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Azoreductases are a family of diverse enzymes found in many pathogenic bacteria as well as distant homologues being present in eukarya. In addition to having azoreductase activity these enzymes are also suggested to have NAD(P)H quinone oxidoreductase activity which leads to a proposed role in plant pathogenesis. Azoreductases have also been suggested to play role in the mammalian pathogenesis of Pseudomonas aeruginosa. In view of the importance of P. aeruginosa as a pathogen, we therefore characterised recombinant enzymes following expression of a group of putative azoreductase genes from P. aeruginosa expressed in Escherichia coli . The enzymes include members of the "Arsenic resistance protein H" (ArsH), "tryptophan repressor binding protein A" (WrbA), "modulator of drug activity B" (MdaB) and YieF families. The ArsH, MdaB and YieF family members all show azoreductase and NAD(P)H quinone oxidoreductase activities. In contrast, WrbA is the first enzyme to show NAD(P)H quinone oxidoreductase activity but does not reduce any of the 11 azo compounds tested under a wide range of conditions. These studies will allow further investigation of the possible role of these enzymes in the pathogenesis of P. aeruginosa .

'Psedomonas aeruginosa' is a prevalent nosocmial pathogen predominantly associated with infections in immune compromised individuals and long term colonisation and pathogenesis in the lungs of Cystic Fibrosis patients. With multi-drug resistant strains increasingly common, the discovery of novel targets for antimicrobial chemotherapy is of utmost importance and expansion of data on 'P. aeruginosa's' complex genome could facilitate this. Azoreductases are a group of enzymes mainly noted for their reductive capacity against azo and quinone compounds. Ubiquitous amongst many classes of organism including prokaryotes and eukaryotes, the primary physiological role of azoreductases remians unclear. This study characterises azoreductase-like enzymes from 'P. aeruginosa' in terms of biochemical properties, substrate specificity and structural analysis. The effect of these enzymes on bacterial physiology in 'P. aeruginosa' is also explored in relation to antibiotic susceptibility.\ud \ud Three azoreductase-like genes from 'P. aeruginosa' (pa1224, pa1225 and pa4975) were overexpressed in 'E. coli' strains following molecular cloning. Recombinant proteins were biochemically characterised by means of Thin Layer Chromatography, Differential Scanning Fluorimetry and ezymatic assays. All enzymes were noted to be selective for FAD as the flavin cofactor and NADPH as the preferred reductant. All three enzymes were confirmed as NAD(P)H dependent quinone oxidoreductases (NQOs) with PA1224 also catalysing reduction of the azo substrate methyl red, albeit at a rate an order of magnitude lower than that observed for the quinone compounds. The preferred flavin cofactor for four previously characterised azoreductase and NQO enzymes (PA2280, PA2580, PA1204 and PA0949) was also explored and PA2280 and PA0949 were observed to select for FMN while PA2580 and PA1204 were selective for FAD. The crystal structure of PA2580 was solved with the nicotinamide group of NADPH bound and was noted to form a homodimer with the same short flavodoxin-like fold as previously described for other members of this enzyme family. Complemented strains of azoreductase-like gene deletion mutants of 'P. aeruginosa' PAO1 were generated via molecular cloning and used to monitor the effects of these enzymes on antibiotic susceptibility. Antimicrobial sensitivity assays were carried out and although the knockout strains displayed increased sensitivity to fluroquinolones, they did not revert to the wild type phenotype upon reinsertion of the genes of interest.\ud \ud This study has for the first time characterised three new NQO's from 'P. aeruginosa' PAO1 and solved the crystal structure of an azoreductase/NQO with nicotinamide bound. With these findings and a library of complemented strains generated, this original study offers a platform for the continued research into the physiological role of these enzymes.