The nitric oxide (NO)-sensing repressor NsrR of Neisseria meningitidis has a compact regulon of genes involved in NO synthesis and detoxification

Metalloregulatory proteins and nitric oxide signalling in bacteria

Bacterial gene regulators containing transition metal cofactors that function as binding sites for small ligands were first described in the 1990s. Since then, numerous metal-containing regulators have been discovered, and our knowledge of the diversity of proteins, their cofactors and the signals that they sense has greatly increased. The present article reviews recent developments, with a particular focus on bacterial sensors of nitric oxide.

Identification of a repressor of a truncated denitrification pathway in Moraxella catarrhalis

Growth of Moraxella catarrhalis in a biofilm resulted in marked upregulation of two open reading frames (ORFs), aniA and norB, predicted to encode a nitrite reductase and a nitric oxide reductase, respectively (W. Wang, L. Reitzer, D. A. Rasko, M. M. Pearson, R. J. Blick, C. Laurence, and E. J. Hansen, Infect. Immun. 75:4959-4971, 2007). An ORF designated nsrR, which was located between aniA and norB, was shown to encode a predicted transcriptional regulator. Inactivation of nsrR resulted in increased expression of aniA and norB in three different M. catarrhalis strains, as measured by both DNA microarray analysis and quantitative reverse transcriptase PCR. Provision of a wild-type nsrR gene in trans in an nsrR mutant resulted in decreased expression of the AniA protein. DNA microarray analysis revealed that two other ORFs (MC ORF 683 and MC ORF 1550) were also consistently upregulated in an nsrR mutant. Consumption of both nitrite and nitric oxide occurred more rapidly with cells of an nsrR mutant than with wild-type cells. However, growth of nsrR mutants was completely inhibited by a low level of sodium nitrite. This inhibition of growth by nitrite was significantly reversed by introduction of an aniA mutation into the nsrR mutant and was completely reversed by the presence of a wild-type nsrR gene in trans. NsrR regulation of the expression of aniA was sensitive to nitrite, whereas NsrR regulation of norB was sensitive to nitric oxide.

Escherichia coli NsrR regulates a pathway for the oxidation of 3-nitrotyramine to 4-hydroxy-3-nitrophenylacetate

Chromatin immunoprecipitation and microarray (ChIP-chip) analysis showed that the nitric oxide (NO)-sensitive repressor NsrR from Escherichia coli binds in vivo to the promoters of the tynA and feaB genes. These genes encode the first two enzymes of a pathway that is required for the catabolism of phenylethylamine (PEA) and its hydroxylated derivatives tyramine and dopamine. Deletion of nsrR caused small increases in the activities of the tynA and feaB promoters in cultures grown on PEA. Overexpression of nsrR severely retarded growth on PEA and caused a marked repression of the tynA and feaB promoters. Both the growth defect and the promoter repression were reversed in the presence of a source of NO. These results are consistent with NsrR mediating repression of the tynA and feaB genes by binding (in an NO-sensitive fashion) to the sites identified by ChIP-chip. E. coli was shown to use 3-nitrotyramine as a nitrogen source for growth, conditions which partially induce the tynA and feaB promoters. Mutation of tynA (but not feaB) prevented growth on 3-nitrotyramine. Growth yields, mutant phenotypes, and analyses of culture supernatants suggested that 3-nitrotyramine is oxidized to 4-hydroxy-3-nitrophenylacetate, with growth occurring at the expense of the amino group of 3-nitrotyramine. Accordingly, enzyme assays showed that 3-nitrotyramine and its oxidation product (4-hydroxy-3-nitrophenylacetaldehyde) could be oxidized by the enzymes encoded by tynA and feaB, respectively. The results suggest that an additional physiological role of the PEA catabolic pathway is to metabolize nitroaromatic compounds that may accumulate in cells exposed to NO.