DNA Gyrase

DNA gyrase is an essential prokaryotic Type IIA topoisomerase discovered in Escherichia coli in 1976 by Martin Gellert, Kiyoshi Mizuuchi, Howard Nash, and Manley Friedman. Assembled as a ~370-kDa A2B2 heterotetramer composed of two GyrA subunits (~97 kDa each, encoded by gyrA) and two GyrB subunits (~90 kDa each, encoded by gyrB), DNA gyrase is unique among all known topoisomerases in its ability to catalyze the active introduction of negative supercoils (−ΔLk) into relaxed closed circular DNA at the thermodynamic expense of ATP hydrolysis.

In bacterial cells, chromosome compaction and the unwinding of duplex templates ahead of advancing DNA replication forks and transcription complexes generate positive superhelical tension (+ΔLk) that would rapidly arrest replisome and RNA polymerase progression if left unrelaxed. DNA gyrase resolves this topological barrier by wrapping ~130–140 base pairs of DNA in a positive right-handed superhelical node around its C-terminal domains, cleaving both strands of the bound gate segment (G-segment), passing a second double-stranded transport segment (T-segment) through the transient 4-bp staggered break, and resealing the phosphodiester backbone—changing the topological linking number (Lk) in strictly quantized steps of ΔLk = −2 per catalytic cycle. Because human and mammalian cells lack DNA gyrase (relying instead on homodimeric Type IIA topoisomerases Topo IIα and Topo IIβ, which only relax supercoils without introducing negative superhelicity), both GyrA and GyrB serve as premier bactericidal targets in drug discovery and antimicrobial resistance surveillance.

Domain Organization of GyrA and GyrB

X-ray crystal and cryo-electron microscopy structures of intact bacterial A2B2 holoenzymes (Escherichia coli, Mycobacterium tuberculosis, and Thermus thermophilus) reveal a symmetrical molecular machine containing three sequential protein interfaces termed the N-gate, the DNA-gate, and the C-gate:

1. The GyrB Subunit (ATPase and TOPRIM Engine):

• N-terminal GHKL ATPase Domain (residues 2–220): Belongs to the Gyrase, Hsp90, histidine Kinase, MutL superfamily; binds and hydrolyzes two molecules of ATP and dimerizes upon nucleotide binding to close the upper N-gate.
• Transducer Domain (residues 221–392): Transmits conformational changes from ATP hydrolysis downward to the cleavage core.
• TOPRIM (Topoisomerase–Primase) Domain (residues 415–804): Coordinates two catalytic divalent magnesium ions (Mg²⁺) via an acidic triad (Glu424, Asp498, Asp500 in E. coli) adjacent to the GyrA active-site tyrosine.

1. The GyrA Subunit (DNA Cleavage, Reunion, and Chiral Wrapping):

• N-terminal Breakage–Reunion Domain (WHDs + Tower + Coiled-Coil): Contains the winged-helix domain (WHD) carrying the catalytic nucleophile Tyr122 (E. coli numbering), which forms the central DNA-gate together with the GyrB TOPRIM domain, and a long α-helical coiled-coil terminating in the lower C-gate.
• C-terminal Domain (CTD β-Pinwheel): Folds into a six-bladed positively charged β-pinwheel characterized by the conserved GyrA-box (QRRGGKG) motif on blade 1. This β-pinwheel wraps ~130 bp of DNA in a chiral right-handed superhelical turn (+1 node), directing the contiguous T-segment directly into the open N-gate above the bound G-segment.

The Two-Gate Strand-Passage Catalytic Cycle (ΔLk = −2)

During each catalytic turnover, the A2B2 heterotetramer binds a duplex G-segment across the central DNA-gate and wraps the contiguous DNA around one GyrA β-pinwheel CTD to present a T-segment in a positive (+) crossover geometry inside the upper cavity. Binding of two ATP molecules to the GyrB GHKL domains triggers dimerization of the N-gate, trapping the T-segment.

Coordinated by the two TOPRIM-bound Mg²⁺ ions (a two-metal-ion mechanism), the phenolic hydroxyl group of Tyr122 on each GyrA monomer executes a nucleophilic transesterification attack on the target phosphodiester backbone, generating a 4-bp staggered double-strand break with each Tyr122 covalently linked via a 5′-phosphotyrosyl ester bond to the cleaved 5′ DNA overhangs (while leaving free 3′-OH termini). The DNA-gate widens by ~20 Å, allowing the trapped T-segment to pass downward through the cleaved G-segment into the lower cavity, converting the +1 chiral node into a −1 negative node (ΔLk = −2). Because transesterification conserves bond enthalpy (ΔG° ≈ 0), the free 3′-OH groups attack the 5′-phosphotyrosyl linkages to reseal the G-segment without consuming ATP; instead, hydrolysis of the two bound ATP molecules drives unidirectional transport and resets the C-gate and N-gate for the next supercoiling cycle.

Fluoroquinolone Cleavage-Complex Stabilization and QRDR Mutations

Synthetic fluoroquinolone antibiotics (ciprofloxacin, levofloxacin, moxifloxacin, and delafloxacin) are bactericidal topoisomerase poisons rather than simple catalytic inhibitors. After GyrA Tyr122 cleaves the G-segment, two fluoroquinolone molecules intercalate into the 4-bp staggered DNA nicks (one drug molecule per cleaved strand), stacking via π–π interactions between the base pairs flanking the scissile phosphate and physically blocking the 3′-OH terminus from attacking the 5′-phosphotyrosyl bond to religate the DNA. When a replication fork or transcription complex collides with this ternary drug–gyrase–DNA cleavage complex, the reversible covalent intermediate is sheared into lethal, irreversible double-strand chromosome breaks.

X-ray structures solved by Pan Chan, Mark Sanderson, James Berger, and Neil Osheroff established that the C3/C4 keto-acid moiety of the fluoroquinolone chelates a non-catalytic Mg²⁺ ion, which is anchored through four coordinated water molecules to two conserved residues inside the Quinolone Resistance-Determining Region (QRDR, residues 67–106 of GyrA): Ser83 and Asp87 in Escherichia coli (corresponding to Ser84 and Glu88 in Staphylococcus aureus, and Ala90 and Asp94 in Mycobacterium tuberculosis). Single missense mutation at Ser83Leu or Asp87Asn/Tyr disrupts this water–metal ion bridge, reducing fluoroquinolone binding affinity 10-fold to 40-fold while leaving catalytic Tyr122 supercoiling intact—representing the primary chromosomal mechanism of clinical fluoroquinolone resistance, alongside GyrB QRDR mutations (Asp426Asn, Lys447Glu) and plasmid-mediated pentapeptide repeat protection proteins (QnrA, QnrB, QnrS).

Aminocoumarin ATPase Inhibition and Self-Resistance Paralogs (gyrB2)

In contrast to fluoroquinolones, aminocoumarin natural products—exemplified by novobiocin, clorobiocin, and coumermycin A₁, produced by soil actinomycetes (Streptomyces niveus and Streptomyces roseochromogenes)—target the N-terminal GHKL ATP-binding pocket of GyrB. The 3-amino-4,7-dihydroxycoumarin and substituted noviose sugar moieties of novobiocin overlap directly with the adenine ring of ATP, forming a critical hydrogen-bond network with Arg136, Asp73, and Asn46 (E. coli GyrB numbering) with an inhibition constant (K_i) of ~10⁻⁸ M—more than three orders of magnitude tighter than ATP (K_m ≈ 0.3 mM).

To avoid committing suicide while synthesizing novobiocin, producer Streptomyces species embed a duplicated, antibiotic-resistant housekeeping paralog—gyrB2 (gyrB^R)—directly inside the novobiocin biosynthetic gene cluster. In GyrB2, the novobiocin-contacting arginine (Arg136) is replaced by histidine or lysine, abolishing aminocoumarin binding while preserving ATP hydrolysis. In modern computational genomics, scanning actinomycete chromosomes for duplicated gyrB2 self-resistance paralogs (Antibiotic Resistant Target Sequestration, ARTS) serves as a high-specificity beacon for discovering novel DNA-gyrase-targeting natural products.

Resolving Enteric and Bacillus Sister Species Beyond 16S rRNA Collapse

Because gyrB (~2.4 kb) is a universally conserved, strictly single-copy protein-coding housekeeping gene essential for bacterial survival and refractory to frequent horizontal gene transfer, Shigeaki Harayama and colleagues introduced gyrB sequencing as a high-resolution molecular marker for bacterial phylogenetics and speciation analysis.

Whereas prokaryotic 16S ribosomal RNA (rrsA–rrsH) is constrained by rigid secondary-structure stem base pairing and suffers from intragenomic copy heterogeneity (up to 7–15 operons per genome) that causes taxonomic collapse between closely related enteric and Bacillus sister species (for example, E. coli vs. Salmonella enterica 16S Kimura 2-parameter distance d_K2P = 0.0078, smaller than intragenomic E. coli rrsA–rrsH divergence of 0.0091), gyrB evolves 5-fold to 10-fold faster at third-codons synonymous sites while remaining under strong purifying selection at nonsynonymous sites (ω = dN/dS < 0.05). Consequently, gyrB achieves a clean inter-species barcoding gap across the Bacillus subtilis group (B. subtilis vs. B. amyloliquefaciens), the Bacillus cereus group, Pseudomonas, Acinetobacter, and Enterobacterales (d_K2P ≈ 0.07–0.18 between sister species), providing unambiguous species-level identification in clinical and environmental metagenomics.

Why is DNA gyrase the only topoisomerase capable of introducing negative supercoils into DNA?

Unlike Topoisomerase IV or eukaryotic Topoisomerase II, the GyrA subunit of DNA gyrase possesses a specialized C-terminal six-bladed β-pinwheel domain containing the positively charged GyrA-box (QRRGGKG). This β-pinwheel wraps ~130–140 base pairs of DNA in a chiral right-handed (+) superhelical loop, forcing the contiguous T-segment over the G-segment in a strictly positive crossover geometry so that every ATP-driven strand-passage event inverts the node and reduces the linking number by ΔLk = −2.

How does the Ser83Leu mutation in GyrA cause fluoroquinolone resistance without killing the bacterium?

Fluoroquinolones like ciprofloxacin do not bind directly to the catalytic Tyr122 residue; instead, their keto-acid group chelates a non-catalytic Mg²⁺ ion that bridges via four water molecules to Ser83 and Asp87 on the GyrA α4 helix (QRDR). Mutating polar Ser83 to hydrophobic leucine (Ser83Leu) abolishes the water–metal ion bridge needed to anchor the drug in the DNA nick, while leaving Tyr122 and the catalytic TOPRIM Mg²⁺ ions completely functional for normal DNA supercoiling.

Why is single-copy gyrB sequencing superior to 16S rRNA for distinguishing closely related bacterial species?

Many bacteria carry 5 to 15 copies of the 16S rRNA gene that undergo incomplete concerted evolution, and the 16S rRNA sequence evolves too slowly (~1% divergence per 50 million years) to separate recently diverged sister species like Escherichia coli and Salmonella enterica or members of the Bacillus subtilis complex. Because gyrB is present in exactly one copy per chromosome and accumulates neutral synonymous substitutions at third-codon positions while maintaining amino acid conservation, it provides 5-fold to 10-fold higher phylogenetic resolution at the species and subspecies levels.

1. Gellert, M., Mizuuchi, K., O'Dea, M. H., & Nash, H. A. (1976). "DNA gyrase: an enzyme that introduces superhelical turns into DNA." Proceedings of the National Academy of Sciences, 73(11), 3872–3876. https://doi.org/10.1073/pnas.73.11.3872
2. Morais Cabral, J. H., Jackson, A. P., Smith, C. V., Shikotra, N., Maxwell, A., & Liddington, R. C. (1997). "Crystal structure of the breakage–reunion domain of DNA gyrase." Nature, 388(6645), 903–906. https://doi.org/10.1038/42294
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4. Yamamoto, S., & Harayama, S. (1995). "PCR amplification and direct sequencing of gyrB genes with universal primers and their application to the detection and taxonomic analysis of Pseudomonas putida strains." Applied and Environmental Microbiology, 61(3), 1104–1109. https://doi.org/10.1128/aem.61.3.1104-1109.1995