The DNA topoisomerases are a group of fascinating enzymes that play an essential but
dangerous game with DNA. They break and rejoin either one or both strands of the double
helix to solve the problems of tangling and linking that occur as a result of DNA
manipulations (replication, transcription and recombination) in all cells. This basic
problem with the DNA structure was recognized by Watson and Crick almost as soon as
the double helix was described (1). As the parental DNA strands are separated at a
replication fork, the double-helical turns are compressed and overwound ahead of the
fork; the resulting torsional stress will prevent further replication if it is not
relieved. This overwinding corresponds to positive supercoiling. Alternatively, any
rotation of the replication fork leads to interwinding of the replicated regions,
ultimately resulting in linking (catenation) of the daughter chromosomes, which must
be removed if partition is to occur without breaking the DNA (2). Transcription can
also result in the generation of both positive and negative supercoiling (3), and
other processes, particularly recombination, can lead to the knotting of DNA strands.
These complexities of double-helical DNA are grouped together under the label of DNA
topology (4). The topological problems of the DNA helix must have arisen very early
in evolution, as soon as DNA genomes became long enough that a simple rotation of
the entire molecule to remove supercoiling became impracticable.
The only viable solution to these difficulties is to untwist, unlink and unknot the
DNA by breaking one or both strands, permitting strands to pass through one another
or allowing rotation at the break point. These strategies are adopted by the different
classes of topoisomerase enzymes, discovered during the 1970s. The type I enzymes
break and rejoin one strand of the helix, and either pass single strands through one
another (type IA) or allow one broken end to rotate about the intact strand (type
IB). Type I enzymes can remove supercoiling from DNA. In contrast, type II topoisomerases
pass one double-helical segment through a double-stranded break in another, in an
ATP-dependent reaction, and can thus unlink (decatenate) linked chromosomes, and remove
knots. One subset of these enzymes, DNA gyrases, can introduce negative supercoiling
(unwinding) into DNA. Most cell types express a suite of topoisomerase enzymes to
regulate the topology of their DNA.
However, these manipulations of the DNA helix come at a cost; the broken DNA strands
must be efficiently rejoined to avoid serious consequences for the cell. The hijacking
of topoisomerase mechanisms to produce stable single-stranded and, particularly, double-stranded
breaks is a feature of a wide variety of natural and synthetic chemotherapeutic agents,
making the topoisomerase enzymes important drug targets (5,6).
During the 1990s, there were regular meetings on DNA topoisomerases in New York and
Amsterdam. However, in recent years these meetings lapsed and we lacked a regular
forum to discuss issues concerned with DNA topology and topoisomerases. Happily, Nynke
Dekker, Paola Arimondo and Mary-Ann Bjornsti organized an excellent topoisomerase
meeting in Fréjus, France in 2007. This re-established the momentum for similar meetings
in the future, including Topo2008, which was held last year in Norwich, UK. Tremendous
advances are being made in this field, which continues to be a fascinating and vibrant
research area. Topics at the meeting ranged from discussions of the intricacies of
DNA knotting to the translation of fundamental work on topoisomerases into drug discovery.
This issue of NAR contains a special collection of Surveys and Summaries that cover
the field of DNA topology and DNA topoisomerases and reflect the content of the Norwich
meeting. Zechiedrich and colleagues discuss how misregulation of topology can lead
to cellular dysfunction and consider how cells can prevent such topological problems
(7). The control of supercoiling in bacterial cells has been extensively studied;
Dorman and Corcoran discuss such studies and the effects of supercoiling on bacterial
virulence and infectious diseases (8). Gadelle and Forterre review the origins and
phylogenies of these enzymes and suggest that they originated in an ancestral virosphere
(9).
Mondragón and colleagues review structural work on type I enzymes, which has led to
a deeper understanding of their reaction mechanisms (10). A key feature of many type
I and type II enzymes is that they require Mg2+ ions in their reaction mechanisms.
Sissi and Palumbo discuss the role of Mg2+ ions in topoisomerase structure and function,
in particular, a proposed two metal ion mechanism for DNA cleavage (11). DNA cleavage
in type II enzymes occurs at a region of the enzyme known as the ‘DNA gate’, and Collins
et al. describe the use of single-molecule fluorescence energy transfer experiments
to probe the dynamics of the DNA gate of type II topoisomerases (12). The double-strand
break mechanism for type II enzymes has important implications for the role of topoisomerase
II in eukaryotic cells, and Roca discusses the implications of this mechanism in the
context of eukaryotic chromatin structure (13).
Bacterial topoisomerase I is a potential, though currently unexploited, target for
antibacterial agents; Tse-Dinh discusses screening for novel agents that target this
enzyme (14). Deweese and Osheroff consider the DNA breakage–reunion reaction of type
II enzymes and how compounds that stabilize the topoisomerase II cleavage complex
can act as cytotoxic agents and be utilized as anti-cancer drugs (15).
This collection of reviews illustrates the breadth of research work being carried
out in the DNA topology/topoisomerase area, and also highlights some of the unsolved
questions that remain. We would like to thank the authors who both participated in
the meeting (Topo2008) and contributed to this excellent set of reviews, which will
hopefully stimulate further enthusiasm for this field. We anticipate that the next
meeting in this series will take place in 2010 in the USA.