Studies on DNA gyrase and quinolone drugs
A study has been conducted aimed at the generation and characterisation of mutations in the Escherichia coli gyrA gene, resistant to the quinolone group of antibacterial agents. Preliminary studies on quinolone-resistant mutants of strains that over-express the DNA gyrA gene, revealed the over-production of a 60 KDa protein which was partially purified. This 60 KDa protein was found to be similar, but not identical to the E. coli heat shock protein GroEL. The gyrA gene has been recloned in the 8.0 kb plasmid pPH3, which contains the gene under the stringent control of the hybrid tac promoter. The E. coli strain JMtacA containing pPH3 exhibits no expression of the gyrA gene in the absence of the inducer (IPTG), but over-produces the protein at greater than 20% of the total soluble cell protein after induction. The optimisation and purification of the GyrA subunit from JMtacA is also described. A fragment was subjected to site-directed mutagenesis which contained the TCG codon for serine-83, which was mutated to alanine (GCG). The mutant showed a 15x increase in MIC50 compared to wild-type. The mutated GyrA subunit was over-produced, complexed with wild-type GyrB subunit and used in various assays for reactions performed by DNA gyrase. The ID50 was determined for supercoiling, decatenation, relaxation of negatively supercoiled DNA, and cleavage of supercoiled DNA. The cleavage reaction mediated by Ca++ was also investigated. The technique of gap-misrepair mutagenesis, geared to the generation of single, random point mutations on a plasmid was also used on the plasmid pPH3, to generate a quinolone-resistant mutant of the gyrA gene. The mutant isolated (GMIOO) was also over-produced and compared to wild-type in various assays. The mutation was determined by DNA sequencing to be glutamine-106 to arginine.