GyrB

GyrB is the B subunit of bacterial DNA gyrase, a type II topoisomerase responsible for introducing negative supercoils into double-stranded DNA. In bacteria, DNA gyrase is a heterotetramer composed of two GyrA and two GyrB subunits (A2B2). The gyrB gene encodes the GyrB protein, which is essential for cell viability in most bacteria and is broadly conserved among bacteria and some plastids.

GyrB contains an N-terminal ATPase domain that belongs to the Bergerat fold of ATPases, which hydrolyzes ATP to fuel the enzyme's strand passage cycle. A C-terminal region contributes to DNA binding and regulation of activity, and the two GyrB subunits cooperate with GyrA to capture, gate, and introduce DNA supercoils.

Mechanism: GyrB ATP hydrolysis drives conformational changes that power the passage of a DNA segment through an enzyme-bridged break in another segment, introduced by GyrA, resulting in negative supercoiling of the genome. The ATPase activity is modulated by GyrA, DNA, and antibiotics.

Clinical significance: DNA gyrase is the primary antibacterial target of fluoroquinolones, which stabilize the gyrase-DNA complex and cause lethal double-strand breaks. Novobiocin and other coumarins inhibit GyrB ATPase activity. Resistance often arises through mutations in gyrB or gyrA, altered drug targets, or efflux.

Taxonomy and evolution: gyrB is widely distributed in bacteria; the protein shows conserved motifs in the ATPase domain. Structural and biochemical studies have advanced understanding of the enzyme’s mechanism. In humans, there is no close ortholog of bacterial gyrase, making GyrB an attractive antibiotic target.