B-258. Identification and Characterization of Moraxella catarrhalis Gene Products Involved in a Truncated Denitrification Pathway

W. Wang1, A. R. Richardson2, F. C. Fang2, E. J. Hansen1;
1Univ. of Texas Southwestern Med. Ctr, Dallas, TX, 2Univ. of Washington, Seattle, WA.

A previous study on the effect of growth in a biofilm on global gene expression by M. catarrhalis ATCC 43617 revealed that the expression of numerous different genes was significantly up-regulated. Among these up-regulated genes were several encoding proteins predicted to be involved in the reduction of nitrogenous compounds, including nitrate reductase (narGHJI), nitrite reductase (aniA), and nitric oxide reductase (norB). These three enzymes would constitute a truncated denitrification pathway. An ORF located between aniA and norB was predicted to encode a putative transcriptional regulatory protein which was similar (35% identity) to the NsrR repressor protein of Neisseria meningitidis. Inactivation of this nsrR homolog in M. catarrhalis strain O35E resulted in up-regulation of both aniA and norB expression as measured by real-time RT-PCR. The presence of a low level of nitrite in the growth medium also resulted in increased expression of both aniA and norB by the wild-type O35E strain and did not affect growth in broth. In contrast, the growth of a M. catarrhalis O35E nsrR mutant was completely inhibited by this same low level of nitrite, and this inhibitory effect could be reversed by complementation with a wild-type nsrR gene in trans. When nitric oxide production in the presence of nitrite was monitored, the level of nitric oxide detectable in wild-type broth cultures was much greater than that detected in cultures of the aniA or nsrR mutants. Whether nitric oxide or nitrous oxide, or both, were responsible for the observed inhibition of growth of the nsrR mutant by nitrite remains to be determined. It is hypothesized that expression of AniA and NorB is kept at a relatively low level by NsrR to allow growth of M. catarrhalis in the presence of low levels of nitrite.