The flavohemoglobin of Escherichia coli confers resistance to a nitrosating agent, a "nitric oxide releaser," and paraquat and is essential for transcriptional responses to oxidative stress

Escherichia coli possesses a flavohemoglobin (Hmp), product of hmp, the fir st microbial globin gene to be sequenced and characterized at the molecular level. Although related proteins occur in numerous prokaryotes and eukaryo tic microorganisms, the function(s) of these proteins have been elusive. He re we report construction of a defined hmp mutation and its use to probe Hm p function. As anticipated from up-regulation of hmp expression by nitric o xide (NO), S-nitrosoglutathione (GSNO) or sodium nitroprusside (SNP), the h mp mutant is hypersensitive to these agents. The hmp promoter is more sensi tive to SNP and S-nitroso-N-penicillamine (SNAP) than is the soxS promoter, consistent with the role of Hmp in protection from reactive nitrogen speci es. Additional functions for Hmp are indicated by (a) parallel sensitivity of the hmp mutant to the redox-cycling agent, paraquat, (b) inability of th e mutant to upregulate fully the soxS and sodA promoters in response to oxi dative stress caused by paraquat, GSNO and SNP, and (c) failure of the muta nt to accumulate reduced paraquat radical after anoxic growth. We conclude that Hmp plays a role in protection from nitrosating agents and NO-related species and oxidative stress. This protective role probably involves direct detoxification of those species and sensing of NO-related and oxidative st ress.