ABSTRACT
Microarray studies of the Escherichia coli response to nitric oxide and nitrosative stress have suggested that additional transcriptional regulators of this response remain to be characterized. We identify here the product of the yjeB gene as a negative regulator of the transcription of the ytfE, hmpA and ygbA genes, all of which are known to be upregulated by nitrosative stress. Transcriptional fusions to the promoters of these genes were expressed constitutively in a yjeB mutant, indicating that all three are targets for repression by YjeB. An inverted repeat sequence that overlaps the −10 element of all three promoters is proposed to be a binding site for the YjeB protein. A similar inverted repeat sequence was identified in the tehA promoter, which is also known to be sensitive to nitrosative stress. The ytfE, hmpA, ygbA, and tehA promoters all caused derepression of a ytfE-lacZ transcriptional fusion when present in the cell in multiple copies, presumably by a repressor titration effect, suggesting the presence of functional YjeB binding sites in these promoters. However, YjeB regulation of tehA was weak, as judged by the activity of a tehA-lacZ fusion, perhaps because YjeB repression of tehA is masked by other regulatory mechanisms. Promoters regulated by YjeB could be derepressed by iron limitation, which is consistent with an iron requirement for YjeB activity. The YjeB protein is a member of the Rrf2 family of transcriptional repressors and shares three conserved cysteine residues with its closest relatives. We propose a regulatory model in which the YjeB repressor is directly sensitive to nitrosative stress. On the basis of similarity to the nitrite-responsive repressor NsrR from Nitrosomonas europaea, we propose that the yjeB gene of E. coli be renamed nsrR.
Nucleotide sequences of the fecBCDE genes and locations of the proteins suggest a periplasmic-binding-protein-dependent transport mechanism for iron(III) dicitrate in Escherichia coli.
