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.