Regulation of the Nitric Oxide Reduction Operon (norRVW) in Escherichia coli

Nitric oxide (NO) induces NO-detoxifying enzymes in Escherichia coli suggesting sensitive mechanisms for coordinate control of NO defense genes in response to NO stress. Exposure of E. coli to sub-micromolar NO levels under anaerobic conditions rapidly induced transcription of the NO reductase (NOR) structural genes, norV and norW, as monitored by lac gene fusions. Disruption ofrpoN (ς54) impaired the NO-mediated induction of norV and norW transcription and NOR expression, whereas disruption of the upstream regulatory gene,norR, completely ablated NOR induction. NOR inducibility was restored to NorR null mutants by expressing NorR intrans. Furthermore, an internal deletion of the N-terminal domain of NorR activated NOR expression independent of NO exposure. Neither NorR nor ς54 was essential for NO-mediated induction of the NO dioxygenase (flavohemoglobin) encoded byhmp. However, elevated NOR activity inhibited NO dioxygenase induction, and, in the presence of dioxygen, NO dioxygenase inhibited norV induction by NO. The results demonstrate the role of NorR as a ς54-dependent regulator ofnorVW expression. A role for the NorR N-terminal domain as a transducer or sensor for NO is suggested. Nitric oxide (NO) induces NO-detoxifying enzymes in Escherichia coli suggesting sensitive mechanisms for coordinate control of NO defense genes in response to NO stress. Exposure of E. coli to sub-micromolar NO levels under anaerobic conditions rapidly induced transcription of the NO reductase (NOR) structural genes, norV and norW, as monitored by lac gene fusions. Disruption ofrpoN (ς54) impaired the NO-mediated induction of norV and norW transcription and NOR expression, whereas disruption of the upstream regulatory gene,norR, completely ablated NOR induction. NOR inducibility was restored to NorR null mutants by expressing NorR intrans. Furthermore, an internal deletion of the N-terminal domain of NorR activated NOR expression independent of NO exposure. Neither NorR nor ς54 was essential for NO-mediated induction of the NO dioxygenase (flavohemoglobin) encoded byhmp. However, elevated NOR activity inhibited NO dioxygenase induction, and, in the presence of dioxygen, NO dioxygenase inhibited norV induction by NO. The results demonstrate the role of NorR as a ς54-dependent regulator ofnorVW expression. A role for the NorR N-terminal domain as a transducer or sensor for NO is suggested. nitric oxide NO reductase NO dioxygenase Luria-Bertani chloramphenicol resistance, Apr, ampicillin resistance tetracycline resistance kanamycin resistance Nitric oxide (NO)1 is a free radical with multiple and diverse biological functions (reviewed in Ref. 1Ignarro L.J. Kidney Int. Suppl. 1996; 55: S2-S5PubMed Google Scholar). NO serves as an intermediate in microbial denitrification (2Zumft W. Microbiol. Mol. Biol. 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Microbes also express NO dioxygenases (NODs) that utilize O2 to convert NO to nitrate (12Gardner P.R. Gardner A.M. Martin L.A. Salzman A.L. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 10378-10383Crossref PubMed Scopus (497) Google Scholar, 13Gardner A.M. Martin L.A. Gardner P.R. Dou Y. Olson J.S. J. Biol. Chem. 2000; 275: 12581-12589Abstract Full Text Full Text PDF PubMed Scopus (139) Google Scholar, 14Gardner P.R. Gardner A.M. Martin L.A. Dou Y. Li T. Olson J.S. Zhu H. Riggs A.F. J. Biol. Chem. 2000; 275: 31581-31587Abstract Full Text Full Text PDF PubMed Scopus (133) Google Scholar, 15Hausladen A. Gow A.J. Stamler J.S. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 14100-14105Crossref PubMed Scopus (255) Google Scholar, 16Liu L. Zeng M. Hausladen A. Heitman J. Stamler J.S. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 4672-4676Crossref PubMed Scopus (166) Google Scholar, 17Ouellet H. Ouellet Y. Richard C. Labarre M. Wittenberg B. Wittenberg J. Guertin M. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 5902-5907Crossref PubMed Scopus (237) Google Scholar, 18Pathania R. Navani N.K. Gardner A.M. Gardner P.R. Dikshit K.L. Mol. Microbiol. 2002; 45: 1303-1314Crossref PubMed Scopus (119) Google Scholar). Escherichia coli employs both of these enzymes. An inducible NOD (flavohemoglobin), encoded by the genehmp (19Vasudevan S.G. Armarego W.L.F. Shaw D.C. Lilley P.E. Dixon N.E. Poole R.K. Mol. Gen. Genet. 1991; 226: 49-58Crossref PubMed Scopus (184) Google Scholar), detoxifies NO under aerobic growth conditions (12Gardner P.R. Gardner A.M. Martin L.A. Salzman A.L. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 10378-10383Crossref PubMed Scopus (497) Google Scholar,15Hausladen A. Gow A.J. Stamler J.S. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 14100-14105Crossref PubMed Scopus (255) Google Scholar, 20Gardner A.M. Gardner P.R. J. Biol. Chem. 2002; 277: 8166-8171Abstract Full Text Full Text PDF PubMed Scopus (147) Google Scholar). An inducible O2-sensitive NOR activity encoded by the norRVW operon detoxifies NO under anaerobic and microaerobic conditions (8Gardner A.M. Helmick R.A. Gardner P.R. J. Biol. Chem. 2002; 277: 8172-8177Abstract Full Text Full Text PDF PubMed Scopus (256) Google Scholar, 20Gardner A.M. Gardner P.R. J. Biol. Chem. 2002; 277: 8166-8171Abstract Full Text Full Text PDF PubMed Scopus (147) Google Scholar). NorV is a di-iron center-containing flavorubredoxin-type NOR with orthologues in the Archeae, strict anaerobes, and facultative anaerobes (21Gomes C.M. Vicente J.B. Wasserfallen A. Teixeira M. Biochemistry. 2000; 39: 16230-16237Crossref PubMed Scopus (60) Google Scholar, 22Wasserfallen A. Ragettli S. Jouanneau J. Leisinger T. Eur. J. Biochem. 1998; 254: 325-332Crossref PubMed Scopus (79) Google Scholar, 23Frazão C. Silva G. Gomes C.M. Matias P. Coelho R. Sieker L. Macedo S. Liu M.-Y. Oliveira S. Teixeira M. Xavier A.V. Rodrigues-Pousada C. Carrondo M.A. LeGall J. Nat. Struct. Biol. 2000; 7: 1041-1045Crossref PubMed Scopus (198) Google Scholar, 24Das A. Coulter E.D. Kurtz Jr., D.M. Ljungdahl L.G. J. Bacteriol. 2001; 183: 1560-1567Crossref PubMed Scopus (68) Google Scholar). It is distinct from the bacterial heme/nonheme iron-containing cytochrome bc-type NORs and the fungal P450-type NOR (2Zumft W. Microbiol. Mol. Biol. Rev. 1997; 61: 533-616Crossref PubMed Scopus (2897) Google Scholar). NorW functions as an NADH:flavorubredoxin oxidoreductase (21Gomes C.M. Vicente J.B. Wasserfallen A. Teixeira M. Biochemistry. 2000; 39: 16230-16237Crossref PubMed Scopus (60) Google Scholar) and is required for maximal flavorubredoxin-catalyzed NO reduction in cells (8Gardner A.M. Helmick R.A. Gardner P.R. J. Biol. Chem. 2002; 277: 8172-8177Abstract Full Text Full Text PDF PubMed Scopus (256) Google Scholar) and in vitro (25Gomes C.M. Giuffrè A. Forte E. Vicente J.B. Saraiva L.M. Brunori M. Teixeira M. J. Biol. Chem. 2002; 277: 25273-25276Abstract Full Text Full Text PDF PubMed Scopus (177) Google Scholar). Together, the O2-dependent NOD and the O2-sensitive NOR (NorVW) detoxify NO throughout the physiological O2 range (7Gardner P.R. Costantino G. Salzman A.L. J. Biol. Chem. 1998; 273: 26528-26533Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar, 8Gardner A.M. Helmick R.A. Gardner P.R. J. Biol. Chem. 2002; 277: 8172-8177Abstract Full Text Full Text PDF PubMed Scopus (256) Google Scholar, 20Gardner A.M. Gardner P.R. J. Biol. Chem. 2002; 277: 8166-8171Abstract Full Text Full Text PDF PubMed Scopus (147) Google Scholar). NORs and NODs are induced by NO or NO-generating agents suggesting fine-tuned mechanisms for the coordination of microbial NO defenses to NO stress levels. In denitrifying Pseudomonas andRhodobacter, cytochrome bc-type NORs are up-regulated by the Fnr-like DnrD/NnrR transcription regulators in response to nanomolar NO (26Vollack K.-U. Zumft W.G. J. Bacteriol. 2001; 183: 2516-2526Crossref PubMed Scopus (102) Google Scholar, 27Kwiatkowski A.V. Shapleigh J.P. J. Biol. Chem. 1996; 271: 24382-24388Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar, 28Hutchings M.I. Shearer N. Wastell S. van Spanning R.J. Spiro S. J. Bacteriol. 2000; 182: 6434-6439Crossref PubMed Scopus (46) Google Scholar). However, unlike Fnr (29Cruz-Ramos H. Crack J. Wu G. Hughes M.N. Scott C. Thomson A.J. Green J. Poole R.K. EMBO J. 2002; 21: 3235-3244Crossref PubMed Scopus (253) Google Scholar), DnrD/NnrR do not bear NO-reactive 4Fe-4S centers, and the NO sensing mechanism is currently unknown (26Vollack K.-U. Zumft W.G. J. Bacteriol. 2001; 183: 2516-2526Crossref PubMed Scopus (102) Google Scholar, 27Kwiatkowski A.V. Shapleigh J.P. J. Biol. Chem. 1996; 271: 24382-24388Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar, 28Hutchings M.I. Shearer N. Wastell S. van Spanning R.J. Spiro S. J. Bacteriol. 2000; 182: 6434-6439Crossref PubMed Scopus (46) Google Scholar, 30Zumft W.G. J. Mol. Microbiol. Biotechnol. 2002; 4: 277-286PubMed Google Scholar). In the denitrifierRalstonia eutropha, the tripartite transcription factor NorR regulates denitrification, norA1B1 transcription, and NOR activity expression in a ς54-dependent mechanism in response to exposures to sodium nitroprusside, the NO donor compound NOC18, or during growth with nitrite or nitrate (31Pohlmann A. Cramm R. Schmelz K. Friedrich B. Mol. Microbiol. 2000; 38: 626-638Crossref PubMed Scopus (98) Google Scholar).E. coli and related microbes contain norRorthologues suggesting a global regulatory role for NorR in controlling defenses (i.e. norVW, norBC, andhmp) against the incipient toxicity of NO and secondarily derived reactive nitrogen species (8Gardner A.M. Helmick R.A. Gardner P.R. J. Biol. Chem. 2002; 277: 8172-8177Abstract Full Text Full Text PDF PubMed Scopus (256) Google Scholar, 31Pohlmann A. Cramm R. Schmelz K. Friedrich B. Mol. Microbiol. 2000; 38: 626-638Crossref PubMed Scopus (98) Google Scholar, 32Hutchings M.I. Mandhana N. Spiro S. J. Bacteriol. 2002; 184: 4640-4643Crossref PubMed Scopus (104) Google Scholar). Recently, Hutchings et al. (32Hutchings M.I. Mandhana N. Spiro S. J. Bacteriol. 2002; 184: 4640-4643Crossref PubMed Scopus (104) Google Scholar) reported NorR-dependent activation of norV transcription by the NO+ donor and NO-generating compound nitroprusside in support of the proposed regulatory function. Interestingly, nitroprusside-elicited norV transcription was increased >5-fold by normoxic O2 suggesting mechanisms for NorR activation involving O2-derived reactive nitrogen intermediates rather than NO per se. The large oxygen enhancement of norV transcription observed with or without nitroprusside exposure has also supported proposals for aerobic functions for the norRVW operon, including O2reduction and the detoxification of O2-derived reactive nitrogen intermediates (25Gomes C.M. Giuffrè A. Forte E. Vicente J.B. Saraiva L.M. Brunori M. Teixeira M. J. Biol. Chem. 2002; 277: 25273-25276Abstract Full Text Full Text PDF PubMed Scopus (177) Google Scholar, 32Hutchings M.I. Mandhana N. Spiro S. J. Bacteriol. 2002; 184: 4640-4643Crossref PubMed Scopus (104) Google Scholar). We now report the rapid and robust induction of norVand norW transcription and NorVW activity by sub-micromolar NO via a NorR- and ς54-dependent mechanism inE. coli. We also show that a deletion within the conserved NorR N-terminal domain activates NorVW expression independent of NO exposure, thus demonstrating the role of the N terminus in NO sensing and signaling. Contrary to the results obtained with nitroprusside (32Hutchings M.I. Mandhana N. Spiro S. J. Bacteriol. 2002; 184: 4640-4643Crossref PubMed Scopus (104) Google Scholar), O2 greatly diminished norV andnorW induction by NO. The results are discussed in light of the proposed NO reduction and detoxification function of thenorRVW operon within the NO defense network. Bovine liver catalase (260,000 units/ml) was purchased from Roche Molecular units/ml) and units/ml) obtained from NO in as (7Gardner P.R. Costantino G. Salzman A.L. J. Biol. Chem. 1998; 273: 26528-26533Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar). of NO in and O2 in obtained from and are in gene of andnorW as (8Gardner A.M. Helmick R.A. Gardner P.R. J. Biol. Chem. 2002; 277: 8172-8177Abstract Full Text Full Text PDF PubMed Scopus (256) Google Scholar). A in the was with and mutants for tetracycline The is a in the and norV in to the from the 2000; PubMed Scopus Google Scholar). a the terminus was as the and the and and was and and the was an of NorR that and a at the A deletion of was by with and coli and in or or B. J. Bacteriol. PubMed Google S. B. J. Bacteriol. PubMed Google from A.M. Helmick R.A. Gardner P.R. J. Biol. Chem. 2002; 277: 8172-8177Abstract Full Text Full Text PDF PubMed Scopus (256) Google A.M. Helmick R.A. Gardner P.R. J. Biol. Chem. 2002; 277: 8172-8177Abstract Full Text Full Text PDF PubMed Scopus (256) Google A.M. Helmick R.A. Gardner P.R. J. Biol. Chem. 2002; 277: 8172-8177Abstract Full Text Full Text PDF PubMed Scopus (256) Google A.M. Gardner P.R. J. Biol. Chem. 2002; 277: 8166-8171Abstract Full Text Full Text PDF PubMed Scopus (147) Google C. C. J. PubMed Scopus Google regulatory and the structural gene a to in gene with internal deletion of in in a at in of with (7Gardner P.R. Costantino G. Salzman A.L. J. Biol. Chem. 1998; 273: 26528-26533Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar). and microaerobic in of in at at and ampicillin as at and growth was monitored by the at and by and of in a was to bacteria per for in and to growth as A.M. Gardner P.R. J. Biol. Chem. 2002; 277: 8166-8171Abstract Full Text Full Text PDF PubMed Scopus (147) Google Scholar). NO at with a NO in the presence or of O2 as (12Gardner P.R. Gardner A.M. Martin L.A. Salzman A.L. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 10378-10383Crossref PubMed Scopus (497) Google Scholar, 20Gardner A.M. Gardner P.R. J. Biol. Chem. 2002; 277: 8166-8171Abstract Full Text Full Text PDF PubMed Scopus (147) Google Scholar). by and in sodium activity was to the of in Molecular Scholar) with the at cells per in sodium and by at for for at in a with of in a a with of is reported in per of per at activity in cells was in anaerobic in aerobic and in aerobic activity in cells of NO exposure and was from the in under growth was with as the Biochem. PubMed Scopus Google Scholar). was the in the norV and norW are in a with the of NorW within the of norV suggesting coordinate transcription and in response to NO stress (8Gardner A.M. Helmick R.A. Gardner P.R. J. Biol. Chem. 2002; 277: 8172-8177Abstract Full Text Full Text PDF PubMed Scopus (256) Google Scholar). In is (8Gardner A.M. Helmick R.A. Gardner P.R. J. Biol. Chem. 2002; 277: 8172-8177Abstract Full Text Full Text PDF PubMed Scopus (256) Google Scholar) and is (32Hutchings M.I. Mandhana N. Spiro S. J. Bacteriol. 2002; 184: 4640-4643Crossref PubMed Scopus (104) Google Scholar). a within and inducible anaerobic NOR activity (8Gardner A.M. Helmick R.A. Gardner P.R. J. Biol. Chem. 2002; 277: 8172-8177Abstract Full Text Full Text PDF PubMed Scopus (256) Google Scholar) was to the of to NO. Exposure of anaerobic to NO in induced activity by within expression of exposure in induction A in norV transcription was observed with NO in induction of activity was observed with NO in was with NO exposure suggesting toxicity of NO under these NO induced in under anaerobic conditions However, the was induced to a than that observed for the a NO of and in a The response of to NO by the of NOR activity from within (8Gardner A.M. Helmick R.A. Gardner P.R. J. Biol. Chem. 2002; 277: 8172-8177Abstract Full Text Full Text PDF PubMed Scopus (256) Google Scholar), thus in a NO It is also that NO levels are required to maximal norW the results demonstrate a and coordinate of norV and in response to levels of NO. The results in demonstrate a of transcription with results obtained nitroprusside as the (32Hutchings M.I. Mandhana N. Spiro S. J. Bacteriol. 2002; 184: 4640-4643Crossref PubMed Scopus (104) Google Scholar). NO-mediated induction of was inhibited by conditions induction for and with in the expression levels a O2 and induction and norV andnorW induction in the presence of O2 can by the in NO levels by the inducible NOD expression by in cells to an NO in O2 for and control and norV and norW are induced under conditions in the O2-sensitive NOR functions (8Gardner A.M. Helmick R.A. Gardner P.R. J. Biol. Chem. 2002; 277: 8172-8177Abstract Full Text Full Text PDF PubMed Scopus (256) Google Scholar, 20Gardner A.M. Gardner P.R. J. Biol. Chem. 2002; 277: 8166-8171Abstract Full Text Full Text PDF PubMed Scopus (147) Google Scholar), and NOD regulates NOR expression. The domain of the tripartite NorR is with ς54-dependent response regulators (31Pohlmann A. Cramm R. Schmelz K. Friedrich B. Mol. Microbiol. 2000; 38: 626-638Crossref PubMed Scopus (98) Google Scholar, T.M. Jr., PubMed Scopus Google Scholar) thus suggesting an role for ς54 in the NO Furthermore, the upstream of the the and and of ς54-dependent L. Microbiol. Mol. Biol. Rev. 2001; PubMed Scopus Google Scholar, H. B. E. 1999; PubMed Scopus Google Scholar). We mutants to the role of ς54 in norV transcription and NOR activity expression. activity was in and a exposure to NO under microaerobic In the of expression was impaired The and to NO under O2 and for anaerobic NOR and aerobic NOD NOR activity was in was of ς54 NOD expression under these conditions The results demonstrate a role for ς54 in norV The induction of norV transcription and NOR activity in the of ς54 for in norV transcription or mechanisms for of NorR from a the NO inducibility of NOR activity in the deletion thus regulatory role of NorR in the activation ofnorVW transcription and NOR activity expression (8Gardner A.M. Helmick R.A. Gardner P.R. J. Biol. Chem. 2002; 277: 8172-8177Abstract Full Text Full Text PDF PubMed Scopus (256) Google Scholar, 32Hutchings M.I. Mandhana N. Spiro S. J. Bacteriol. 2002; 184: 4640-4643Crossref PubMed Scopus (104) Google Scholar). In the of was NOR activity that of NorR not by NorVW expression. However, internal deletion of the domain (31Pohlmann A. Cramm R. Schmelz K. Friedrich B. Mol. Microbiol. 2000; 38: 626-638Crossref PubMed Scopus (98) Google Scholar, T.M. Jr., PubMed Scopus Google Scholar), the domain and the induced NOR activity in the of NO deletion of NorR to part of the not activity not thus the for ς54 with NorR for Interestingly, the NO-mediated induction of NOD activity was in expressing NorR and elevated NOR activity thus suggesting an role for NorR and NOR in NOD expression by NO levels. results demonstrate the function of the N-terminal domain of the E. coli transcription regulator NorR to that for tripartite regulators (31Pohlmann A. Cramm R. Schmelz K. Friedrich B. Mol. Microbiol. 2000; 38: 626-638Crossref PubMed Scopus (98) Google Scholar, A.M. Rev. Biochem. 2000; PubMed Scopus Google Scholar). In the results demonstrate that NorR nor ς54 is in the NO-mediated of the E. coli NOD demonstrate that the exposure of E. coli to NO induces transcription of the norV and norW genes via a NorR and ς54-dependent The the results of Hutchings et al. (32Hutchings M.I. Mandhana N. Spiro S. J. Bacteriol. 2002; 184: 4640-4643Crossref PubMed Scopus (104) Google Scholar) demonstrating activation of norV transcription by the NO+ donor nitroprusside, or nitrate in a NorR-dependent the of NO required for anaerobic norV induction NO is the physiological signal NorR and results from of Hutchings et al. (32Hutchings M.I. Mandhana N. Spiro S. J. Bacteriol. 2002; 184: 4640-4643Crossref PubMed Scopus (104) Google Scholar) reported that and induced norV transcription was in the presence of for the is that NO and Hutchings et al. (32Hutchings M.I. Mandhana N. Spiro S. J. Bacteriol. 2002; 184: 4640-4643Crossref PubMed Scopus (104) Google Scholar) nitroprusside as a NO+ donor and NO-generating have transcription NO at levels in aerobic NO is and is for of the of NO NO defense gene The of NO for the of gene expression also of the of multiple for rapid and inducible NO and of the incipient toxicity of NO. the that the NO levels required for norV induction with levels to the proposed role of the norRVW operon in NO reduction and detoxification. NO in E. coli and and inhibited growth in the of the induced NorVW activity (6Gardner P.R. Costantino G. Szabó C. Salzman A.L. J. Biol. Chem. 1997; 272: 25071-25076Abstract Full Text Full Text PDF PubMed Scopus (210) Google Scholar, 8Gardner A.M. Helmick R.A. Gardner P.R. J. Biol. Chem. 2002; 277: 8172-8177Abstract Full Text Full Text PDF PubMed Scopus (256) Google Scholar). Furthermore, the of NO norV transcription the (NO) of for NO reduction (8Gardner A.M. Helmick R.A. Gardner P.R. J. Biol. Chem. 2002; 277: 8172-8177Abstract Full Text Full Text PDF PubMed Scopus (256) Google Scholar). results the of a function of the operon in O2 detoxification or in the detoxification of reactive nitrogen intermediates from nitroprusside or NO exposure as (25Gomes C.M. Giuffrè A. Forte E. Vicente J.B. Saraiva L.M. Brunori M. Teixeira M. J. Biol. Chem. 2002; 277: 25273-25276Abstract Full Text Full Text PDF PubMed Scopus (177) Google Scholar, 32Hutchings M.I. Mandhana N. Spiro S. J. Bacteriol. 2002; 184: 4640-4643Crossref PubMed Scopus (104) Google Scholar). A of with the N-terminal of NorR NorR orthologues in E. NorR orthologues in are upstream of Interestingly, NorR orthologues in are from genes suggesting a role for NorR in NODs in response to NO. In is that NorR was not required for the induction of NOD activity in response to NO in E. coli thus demonstrating the of or of in E. coli. NorR to the of response regulators A.M. Rev. Biochem. 2000; PubMed Scopus Google Scholar). to tripartite regulators in deletion of the N-terminal or domain of NorR activated NorVW expression independent of NO Furthermore, conserved in the NorR N-terminal domain the for by a sensor to that for the B. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). In and are in to and the conserved at to J.B. A.J. A.M. Microbiol. Rev. PubMed Google Scholar). the NorR N-terminal domain transcription by with a signal as for N. Dixon R.A. Mol. Gen. Genet. Scopus Google Scholar) or by NO as the transcription regulator S. A. J. Bacteriol. PubMed Google Scholar). The NorR N-terminal domain and that the NO sensor NorR an of the in the of Nat. Struct. Biol. 2000; 7: PubMed Scopus Google Scholar). of the NO defense in E. coli. NO exposure the of enzymes, NOD and NOR (NorVW) by transcription of genes, The of to NO detoxification the of NOD is under aerobic and microaerobic conditions A.M. Martin L.A. Gardner P.R. Dou Y. Olson J.S. J. Biol. Chem. 2000; 275: 12581-12589Abstract Full Text Full Text PDF PubMed Scopus (139) Google Scholar, 14Gardner P.R. Gardner A.M. Martin L.A. Dou Y. Li T. Olson J.S. Zhu H. Riggs A.F. J. Biol. Chem. 2000; 275: 31581-31587Abstract Full Text Full Text PDF PubMed Scopus (133) Google Scholar). The NOR activity of NorVW is in that to O2 NO function to anaerobic or microaerobic conditions (8Gardner A.M. Helmick R.A. Gardner P.R. J. Biol. Chem. 2002; 277: 8172-8177Abstract Full Text Full Text PDF PubMed Scopus (256) Google Scholar, 20Gardner A.M. Gardner P.R. J. Biol. Chem. 2002; 277: 8166-8171Abstract Full Text Full Text PDF PubMed Scopus (147) Google Scholar). The results also support a in and are by O2 O2 levels NorVW and NOD NO levels and the activity of transcription regulators as NorR and Fnr (29Cruz-Ramos H. Crack J. Wu G. Hughes M.N. Scott C. Thomson A.J. Green J. Poole R.K. EMBO J. 2002; 21: 3235-3244Crossref PubMed Scopus (253) Google Scholar, R.K. J. Kim S.O. Hughes M.N. J. Bacteriol. 1996; PubMed Scopus Google Scholar). Interestingly, nor have proposed to critical NO stress response B. Mol. Biochem. 2002; PubMed Scopus Google Scholar, S.O. K. R. Jr., T. J. Hausladen A. Stamler J.S. 2002; Full Text Full Text PDF PubMed Scopus Google Scholar) to in the of in E. coli R.K. J. Kim S.O. Hughes M.N. J. Bacteriol. 1996; PubMed Scopus Google Scholar). M. Gardner and P. R. to the diverse and mechanisms of NO defense genes, enzymes, and regulators in microbial to vitro and in of We and for and in these