarL, but instead it is directly regulated by NsrR in response to NO (Chismon et al., 2010). Nitrosative damage to iron-sulfur centers, and possibly other iron proteins as well, is repaired in E. coli by YtfE or Adenylyl cyclase RIC – for repair of iron centers (Justino et al., 2007; Overton et al., 2008). It is not known whether damaged iron-sulfur centers are repaired directly by the removal of the NO moiety, or whether iron is released followed by the reconstruction of the active redox center. If the former is correct, is an acceptor molecule nitrosated in the process and, if so, what is this acceptor and how is that regenerated? Synthesis of three further proteins is strongly up-regulated by nitrate-activated NarL during anaerobic growth in the presence of nitrate but is not dependent on activation by the FNR protein. These are the O6-methyl-guanine methyl transferase, Ogt; and the products of the two-gene operon, yeaR yoaG.