The enteric bacterium \(Escherichia\) \(coli\) is exposed to nitric oxide (NO) in its oxygen-limited environment. Various transcription factors regulate gene expression to provide protection against nitrosative stress, but their respective roles remain controversial. Key questions answered in this thesis were whether S-nitrosylated OxyR directly regulates the expression of NO-regulated genes; whether NsrR is required for the synthesis of an important protective system; and whether FNR is a physiologically relevant sensor of environmental NO. Transcription from the NO-activated \(hcp\) promoter was almost totally dependent upon a functional FNR protein, which is inactivated by oxidative stress, but unaffected by deletion of the \(oxyR\) gene. This indicated that the effects of an \(oxyR\) mutation on resistance to nitrosative stress are indirectly due to inactivation of FNR rather than to direct activation of \(hcp\) transcription by S-nitrosylated OxyR. The NO-sensitive repressor, NsrR, is not essential for protection against nitrosative stress. Nitric oxide produced during nitrite reduction had no effect on the ability of FNR to activate or repress gene expression, which is primarily due to NO sensing by NsrR, not by FNR. Some NO accumulates in the \(E. coli\) cytoplasm even in mutants that lack known sources of NO. The source of most of this residual NO is the NsrR-regulated protein, YtfE. Contrary to the proposal by others that YtfE repairs iron-sulfur centres by replacing iron atoms released during nitrosative stress, it is proposed that YtfE releases NO from nitrosylated iron-sulfur proteins, and that Hcp reduces this NO to the less toxic N2O.