The small FNR regulon of Neisseria gonorrhoeae: comparison with the larger Escherichia coli FNR regulon and interaction with the NarQ-NarP regulon

Microarray experiments show that N. gonorrhoeae FNR controls a much smaller regulon than its E. coli counterpart; it activates transcription of aniA and thirteen other genes, and represses transcription of six genes that include dnrN and norB. Having previously shown that a single amino acid substitution is sufficient to enable the gonococcal FNR to complement an E. coli fnr mutation, we investigated whether the gonococcal NarQ-NarP can substitute for E. coli NarX-NarL or NarQ-NarP. A plasmid expressing gonococcal narQ-narP was unable to complement E. coli narQP or narXL mutants, and was insensitive to nitrate or nitrite. Mutations that progressively changed the periplasmic nitrate sensing region, the P box, of E. coli NarQ to the sequence of the corresponding region of gonococcal NarQ resulted in loss of transcription activation in response to the availability of either nitrate or nitrite. However, the previously reported ligand-insensitive ability of gonococcal NarQ, the "locked on" phenotype, to activate either E. coli NarL or NarP was confirmed.Despite the sequence similarities between transcription activators of E. coli and N. gonorrhoeae, these results emphasise the fundamental differences in transcription regulation between these two types of pathogenic bacteria.Neisseria gonorrhoeae is an obligate human pathogen with no known environmental reservoirs. It can be isolated from gonorrhoea patients in clinical samples in which obligately anaerobic bacteria are abundant [1,2], suggesting that gonococci encounter and survive without oxygen in their natural habitat. Clark and her colleagues have shown that gonococci can grow anaerobically using a truncated denitrification pathway in which nitrite is reduced to nitrous oxide, catalysed by the copper-containing nitrite reductase, AniA, and the single subunit nitric oxide reductase, NorB [2-5]. Nitrite reduction is severely repressed by oxygen, but is induced during anaerobic growth by the global transcription factor,