All tangled up: how cells direct, manage and exploit topoisomerase function.

Topoisomerases are complex molecular machines that modulate DNA topology to maintain chromosome superstructure and integrity. Although capable of stand-alone activity in vitro, topoisomerases are frequently linked to larger pathways and systems that resolve specific DNA superstructures and intermediates arising from cellular processes such as DNA repair, transcription, replication and chromosome compaction. Topoisomerase activity is indispensible to cells, but requires the transient breakage of DNA strands. This property has been exploited, often for significant clinical benefit, by various exogenous agents that interfere with cell proliferation. Despite decades of study, surprising findings involving topoisomerases continue to emerge with respect to their cellular function, regulation and utility as therapeutic targets.

Dynamic coupling between conformations and nucleotide states in DNA gyrase.

Gyrase is an essential bacterial molecular motor that supercoils DNA using a conformational cycle in which chiral wrapping of > 100 base pairs confers directionality on topoisomerization. To understand the mechanism of this nucleoprotein machine, global structural transitions must be mapped onto the nucleotide cycle of ATP binding, hydrolysis and product release. Here we investigate coupling mechanisms using single-molecule tracking of DNA rotation and contraction during Escherichia coli gyrase activity under varying nucleotide conditions. We find that ADP must be exchanged for ATP to drive the rate-limiting remodeling transition that generates the chiral wrap. ATP hydrolysis accelerates subsequent duplex strand passage and is required for resetting the enzyme and recapturing transiently released DNA. Our measurements suggest how gyrase coordinates DNA rearrangements with the dynamics of its ATP-driven protein gate, how the motor minimizes futile cycles of ATP hydrolysis and how gyrase may respond to changing cellular energy levels to link gene expression with metabolism.

Gold rotor bead tracking for high-speed measurements of DNA twist, torque and extension.

Single-molecule measurements of DNA twist and extension have been used to reveal physical properties of the double helix and to characterize structural dynamics and mechanochemistry in nucleoprotein complexes. However, the spatiotemporal resolution of twist measurements has been limited by the use of angular probes with high rotational drag, which prevents detection of short-lived intermediates or small angular steps. We introduce gold rotor bead tracking (AuRBT), which yields >100× improvement in time resolution over previous techniques. AuRBT employs gold nanoparticles as bright low-drag rotational and extensional probes, which are monitored by instrumentation that combines magnetic tweezers with objective-side evanescent darkfield microscopy. Our analysis of high-speed structural dynamics of DNA gyrase using AuRBT revealed an unanticipated transient intermediate. AuRBT also enables direct measurements of DNA torque with >50× shorter integration times than previous techniques; we demonstrated high-resolution torque spectroscopy by mapping the conformational landscape of a Z-forming DNA sequence.

Mechanisms for defining supercoiling set point of DNA gyrase orthologs: II. The shape of the GyrA subunit C-terminal domain (CTD) is not a sole determinant for controlling supercoiling efficiency.

DNA topoisomerases are essential enzymes that can overwind, underwind, and disentangle double-helical DNA segments to maintain the topological state of chromosomes. Nearly all bacteria utilize a unique type II topoisomerase, gyrase, which actively adds negative supercoils to chromosomes using an ATP-dependent DNA strand passage mechanism; however, the specific activities of these enzymes can vary markedly from species to species. Escherichia coli gyrase is known to favor supercoiling over decatenation (Zechiedrich, E. L., Khodursky, A. B., and Cozzarelli, N. R. (1997) Genes Dev. 11, 2580-2592), whereas the opposite has been reported for Mycobacterium tuberculosis gyrase (Aubry, A., Fisher, L. M., Jarlier, V., and Cambau, E. (2006) Biochem. Biophys. Res. Commun. 348, 158-165). Here, we set out to understand the molecular basis for these differences using structural and biochemical approaches. Contrary to expectations based on phylogenetic inferences, we find that the dedicated DNA wrapping domains (the C-terminal domains) of both gyrases are highly similar, both architecturally and in their ability to introduce writhe into DNA. However, the M. tuberculosis enzyme lacks a C-terminal control element recently uncovered in E. coli gyrase (see accompanying article (Tretter, E. M., and Berger, J. M. (2012) J. Biol. Chem. 287, 18636-18644)) and turns over ATP at a much slower rate. Together, these findings demonstrate that C-terminal domain shape is not the sole regulatory determinant of gyrase activity and instead indicate that an inability to tightly couple DNA wrapping to ATP turnover is why M. tuberculosis gyrase cannot supercoil DNA to the same extent as its γ-proteobacterial counterpart. Our observations demonstrate that gyrase has been modified in multiple ways throughout evolution to fine-tune its specific catalytic properties.

Mechanisms for defining supercoiling set point of DNA gyrase orthologs: I. A nonconserved acidic C-terminal tail modulates Escherichia coli gyrase activity.

DNA topoisomerases manage chromosome supercoiling and organization in all cells. Gyrase, a prokaryotic type IIA topoisomerase, consumes ATP to introduce negative supercoils through a strand passage mechanism. All type IIA topoisomerases employ a similar set of catalytic domains for function; however, the activity and specificity of gyrase are augmented by a specialized DNA binding and wrapping element, termed the C-terminal domain (CTD), which is appended to its GyrA subunit. We have discovered that a nonconserved, acidic tail at the extreme C terminus of the Escherichia coli GyrA CTD has a dramatic and unexpected impact on gyrase function. Removal of the CTD tail enables GyrA to introduce writhe into DNA in the absence of GyrB, an activity exhibited by other GyrA orthologs, but not by wild-type E. coli GyrA. Strikingly, a "tail-less" gyrase holoenzyme is markedly impaired for DNA supercoiling capacity, but displays normal ATPase function. Our findings reveal that the E. coli GyrA tail regulates DNA wrapping by the CTD to increase the coupling efficiency between ATP turnover and supercoiling, demonstrating that CTD functions can be fine-tuned to control gyrase activity in a highly sophisticated manner.

A naturally chimeric type IIA topoisomerase in Aquifex aeolicus highlights an evolutionary path for the emergence of functional paralogs.

Bacteria frequently possess two type IIA DNA topoisomerases, gyrase and topo IV, which maintain chromosome topology by variously supercoiling, relaxing, and disentangling DNA. DNA recognition and functional output is thought to be controlled by the C-terminal domain (CTD) of the topoisomerase DNA binding subunit (GyrA/ParC). The deeply rooted organism Aquifex aeolicus encodes one type IIA topoisomerase conflictingly categorized as either DNA gyrase or topo IV. To resolve this enzyme's catalytic properties and heritage, we conducted a series of structural and biochemical studies on the isolated GyrA/ParC CTD and the holoenzyme. Whereas the CTD displays a global structure similar to that seen in bone fide GyrA and ParC paralogs, it lacks a key functional motif (the "GyrA-box") and fails to wrap DNA. Biochemical assays show that the A. aeolicus topoisomerase cannot supercoil DNA, but robustly removes supercoils and decatenates DNA, two hallmark activities of topo IV. Despite these properties, phylogenetic analyses place all functional domains except the CTD squarely within a gyrase lineage, and the A. aeolicus GyrB subunit is capable of supporting supercoiling with Escherichia coli GyrA, but not DNA relaxation with E. coli ParC. Moreover, swapping the A. aeolicus GyrA/ParC CTD with the GyrA CTD from Thermotoga maritima creates an enzyme that negatively supercoils DNA. These findings identify A. aeolicus as the first bacterial species yet found to exist without a functional gyrase, and suggest an evolutionary path for generation of bacterial type IIA paralogs.