Characterization of simian virus 40 T-antigen double hexamers bound to a replication fork. The active form of the helicase

Replication-fork dynamics

The proliferation of all organisms depends on the coordination of enzymatic events within large multiprotein replisomes that duplicate chromosomes. Whereas the structure and function of many core replisome components have been clarified, the timing and order of molecular events during replication remains obscure. To better understand the replication mechanism, new methods must be developed that allow for the observation and characterization of short-lived states and dynamic events at single replication forks. Over the last decade, great progress has been made toward this goal with the development of novel DNA nanomanipulation and fluorescence imaging techniques allowing for the direct observation of replication-fork dynamics both reconstituted in vitro and in live cells. This article reviews these new single-molecule approaches and the revised understanding of replisome operation that has emerged.

Bypass of a protein barrier by a replicative DNA helicase

Replicative DNA helicases generally unwind DNA as a single hexamer that encircles and translocates along one strand of the duplex while excluding the complementary strand (“steric exclusion”). In contrast, large T antigen (T-ag), the replicative DNA helicase of the Simian Virus 40 (SV40), is reported to function as a pair of stacked hexamers that pumps double-stranded DNA through its central channel while laterally extruding single-stranded DNA. Here, we use single-molecule and ensemble assays to show that T-ag assembled on the SV40 origin unwinds DNA efficiently as a single hexamer that translocates on single-stranded DNA in the 3′ to 5′ direction. Unexpectedly, T-ag unwinds DNA past a DNA-protein crosslink on the translocation strand, suggesting that the T-ag ring can open to bypass bulky adducts. Together, our data underscore the profound conservation among replicative helicase mechanisms while revealing a new level of plasticity in their interactions with DNA damage.

Uncoupling of sister replisomes during eukaryotic DNA replication

The duplication of eukaryotic genomes involves the replication of DNA from multiple origins of replication. In S phase, two sister replisomes assemble at each active origin, and they replicate DNA in opposite directions. Little is known about the functional relationship between sister replisomes. Some data imply that they travel away from one another and thus function independently. Alternatively, sister replisomes may form a stationary, functional unit that draws parental DNA toward itself. If this "double replisome" model is correct, a constrained DNA molecule should not undergo replication. To test this prediction, lambda DNA was stretched and immobilized at both ends within a microfluidic flow cell. Upon exposure to Xenopus egg extracts, this DNA underwent extensive replication by a single pair of diverging replisomes. The data show that there is no obligatory coupling between sister replisomes and, together with other studies, imply that genome duplication involves autonomously functioning replisomes.

Insights into the MCM functional mechanism: lessons learned from the archaeal MCM complex

The helicase function of the minichromosome maintenance protein (MCM) is essential for genomic DNA replication in archaea and eukaryotes. There has been rapid progress in studies of the structure and function of MCM proteins from different organisms, leading to better understanding of the MCM helicase mechanism. Because there are a number of excellent reviews on this topic, we will use this review to summarize some of the recent progress, with particular focus on the structural aspects of MCM and their implications for helicase function. Given the hexameric and double hexameric architecture observed by X-ray crystallography and electron microscopy of MCMs from archaeal and eukaryotic cells, we summarize and discuss possible unwinding modes by either a hexameric or a double hexameric helicase. Additionally, our recent crystal structure of a full length archaeal MCM has provided structural information on an intact, multi-domain MCM protein, which includes the salient features of four unusual beta-hairpins from each monomer, and the side channels of a hexamer/double hexamer. These new structural data enable a closer examination of the structural basis of the unwinding mechanisms by MCM.

Origin remodeling and opening in bacteria rely on distinct assembly states of the DnaA initiator

The initiation of DNA replication requires the melting of chromosomal origins to provide a template for replisomal polymerases. In bacteria, the DnaA initiator plays a key role in this process, forming a large nucleoprotein complex that opens DNA through a complex and poorly understood mechanism. Using structure-guided mutagenesis, biochemical, and genetic approaches, we establish an unexpected link between the duplex DNA-binding domain of DnaA and the ability of the protein to both self-assemble and engage single-stranded DNA in an ATP-dependent manner. Intersubunit cross-talk between this domain and the DnaA ATPase region regulates this link and is required for both origin unwinding in vitro and initiator function in vivo. These findings indicate that DnaA utilizes at least two different oligomeric conformations for engaging single- and double-stranded DNA, and that these states play distinct roles in controlling the progression of initiation.

Simian virus 40 large T antigen can specifically unwind the central palindrome at the origin of DNA replication

The hydrophilic channels between helicase domains of simian virus 40 (SV40) large T antigen play a critical role in DNA replication. Previous mutagenesis of residues in the channels identified one class of mutants (class A: D429A, N449S, and N515S) with normal DNA binding and ATPase and helicase activities but with a severely reduced ability to unwind origin DNA and to support SV40 DNA replication in vitro. Here, we further studied these mutants to gain insights into how T antigen unwinds the origin. We found that the mutants were compromised in melting the imperfect palindrome (EP) but normal in untwisting the AT-rich track. However, the mutants' defect in EP melting was not the major reason they failed to unwind the origin because supplying an EP region as a mismatched bubble, or deleting the EP region altogether, did not rescue their unwinding deficiency. These results suggested that specific separation of the central palindrome of the origin (site II) is an essential step in unwinding origin DNA by T antigen. In support of this, wild-type T antigen was able to specifically unwind a 31-bp DNA containing only site II in an ATPase-dependent reaction, whereas D429A and N515S failed to do so. By performing a systematic mutagenesis of 31-bp site II DNA, we identified discrete regions in each pentanucleotide necessary for normal origin unwinding. These data indicate that T antigen has a mechanism to specifically unwind the central palindrome. Various models are proposed to illustrate how T antigen could separate the central origin.

AAA+ ATPases in the initiation of DNA replication

All cellular organisms and many viruses rely on large, multi-subunit molecular machines, termed replisomes, to ensure that genetic material is accurately duplicated for transmission from one generation to the next. Replisome assembly is facilitated by dedicated initiator proteins, which serve to both recognize replication origins and recruit requisite replisomal components to the DNA in a cell-cycle coordinated manner. Exactly how imitators accomplish this task, and the extent to which initiator mechanisms are conserved among different organisms have remained outstanding issues. Recent structural and biochemical findings have revealed that all cellular initiators, as well as the initiators of certain classes of double-stranded DNA viruses, possess a common adenine nucleotide-binding fold belonging to the ATPases Associated with various cellular Activities (AAA+) family. This review focuses on how the AAA+ domain has been recruited and adapted to control the initiation of DNA replication, and how the use of this ATPase module underlies a common set of initiator assembly states and functions. How biochemical and structural properties correlate with initiator activity, and how species-specific modifications give rise to unique initiator functions, are also discussed.

Cell cycle regulation of DNA replication

Eukaryotic DNA replication is regulated to ensure all chromosomes replicate once and only once per cell cycle. Replication begins at many origins scattered along each chromosome. Except for budding yeast, origins are not defined DNA sequences and probably are inherited by epigenetic mechanisms. Initiation at origins occurs throughout the S phase according to a temporal program that is important in regulating gene expression during development. Most replication proteins are conserved in evolution in eukaryotes and archaea, but not in bacteria. However, the mechanism of initiation is conserved and consists of origin recognition, assembly of prereplication (pre-RC) initiative complexes, helicase activation, and replisome loading. Cell cycle regulation by protein phosphorylation ensures that pre-RC assembly can only occur in G1 phase, whereas helicase activation and loading can only occur in S phase. Checkpoint regulation maintains high fidelity by stabilizing replication forks and preventing cell cycle progression during replication stress or damage.

Quantitative analysis of the binding of simian virus 40 large T antigen to DNA

SV40 large T antigen (T-ag) is a multifunctional protein that successively binds to 5'-GAGGC-3' sequences in the viral origin of replication, melts the origin, unwinds DNA ahead of the replication fork, and interacts with host DNA replication factors to promote replication of the simian virus 40 genome. The transition of T-ag from a sequence-specific binding protein to a nonspecific helicase involves its assembly into a double hexamer whose formation is likely dictated by the propensity of T-ag to oligomerize and its relative affinities for the origin as well as for nonspecific double- and single-stranded DNA. In this study, we used a sensitive assay based on fluorescence anisotropy to measure the affinities of wild-type and mutant forms of the T-ag origin-binding domain (OBD), and of a larger fragment containing the N-terminal domain (N260), for different DNA substrates. We report that the N-terminal domain does not contribute to binding affinity but reduces the propensity of the OBD to self-associate. We found that the OBD binds with different affinities to its four sites in the origin and determined a consensus binding site by systematic mutagenesis of the 5'-GAGGC-3' sequence and of the residue downstream of it, which also contributes to affinity. Interestingly, the OBD also binds to single-stranded DNA with an approximately 10-fold higher affinity than to nonspecific duplex DNA and in a mutually exclusive manner. Finally, we provide evidence that the sequence specificity of full-length T-ag is lower than that of the OBD. These results provide a quantitative basis onto which to anchor our understanding of the interaction of T-ag with the origin and its assembly into a double hexamer.

Role of the hydrophilic channels of simian virus 40 T-antigen helicase in DNA replication

The simian virus 40 (SV40) hexameric helicase consists of a central channel and six hydrophilic channels located between adjacent large tier domains within each hexamer. To study the function of the hydrophilic channels in SV40 DNA replication, a series of single-point substitutions were introduced at sites not directly involved in protein-protein contacts. The mutants were characterized biochemically in various ways. All mutants oligomerized normally in the absence of DNA. Interestingly, 8 of the 10 mutants failed to unwind an origin-containing DNA fragment and nine of them were totally unable to support SV40 DNA replication in vitro. The mutants fell into four classes based on their biochemical properties. Class A mutants bound DNA normally and had normal ATPase and helicase activities but failed to unwind origin DNA and support SV40 DNA replication. Class B mutants were compromised in single-stranded DNA and origin DNA binding at low protein concentrations. They were defective in helicase activity and unwinding of the origin and in supporting DNA replication. Class C and D mutants possessed higher-than-normal single-stranded DNA binding activity at low protein concentrations. The class C mutants failed to separate origin DNA and support DNA replication. The class D mutants unwound origin DNA normally but were compromised in their ability to support DNA replication. Taken together, these results suggest that the hydrophilic channels have an active role in the unwinding of SV40 DNA from the origin and the placement of the resulting single strands within the helicase.

Structural mechanism of RPA loading on DNA during activation of a simple pre-replication complex

We report that during activation of the simian virus 40 (SV40) pre-replication complex, SV40 T antigen (Tag) helicase actively loads replication protein A (RPA) on emerging single-stranded DNA (ssDNA). This novel loading process requires physical interaction of Tag origin DNA-binding domain (OBD) with the RPA high-affinity ssDNA-binding domains (RPA70AB). Heteronuclear NMR chemical shift mapping revealed that Tag-OBD binds to RPA70AB at a site distal from the ssDNA-binding sites and that RPA70AB, Tag-OBD, and an 8-nucleotide ssDNA form a stable ternary complex. Intact RPA and Tag also interact stably in the presence of an 8-mer, but Tag dissociates from the complex when RPA binds to longer oligonucleotides. Together, our results imply that an allosteric change in RPA quaternary structure completes the loading reaction. A mechanistic model is proposed in which the ternary complex is a key intermediate that directly couples origin DNA unwinding to RPA loading on emerging ssDNA.

The oligomeric Rep protein of Mungbean yellow mosaic India virus (MYMIV) is a likely replicative helicase

Geminiviruses replicate by rolling circle mode of replication (RCR) and the viral Rep protein initiates RCR by the site-specific nicking at a conserved nonamer (TAATATT downward arrow AC) sequence. The mechanism of subsequent steps of the replication process, e.g. helicase activity to drive fork-elongation, etc. has largely remained obscure. Here we show that Rep of a geminivirus, namely, Mungbean yellow mosaic India virus (MYMIV), acts as a replicative helicase. The Rep-helicase, requiring > or =6 nt space for its efficient activity, translocates in the 3'-->5' direction, and the presence of forked junction in the substrate does not influence the activity to any great extent. Rep forms a large oligomeric complex and the helicase activity is dependent on the oligomeric conformation ( approximately 24mer). The role of Rep as a replicative helicase has been demonstrated through ex vivo studies in Saccharomyces cerevisiae and in planta analyses in Nicotiana tabacum. We also establish that such helicase activity is not confined to the MYMIV system alone, but is also true with at least two other begomoviruses, viz., Mungbean yellow mosaic virus (MYMV) and Indian cassava mosaic virus (ICMV).

Analyses of the interaction between the origin binding domain from simian virus 40 T antigen and single-stranded DNA provide insights into DNA unwinding and initiation of DNA replication

DNA helicases are essential for DNA metabolism; however, at the molecular level little is known about how they assemble or function. Therefore, as a model for a eukaryotic helicase, we are analyzing T antigen (T-ag) the helicase encoded by simian virus 40. In this study, nuclear magnetic resonance (NMR) methods were used to investigate the transit of single-stranded DNA (ssDNA) through the T-ag origin-binding domain (T-ag OBD). When the residues that interact with ssDNA are viewed in terms of the structure of a hexamer of the T-ag OBD, comprised of residues 131 to 260, they indicate that ssDNA passes over one face of the T-ag OBD and then transits through a gap in the open ring structure. The NMR-based conclusions are supported by an analysis of previously described mutations that disrupt critical steps during the initiation of DNA replication. These and related observations are discussed in terms of the threading of DNA through T-ag hexamers and the initiation of viral DNA replication.

DNA helicase activity is associated with the replication initiator protein rep of tomato yellow leaf curl geminivirus

The Rep protein of tomato yellow leaf curl Sardinia virus (TYLCSV), a single-stranded DNA virus of plants, is the replication initiator essential for virus replication. TYLCSV Rep has been classified among ATPases associated with various cellular activities (AAA+ ATPases), in superfamily 3 of small DNA and RNA virus replication initiators whose paradigmatic member is simian virus 40 large T antigen. Members of this family are DNA- or RNA-dependent ATPases with helicase activity necessary for viral replication. Another distinctive feature of AAA+ ATPases is their quaternary structure, often composed of hexameric rings. TYLCSV Rep has ATPase activity, but the helicase activity, which is instrumental in further characterization of the mechanism of rolling-circle replication used by geminiviruses, has been a longstanding question. We present results showing that TYLCSV Rep lacking the 121 N-terminal amino acids has helicase activity comparable to that of the other helicases: requirements for a 3' overhang and 3'-to-5' polarity of unwinding, with some distinct features and with a minimal AAA+ ATPase domain. We also show that the helicase activity is dependent on the oligomeric state of the protein.

Pumps, paradoxes and ploughshares: mechanism of the MCM2-7 DNA helicase

In eukaryotes, numerous lines of evidence have coalesced into a convincing case that the MCM2-7 complex - a heterohexameric ATPase - is the replicative DNA helicase. However, almost nothing is known about how this enzyme functions in a cellular context. Some models for the mechanism of the MCM2-7 helicase envision that it translocates along single-stranded DNA (ssDNA), whereas, more recently, it is has been suggested that it pumps double-stranded DNA (dsDNA) through its central channel. In particular, one model in which a double hexamer of MCM2-7 pumps dsDNA towards the hexamer interface and extrudes ssDNA laterally as a result of torsional strain is gaining popularity. Here, we discuss existing models and propose a new variation in which a single hexamer is the functional unit of the helicase. Duplex DNA is pumped into MCM2-7 and, as it emerges from the complex, a rigid protein that we term the 'ploughshare' splits the duplex.

Structural studies of the archaeal MCM complex in different functional states

The primary candidate for the eukaryotic replicative helicase is the MCM2-7 complex, a hetero-oligomer formed by six AAA+ paralogous polypeptides. A simplified model for structure-function studies is the homo-oligomeric orthologue from the archaeon Methanothermobacter thermoautotrophicus. The crystal structure of the DNA-interacting N-terminal domain of this homo-oligomer revealed a double hexamer in a head-to-head configuration; single-particle electron microscopy studies have shown that the full-length protein complex can form both single and double rings, in which each ring can consist of a cyclical arrangement of six or seven subunits. Using single-particle techniques and especially multivariate statistical symmetry analysis, we have assessed the changes in stoichiometry that the complex undergoes when treated with various nucleotide analogues or when binding a double-stranded DNA fragment. We found that the binding of nucleotides or of double-stranded DNA leads to the preferred formation of double-ring structures. Specifically, the protein complex is present as a double heptamer when treated with a nucleotide analogue, but it is rather found as a double hexamer when complexed with double-stranded DNA. The possible physiological role of the various stoichiometries of the complex is discussed in the light of the proposed mechanisms of helicase activity.

Understanding helicases as a means of virus control

Helicases are promising antiviral drug targets because their enzymatic activities are essential for viral genome replication, transcription, and translation. Numerous potent inhibitors of helicases encoded by herpes simplex virus, severe acute respiratory syndrome coronavirus, hepatitis C virus, Japanese encephalitis virus, West Nile virus, and human papillomavirus have been recently reported in the scientific literature. Some inhibitors have also been shown to decrease viral replication in cell culture and animal models. This review discusses recent progress in understanding the structure and function of viral helicases to help clarify how these potential antiviral compounds function and to facilitate the design of better inhibitors. The above helicases and all related viral proteins are classified here based on their evolutionary and functional similarities, and the key mechanistic features of each group are noted. All helicases share a common motor function fueled by ATP hydrolysis, but differ in exactly how the motor moves the protein and its cargo on a nucleic acid chain. The helicase inhibitors discussed here influence rates of helicase-catalyzed DNA (or RNA) unwinding by preventing ATP hydrolysis, nucleic acid binding, nucleic acid release, or by disrupting the interaction of a helicase with a required cofactor.

Double hexamer disruption and biochemical activities of Methanobacterium thermoautotrophicum MCM

Methanobacterium thermoautotrophicum MCM (mtMCM) is a helicase required for DNA replication. Previous electron microscopy studies have shown mtMCM in several oligomeric forms. However, biochemical studies suggest that mtMCM is a dodecamer, likely a double hexamer (dHex). The crystal structure of the N-terminal fragment of mtMCM reveals a stable dHex architecture. To further confirm that the dHex is not an artifact of crystal packing of two hexamers, we investigated the relevance of the dHex by disrupting the hexamer-hexamer interactions seen in the crystal structure via site-directed mutagenesis and examining various biochemical activities of the mutants in vitro. Using a combination of biochemical and structural assays, we demonstrated that changing arginine to alanine at amino acid position 161 or the insertion of a six-aminoacid peptide at the hexamer-hexamer interface disrupted dHex formation and produced stable single hexamers (sHex). Furthermore, we showed that the sHex mutants retained wild-type level of ATPase and DNA binding activities but had decreased helicase activity when compared with the wild type dHex protein. These biochemical properties of mtMCM are reminiscent of those of SV40 large T antigen, suggesting that the dHex form of mtMCM may be the active helicase for DNA unwinding during the bidirectional DNA replication.

Polymorphism and double hexamer structure in the archaeal minichromosome maintenance (MCM) helicase from Methanobacterium thermoautotrophicum

Methanobacterium thermoautotrophicum minichromosome maintenance complex (mtMCM), a cellular replicative helicase, is a useful model for the more complex eukaryotic MCMs. Biochemical and crystallographic evidence indicates that mtMCM assembles as a double hexamer (dHex), but previous electron microscopy studies reported only the presence of single heptamers or single hexamers (Pape, T., Meka, H., Chen, S., Vicentini, G., Van Heel, M., and Onesti, S. (2003) EMBO Rep. 4, 1079-1083; Yu, X., VanLoock, M. S., Poplawski, A., Kelman, Z., Xiang, T., Tye, B. K., and Egelman, E. H. (2002) EMBO Rep. 3, 792-797). Here we present the first three-dimensional electron microscopy reconstruction of the full-length mtMCM dHex in which two hexamers contact each other via the structurally well defined N-terminal domains. The dHex has obvious side openings that resemble the side channels of LTag (large T antigen). 6-fold and 7-fold rings were observed in the same mtMCM preparation, but we determined that assembly as a double ring favors 6-fold structures. Additionally, open rings were also detected, which suggests a direct mtMCM loading mechanism onto DNA.

TrwB, the coupling protein involved in DNA transport during bacterial conjugation, is a DNA-dependent ATPase

Bacterial conjugation is an example of macromolecular trafficking between cells, based on the translocation of single-stranded DNA across membranes through a type IV secretion system. TrwBDeltaN70 is the soluble domain of TrwB, an essential integral membrane protein that couples the relaxosome (a nucleoprotein complex) to the DNA transport apparatus in plasmid R388 conjugation. TrwBDeltaN70 crystallographic structure revealed a hexamer with six equivalent subunits and a central channel. In this work, we characterize a DNA-dependent ATPase activity for TrwBDeltaN70. The protein displays positive cooperativity for ATP hydrolysis, with at least three catalytic sites involved. The activity is sensitive to pH and salt concentration, being more active at low pH values. The effective oligonucleotide size required for activation of the ATPase function is between 40 and 45 nucleotides, and the same length is required for the formation of high-molecular-weight TrwBDeltaN70-DNA complexes, as observed by gel filtration chromatography. A mutation in a tryptophan residue (W216A), placed in the central pore formed by the hexameric structure, resulted in a protein that did not hydrolyze ATP. In addition, it exerted a dominant negative effect, both on R388 conjugation frequency and ATP hydrolysis, underscoring the multimeric state of the protein. ATP hydrolysis was not coupled to a DNA unwinding activity under the tested conditions, which included forked DNA substrates. These results, together with TrwB structural similarity to F1-ATPase, lead us to propose a mechanism for TrwB as a DNA-translocating motor.

The X-ray structure of the papillomavirus helicase in complex with its molecular matchmaker E2

DNA replication of the papillomaviruses is specified by cooperative binding of two proteins to the ori site: the enhancer E2 and the viral initiator E1, a distant member of the AAA+ family of proteins. Formation of this prereplication complex is an essential step toward the construction of a functional, multimeric E1 helicase and DNA melting. To understand how E2 interacts with E1 to regulate this process, we have solved the X-ray structure of a complex containing the HPV18 E2 activation domain bound to the helicase domain of E1. Modeling the monomers of E1 to a hexameric helicase shows that E2 blocks hexamerization of E1 by shielding a region of the E1 oligomerization surface and stabilizing a conformation of E1 that is incompatible with ATP binding. Further biochemical experiments and structural analysis show that ATP is an allosteric effector of the dissociation of E2 from E1. Our data provide the first molecular insights into how a protein can regulate the assembly of an oligomeric AAA+ complex and explain at a structural level why E2, after playing a matchmaker role by guiding E1 to the DNA, must dissociate for subsequent steps of initiation to occur. Building on previously proposed ideas, we discuss how our data advance current models for the conversion of E1 in the prereplication complex to a hexameric helicase assembly.

Interactions required for binding of simian virus 40 T antigen to the viral origin and molecular modeling of initial assembly events

The purified T-antigen origin binding domain binds site specifically to site II, the central region of the simian virus 40 core origin. However, in the context of full-length T antigen, the origin binding domain interacts poorly with DNA molecules containing just site II. Here we investigate the contributions of additional core origin regions, termed the flanking sequences, to origin recognition and the assembly of T-antigen hexamers and double hexamers. Results from these studies indicate that in addition to site-specific binding of the T-antigen origin binding domain to site II, T-antigen assembly requires non-sequence-specific interactions between a basic finger in the helicase domain and particular flanking sequences. Related studies demonstrate that the assembly of individual hexamers is coupled to the distortions in the proximal flanking sequence. In addition, the point in the double-hexamer assembly process that is regulated by phosphorylation of threonine 124, the sole posttranslational modification required for initiation of DNA replication, was further analyzed. Finally, T-antigen structural information is used to model various stages of T-antigen assembly on the core origin and the regulation of this process.

Nonspecific double-stranded DNA binding activity of simian virus 40 large T antigen is involved in melting and unwinding of the origin

Helicase activity is required for T antigen to unwind the simian virus 40 origin. We previously mapped this activity to residues 131 and 616. In this study, we generated a series of mutants with single-point substitutions in the helicase domain to discover other potential activities required for helicase function. A number of DNA unwinding-defective mutants were generated. Four of these mutants (456RA, 460ED, 462GA, and 499DA) were normal in their ability to hydrolyze ATP and were capable of associating into double hexamers in the presence of origin DNA. Furthermore, they possessed normal ability to bind to single-stranded DNA. However, they were severely impaired in unwinding origin-containing DNA fragments and in carrying out a helicase reaction with an M13 partial duplex DNA substrate. Interestingly, these mutants retained some ability to perform a helicase reaction with artificial replication forks, indicating that their intrinsic helicase activity was functional. Intriguingly, these mutants had almost completely lost their ability to bind to double-stranded DNA nonspecifically. The mutants also failed to melt the early palindrome region of the origin. Taken together, these results indicate that the mutations have destroyed a novel activity required for unwinding of the origin. This activity depends on the ability to bind to DNA nonspecifically, and in its absence, T antigen is unable to structurally distort and subsequently unwind the origin.

Initiation of DNA replication: lessons from viral initiator proteins

Initiator proteins are key components of the DNA replication machinery that determine where initiation will occur. In the past few years, due to a greatly improved understanding of what viral initiators look like and how they function, we can now identify the basic tasks that are required of initiators, as well as begin to comprehend what activities are required to perform these tasks. The improved knowledge of the viral initiators also demonstrates an unexpected level of conservation between different viral initiators, which might extend also to their cellular counterparts.