The Universal Stress Proteins (USPs) are an ancient group of proteins that mediate biotic and abiotic stress responses across the tree of life. Some USPs have been observed co-crystalising with ATP, AMP or cAMP, but the significance of the USP fold and ligand interactions is currently unknown. Inspired by phenotypic data from the bacterium Pseudomonas aeruginosa, we are investigating the nucleotide binding behaviour and mutant phenotypes of USPs in P. aeruginosa. P. aeruginosa is a major human pathogen, infecting the mucosal fluid of patients with incurable respiratory illnesses such as cystic fibrosis. This bacterium is resistant to medical antibiotics and requirements ongoing discovery for drug targeting.
UspN (PA4352) mutants in P. aeruginosa PA14 fail to persist in stationary phase cultures under anoxic conditions. This phenotype however can be rescued by the presence of nitrate, which facilitates bacterial anaerobic respiration by providing alternate terminal electron acceptors and is crucial to biofilm metabolism. UspN mutants also appear to be highly susceptible to proton-motive force dissipators. This suggests that UspN is involved in starvation or anaerobic signalling. The stringent response, mediated largely by the nucleotide guanosine tetraphosphate, has also been linked to UspN expression.
UspN, a tandem monomer consisting of two canonical USP domains, appears to have a conserved G-(2X)-G-(9X)-G(S/T) nucleotide binding motif sequence and we have attempted to produce ligand-UspN crystals. We suggest that a higher oligomeric organisation underlies UspN function in vivo, and that a range of nucleotide binding partners may be involved.
In a multi-disciplinary approach, we present novel structural, biochemical, and phenotypic data concerning the USPs of P. aeruginosa. In combination, these data offer advancements to various theories of USP function and bacterial metabolism. This understanding may yet have important applications for the treatment of Pseudomonas aeruginosa and drug targeting.