GyrAB

GyrAB refers to the bacterial DNA gyrase enzyme complex, a type II topoisomerase that introduces and maintains negative supercoils in chromosomal DNA. The functional enzyme is a heterotetramer consisting of two GyrA and two GyrB subunits (GyrA2GyrB2). The GyrA subunits form the DNA breakage–reunion core and contain the active-site tyrosine that forms a transient covalent linkage with DNA during strand passage. The GyrB subunits harbor the ATPase activity in their N-terminal domain and provide the energy required for the strand passage cycle through ATP hydrolysis. The two subunits cooperate to capture a DNA segment, cleave one duplex, pass another duplex through the break, and reseal the break, using the energy from ATP hydrolysis to drive the conformational changes.

Genetic encoding and regulation: gyrA and gyrB genes are essential in many bacteria and are tightly coordinated to maintain proper subunit stoichiometry for activity. Distribution is wide among bacterial species, with variations in regulatory control and antibiotic susceptibility.

Clinical relevance: GyrA and GyrB are primary targets of fluoroquinolone antibiotics, which stabilize the transient DNA–enzyme complex and induce lethal DNA damage. Resistance can arise through mutations in the gyrA or gyrB quinolone resistance-determining regions, as well as through plasmid-mediated mechanisms and efflux.

Evolution and structure: GyrA and GyrB are conserved components of type II topoisomerases, with structural and functional parallels to related bacterial enzymes. Structural studies have illuminated how the GyrA–GyrB interface coordinates DNA cleavage, passage, and resealing during the catalytic cycle.