Resolution of poxvirus telomeres: processing of vaccinia virus concatemer junctions by conservative strand exchange

The vaccinia virus DNA polymerase and its processivity factor

Vaccinia virus is the prototypic poxvirus. The 192 kilobase double-stranded DNA viral genome encodes most if not all of the viral replication machinery. The vaccinia virus DNA polymerase is encoded by the E9L gene. Sequence analysis indicates that E9 is a member of the B family of replicative polymerases. The enzyme has both polymerase and 3'-5' exonuclease activities, both of which are essential to support viral replication. Genetic analysis of E9 has identified residues and motifs whose alteration can confer temperature-sensitivity, drug resistance (phosphonoacetic acid, aphidicolin, cytosine arabinsode, cidofovir) or altered fidelity. The polymerase is involved both in DNA replication and in recombination. Although inherently distributive, E9 gains processivity by interacting in a 1:1 stoichiometry with a heterodimer of the A20 and D4 proteins. A20 binds to both E9 and D4 and serves as a bridge within the holoenzyme. The A20/D4 heterodimer has been purified and can confer processivity on purified E9. The interaction of A20 with D4 is mediated by the N'-terminus of A20. The D4 protein is an enzymatically active uracil DNA glycosylase. The DNA-scanning activity of D4 is proposed to keep the holoenzyme tethered to the DNA template but allow polymerase translocation. The crystal structure of D4, alone and in complex with A20_1-50 and/or DNA has been solved. Screens for low molecular weight compounds that interrupt the A20_1-50/D4 interface have yielded hits that disrupt processive DNA synthesis in vitro and/or inhibit plaque formation. The observation that an active DNA repair enzyme is an integral part of the holoenzyme suggests that DNA replication and repair may be coupled.

Sgs1 RecQ helicase inhibits survival of Saccharomyces cerevisiae cells lacking telomerase and homologous recombination

In yeast telomerase mutants, the Sgs1 RecQ helicase slows the rate of senescence and also facilitates the appearance of certain types of survivors of critical telomere shortening via mechanisms dependent on Rad52-dependent homologous recombination (HR). Here we describe a third function for Sgs1 in telomerase-deficient cells, inhibition of survivors that grow independent of Rad52. Unlike tlc1 rad52 double mutants, which do not form survivors of telomere dysfunction, tlc1 rad52 sgs1 triple mutants readily generated survivors. After emerging from growth crisis, the triple mutants progressively lost telomeric and subtelomeric sequences, yet grew for more than 1 year. Analysis of cloned chromosome termini and of copy number changes of loci genome-wide using tiling arrays revealed terminal deletions extending up to 57 kb, as well as changes in Ty retrotransposon copy numbers. Amplification of the remaining terminal sequences generated large palindromes at some chromosome termini. Sgs1 helicase activity but not checkpoint function was essential for inhibiting the appearance of the survivors, and the continued absence of Sgs1 was required for the growth of the established survivors. Thus, in addition to facilitating the maintenance of telomere repeat sequences via HR-dependent mechanisms, a RecQ helicase can prevent the adoption of HR-independent mechanisms that stabilize chromosome termini without the use of natural telomere sequences. This provides a novel mechanism by which RecQ helicases may help maintain genome integrity and thus prevent age-related diseases and cancer.

DNA cleavage by the A22R resolvase of vaccinia virus

Vaccinia virus encodes an enzyme, A22R, required during DNA replication for cleaving viral DNA concatamers to yield unit-length viral genomes. The concatamer junctions contain inverted repeat sequences that can be extruded as cruciforms, yielding Holliday junctions. Previous work indicated that A22R can cleave Holliday junctions in vitro. To investigate the mechanism of action of A22R, we have optimized reaction conditions and characterized the sequence specificity of cleavage. We found that addition of 20% dimethylsulfoxide boosted product formation six-fold, resulting in improved sensitivity of cleavage assays. To analyze cleavage specificity, we took advantage of mobile Holliday junctions, in which branch migration allowed sampling of many DNA sequences. We found that A22R weakly favors cleavage at the sequence 5'-(G/C) downward arrow(A/T)-3', and so is much less sequence specific than its Escherichia coli relative, RuvC. Analysis of the reaction products revealed that A22R cleaves to leave a 3' hydroxyl at the cleaved phosphodiester bond.

Vaccinia virus nicking-joining enzyme is encoded by K4L (VACWR035)

Vaccinia virus encodes an enzyme with DNA modifying activity that cleaves and inefficiently cross-links cruciformic DNA. This enzyme is contained within the virion, expressed at late times postinfection, and processes DNA in an energy-independent, Mg2+ ion-independent manner. Viral nuclease activity was measured in extracts from cells infected with well-defined viral mutants. Since some viral extracts lacked nuclease activity, the gene encoding the activity was postulated to be one of the open reading frames absent in the viruses lacking activity. Inducible expression of each candidate open reading frame revealed that only the gene VACWR035, or K4L, was required for nuclease activity. A recombinant virus missing only the open reading frame for K4L lacked nuclease activity. Extracts from a recombinant virus expressing K4L linked to a FLAG polypeptide were able to cleave and cross-link cruciformic DNA. There were no significant differences between the virus lacking K4L and wild-type vaccinia virus WR with respect to infectivity, growth characteristics, or processing of viral replicative intermediate DNA, including both telomeric and cross-linked forms. Purification of the K4L FLAG polypeptide expressed in bacteria yielded protein containing nicking-joining activity, implying that K4L is the only vaccinia virus protein required for the nicking-joining enzymatic activity.

Quaternary structure and cleavage specificity of a poxvirus holliday junction resolvase

Recently, poxviruses were found to encode a protein with signature motifs present in the RuvC family of Holliday junction (HJ) resolvases, which have a key role in homologous recombination in bacteria. The vaccinia virus homolog A22 specifically cleaved synthetic HJ DNA in vitro and was required for the in vivo resolution of viral DNA concatemers into unit-length genomes with hairpin telomeres. It was of interest to further characterize a poxvirus resolvase in view of the low sequence similarity with RuvC, the absence of virus-encoded RuvA and RuvB to interact with, and the different functions of the viral and bacterial resolvases. Because purified A22 aggregated severely, studies were carried out with maltose-binding protein fused to A22 as well as to RuvC. Using gel filtration, chemical cross-linking, analytical ultracentrifugation, and light scattering, we demonstrated that A22 and RuvC are homodimers in solution. Furthermore, the dimeric form of the resolvase associated with HJ DNA, presumably facilitating the symmetrical cleavage of such structures. Like RuvC, A22 symmetrically cleaved fixed HJ junctions as well as junctions allowing strand mobility. Unlike RuvC and other members of the family, however, the poxvirus enzyme exhibited little cleavage sequence specificity. Structural and enzymatic similarities of poxvirus, bacterial, and fungal mitochondrial HJ resolvases are consistent with their predicted evolutionary relationship based on sequence analysis. The absence of a homologous resolvase in mammalian cells makes these microbial enzymes excellent potential therapeutic targets.

Chlorella viruses as a source of novel enzymes

A special advantage has been conferred upon Chlorella cells as tools in biotechnology when viruses (Phycodnaviridae) infecting Chlorella cells were discovered and isolated. The viruses are large icosahedral particles (150-200 nm in diameter), containing a giant, 330-380 kbp long, linear dsDNA genome. Recently, the nucleotide sequence of the 330,740-bp genome of PBCV-1, the prototype virus of Phycodnaviridae, was determined, and up to 702 open reading frames (ORFs) were identified along the genome. The possible genes present include those encoding a variety of enzymes involved in the modification of DNA, RNA, protein and polysaccharides as well as those involved in the metabolism of sugars, amino acids, lipids, nucleotides and nucleosides. Many of these genes are actually expressed during viral infection, with functional enzymes detected in the host cytoplasm or incorporated into the virion. The successful utilization of these viral enzymes as various DNA restriction and modification enzymes (Cvi enzymes) that are now commercially available is well documented. Also noteworthy are virion-associated chitinase and chitosanase activities that have potentially important applications in the recycling of natural resources. The virions of Chlorella viruses contain more than 50 different structural proteins, ranging in size from 10 to 200 kDa. Some of these proteins may be replaced with useful foreign proteins using recombinant DNA technology. The proteins of interest can be recovered easily from the viral particles, and collected by centrifugation after complete lysis of the host Chlorella cells.

Potential for broad-spectrum protection against feline calicivirus using an attenuated myxoma virus expressing a chimeric FCV capsid protein

It has previously been demonstrated that recombinant myxoma viruses expressing FCV capsid protein are capable of eliciting protective responses against virulent FCV challenge, following vaccination, in cats. An attempt was made to produce a bivalent myxoma recombinant expressing the capsid protein genes of both FCV strains F9 and LS015. The FCV capsid protein genes were inserted into the myxoma growth factor gene (MGF) locus, and the serine protease inhibitor (SERP 2) gene locus. Subsequent recombination between myxoma-FCV viruses resulted in a recombinant expressing a chimeric form of the capsid protein. Nonetheless, cats immunised with this myxoma-FCV recombinant demonstrate high levels of serum neutralising antibodies against both F9 and LS015 strains. Such a chimeric vaccine may provide effective protection against a wide range of FCV strains.

Origin-independent plasmid replication occurs in vaccinia virus cytoplasmic factories and requires all five known poxvirus replication factors

BACKGROUND

Replication of the vaccinia virus genome occurs in cytoplasmic factory areas and is dependent on the virus-encoded DNA polymerase and at least four additional viral proteins. DNA synthesis appears to start near the ends of the genome, but specific origin sequences have not been defined. Surprisingly, transfected circular DNA lacking specific viral sequences is also replicated in poxvirus-infected cells. Origin-independent plasmid replication depends on the viral DNA polymerase, but neither the number of additional viral proteins nor the site of replication has been determined.

RESULTS

Using a novel real-time polymerase chain reaction assay, we detected a >400-fold increase in newly replicated plasmid in cells infected with vaccinia virus. Studies with conditional lethal mutants of vaccinia virus indicated that each of the five proteins known to be required for viral genome replication was also required for plasmid replication. The intracellular site of replication was determined using a plasmid containing 256 repeats of the Escherichia coli lac operator and staining with an E. coli lac repressor-maltose binding fusion protein followed by an antibody to the maltose binding protein. The lac operator plasmid was localized in cytoplasmic viral factories delineated by DNA staining and binding of antibody to the viral uracil DNA glycosylase, an essential replication protein. In addition, replication of the lac operator plasmid was visualized continuously in living cells infected with a recombinant vaccinia virus that expresses the lac repressor fused to enhanced green fluorescent protein. Discrete cytoplasmic fluorescence was detected in cytoplasmic juxtanuclear sites at 6 h after infection and the area and intensity of fluorescence increased over the next several hours.

CONCLUSION

Replication of a circular plasmid lacking specific poxvirus DNA sequences mimics viral genome replication by occurring in cytoplasmic viral factories and requiring all five known viral replication proteins. Therefore, small plasmids may be used as surrogates for the large poxvirus genome to study trans-acting factors and mechanism of viral DNA replication.

Extrusion of an imperfect palindrome to a cruciform in superhelical DNA: complete determination of energetics using a statistical mechanical model

We present a detailed study of the extrusion of an imperfect palindrome, derived from the terminal regions of vaccinia virus DNA and contained in a superhelical plasmid, into a cruciform containing bulged bases. We monitor the course of extrusion by two-dimensional gel electrophoresis experiments as a function of temperature and linking number. We find that extrusion pauses at partially extruded states as negative superhelicity increases. To understand the course of extrusion with changes in linking number, DeltaLk, we present a rigorous semiempirical statistical mechanical analysis that includes complete coupling between DeltaLk, cruciform extrusion, formation of extrahelical bases, and temperature-dependent denaturation. The imperfections in the palindrome are sequentially incorporated into the cruciform arms as hairpin loops, single unpaired bases, and complex local regions containing several unpaired bases. We analyze the results to determine the free energies, enthalpies and entropies of formation of all local structures involved in extrusion. We find that, for each unpaired structure, the DeltaG, DeltaH and DeltaS of formation are all approximately proportional to the number of unpaired bases contained therein. This surprising result holds regardless of the arrangement or composition of unpaired bases within a particular structure. Imperfections have major effects on the overall energetics of cruciform extrusion and on the course of this transition. In particular, the extent of the DeltaLk change necessary to extrude an imperfect palindrome is considerably greater than that required for extrusion of the underlying perfect palindrome. Our analysis also suggests that, at higher temperatures, significant denaturation at the base of an imperfect cruciform can successfully compete with extension of the cruciform arms.

Resolution of a Holliday junction by vaccinia topoisomerase requires a spacer DNA segment 3' of the CCCTT/ cleavage sites

Vaccinia virus DNA topoisomerase catalyzes resolution of synthetic Holliday junctions in vitro. The mechanism entails concerted transesterifications at two recognition sites, 5'-CCCTT/, that are opposed within a partially mobile four-way junction. Efficient resolution occurs on a junction with a 10 bp segment of branch mobility (5'-GCCCTTATCG) that extends 4 bp 3' of the scissile phosphate. Here we report that resolution is decreased when branch mobility is limited to an 8 bp segment extending 2 bp 3' of the cleavage site and then eliminated when branch mobility is confined to the 6 bp GCCCTT sequence 5' of the scissile phosphate. We surmise that a spacer region 3' of CCCTT is needed for simultaneous cleavage at two opposing sites at the junction. Branch mobility is not required for reaction chemistry at a junction, because topoisomerase cleaves a single CCCTT site in a non-mobile four-way junction where the scissile phosphate is at the crossover point. The junction resolvase activity of topo-isomerase may be involved in forming the hairpin telomeres of the vaccinia genome.

Shope fibroma virus DNA topoisomerase catalyses holliday junction resolution and hairpin formation in vitro

The telomeres of poxviral chromosomes comprise covalently closed hairpin structures bearing mismatched bases. These hairpins are formed as concatemeric replication intermediates and are processed into mature, unit-length genomes. The structural transitions and enzymes involved in telomere resolution are poorly understood. Here we show that the type I topoisomerase of Shope fibroma virus (SFV) can promote a recombination reaction which converts cloned SFV replication intermediates into hairpin-ended molecules resembling mature poxviral telomeres. Recombinant SFV topoisomerase linearised a palindromic plasmid bearing 1.5 kb of DNA encoding the SFV concatemer junction, at a site near the centre of inverted-repeat symmetry. Most of these linear reaction products bore hairpin tips as judged by denaturing gel electrophoresis. The resolution reaction required palindromic SFV DNA sequences and was inhibited by compounds which block branch migration (MgCl2) or poxviral topoisomerases. The resolution reaction was also slow, needed substantial quantities of topoisomerase, and required that the palindrome be extruded in a cruciform configuration. DNA cleavage experiments identified a pair of suitably oriented topoisomerase recognition sites, 90 bases from the centre of the cloned SFV terminal inverted repeat, which may mark the resolution site. These data suggest a resolution scheme in which branch migration of a Holliday junction through a site occupied by covalently bound topoisomerase molecules, could lead to telomere resolution.

Vaccinia virus DNA topoisomerase: a model eukaryotic type IB enzyme

Vaccinia topoisomerase has proven to be an instructive model system for mechanistic studies of the type IB family of DNA topoisomerases. The catalytically relevant functional groups at the active site and the circumferential topoisomerase-DNA interface were correctly surmised by mutational and footprint analysis of vaccinia topoisomerase in advance of structure determinations by X-ray crystallography. It is now evident from multiple crystal structures that the catalytic domains of type IB topoisomerases and site specific recombinases derive from a common ancestral strand transferase capable of forming a DNA-(3'-phosphotyrosyl)-enzyme intermediate. A constellation of conserved amino acids catalyzes attack of the tyrosine nucleophile on the scissile phosphate. Domain dynamics and DNA-induced conformational changes within the catalytic domain are likely to play a role in triggering strand scission and coordinating the strand exchange or strand passage steps.

Replication at the telomeres of the Streptomyces linear plasmid pSLA2

The Streptomyces linear plasmid pSLA2 initiates DNA replication bidirectionally towards its telomeres from a site located near the centre of the molecule; at the telomeres, the recessed ends of lagging strands are filled in by non-displacing DNA synthesis. Here, we report experiments that test three proposed mechanisms for lagging-strand fill-in. We present data inconsistent with recombinational or terminal hairpin models for the formation of full-length duplex pSLA2 DNA. Instead, we find that deletions in short, distantly separated homologous palindromes in the leading-strand 3' overhang prevent propagation of linear pSLA2 DNA, implicating a mechanism of palindrome-mediated leading-strand fold-back in telomere replication. We further show that circularized pSLA2 DNA molecules are opened in vivo precisely at the terminal nucleotides of telomeres, generating functional linear replicons containing native telomeres covalently bound to a protein at their 5' DNA termini. Together, our results support a model in which pairing of multiple widely separated pSLA2 palindromes anchors the 3' end of the leading-strand overhang to a site near the overhang's base -- providing a recognition site for terminal-protein-primed DNA synthesis and subsequent endonucleolytic processing. Thus, the replication of Streptomyces plasmid telomeres may have features in common with the mechanism proposed for telomere replication in autonomous parvoviruses.

Novobiocin inhibits vaccinia virus replication by blocking virus assembly

Novobiocin inhibits the replication of vaccinia virus in cultured BSC40 cells. All classes of viral proteins were synthesized during synchronous infection in the presence of drug. The onset of DNA replication was delayed slightly, yet the extent of DNA replication in the presence of novobiocin was comparable to that of a control infection. A delay in the temporal transition to late viral protein synthesis was in keeping with the effects on DNA replication. Although the precursor forms of the major viral structural proteins were synthesized normally at late times, the proteolytic processing of these polypeptides was inhibited, which suggested an impediment to virus assembly. Electron microscopy revealed that novobiocin blocked virus morphogenesis at an early stage. Conversion of the concatemeric DNA replication intermediates into hairpin telomeres occurred in the presence of novobiocin, confirming that telomere resolution was not coupled to virus assembly. Novobiocin is the latest addition to a class of antipoxviral agents, which includes rifampin and IMCBH, that arrest morphogenesis.

Complications and implications of linear bacterial chromosomes

The emergence of linear bacterial chromosomes has overthrown the dogma of universal circularity of the bacterial chromosomes, and posed mechanistic and evolutionary implications not previously anticipated.

Resolution of Holliday junctions by eukaryotic DNA topoisomerase I

The Holliday junction, a key intermediate in both homologous and site-specific recombination, is generated by the reciprocal exchange of single strands between two DNA duplexes. Resolution of the junctions can occur in two directions with respect to flanking markers, either restoring the parental DNA configuration or generating a genetic crossover. Recombination can be regulated, in principle, by factors that influence the directionality of the resolution step. We demonstrate that the vaccinia virus DNA topoisomerase, a eukaryotic type I enzyme, catalyzes resolution of synthetic Holliday junctions in vitro. The mechanism entails concerted transesterifications at two recognition sites, 5'-CCCTT decreases, that are opposed within a partially mobile four-way junction. Cruciforms are resolved unidirectionally and with high efficiency into two linear duplexes. These findings suggest a model whereby type I topoisomerases may either promote or suppress genetic recombination in vivo.

Characterization of the terminal inverted repeats and their neighboring tandem repeats in the Chlorella CVK1 virus genome

A unique group of large icosahedral viruses that infect a unicellular green alga (Chlorella sp. NC64A) were isolated from freshwater sources in Japan. These viruses contain a linear double-stranded DNA (dsDNA) genome with hairpin ends. A physical map was constructed for the genomic DNA of CVK1 (Chlorella virus isolated in Kyoto, no. 1) by pulsed-field gel electrophoresis of restriction fragments. The nucleotide sequences around both termini of the CVK1 DNA revealed the presence of inverted terminal repeats (ITR) of approximately 1.0 kb. Adjacent to the ITR, unique sequence elements of 10 to 20 bp were directly repeated 20 to 30 times in tandem array. Several copies of these repeat elements were deleted in virus mutants that were occasionally generated from Chlorella cells that were in a putative CVK1 carrier state. These repeats might represent a hot spot of rearrangement in the CVK1 genome.

Genome organization in prokaryotes

Most of the well-characterized prokaryotic genomes consist of double-stranded DNA organized as a single circular chromosome 0.6-10 Mb in length and one or more circular plasmid species of 2 kb-1.7 Mb. The past few years, however, have revealed some major variations in genome organization. In addition, a recent accumulation of data has shown that the location and orientation of the genes and repeated sequences (including prophages and transposons) on and among these elements is not always random. Some of the non-randomness is probably the result of unique historical events; in other cases it reflects selection for the optimization of function.

In vitro resolution of poxvirus replicative intermediates into linear minichromosomes with hairpin termini by a virally induced Holliday junction endonuclease

Available evidence suggests that one or more late viral gene products are involved in processing poxvirus replicative intermediates into mature progeny hairpin-terminated genomes. Cloned versions of the Shope fibroma virus (SFV) replicated telomere in the inverted repeat configuration were used as substrates to assay lysates from poxvirus-infected cells for protein fractions that participate in the resolution of the circular substrate plasmid into a linear minichromosome with viral hairpin termini. An activity in a crude protein fraction obtained from vaccinia virus-infected cells at late times during the replicative cycle was capable of accurately resolving all poxviral inverted repeat replicative intermediates tested. The resolved linear products are identical to the products of in vivo resolution and possessed symmetrical nicks which mapped at the borders of the inverted repeat sequence. Strand-specific nicks were also identified, which mapped within the telomere resolution target sequence known to be required for telomere resolution in vivo. The resolving activity that we have identified is specific to virus-infected cells at late times during replication and cleaves cloned poxviral telomeric substrates in a fashion expected of a classic Holliday junction-resolving enzyme in addition to possessing a telomere resolution target-specific nicking activity. Although a Holliday junction-resolving activity would also be expected to play a role in the recombination induced by poxvirus infection, the appearance of the activity described here only after the commencement of viral late protein synthesis suggests that it functions strictly at late times. Other non-viral Holliday junction analogs can also be cleaved by this extract, suggesting that this component of the resolution activity may also play a role in other viral processes that require cleavage of a branched DNA structure. Thus, we have identified a poxviral activity that may be a part of a protein complex which resolves concatemeric replicative intermediates of viral DNA as well as participate in general recombination late during infection.

Transcription of the terminal loop region of vaccinia virus DNA is initiated from the telomere sequences directing DNA resolution

The telomeres of vaccinia virus DNA are transcribed at late times after infection. Analysis of cDNAs of RNA transcripts of the terminal loop region of the viral DNA shows that both inverted and complementary forms of the terminal loop region are transcribed. These late RNAs, which contain 5' poly(A) sequences, do not appear to encode any proteins. The transcriptional start sites for most of these RNAs are within the sequences that direct the resolution of concatemeric DNA replication intermediates (M. Merchlinsky and B. Moss, 1989, J. Virol. 63, 4354-4361). This suggests that the process of DNA resolution may involve transcription initiated from the telomere sequences required for resolution.

The target DNA sequence for resolution of poxvirus replicative intermediates is an active late promoter

The linear double-stranded genomes of poxviruses such as Shope fibroma virus (SFV) replicate autonomously within the cytoplasm of infected cells, and it is believed that all of the replication functions are virally encoded. During DNA replication the incompletely base-paired terminal hairpin loops of the viral genome transiently exist in the form of inverted repeat replicative intermediates. These inverted repeat structures form the target for telomere resolution events that include sequence-specific cleavage and directed strand exchange to form the hairpin termini of progeny virus genomes. The terminal sequence domain which forms the telomere resolution target (TRT) shares considerable sequence similarity with viral late promoters. In this study we demonstrate that the TRT of SFV is capable of functioning as a strong viral promoter late in infection. A spectrum of TRT mutations affects telomere resolution and late transcription in a strictly concordant fashion, suggesting that the two activities may be inextricably linked. Further support for this concept comes from the demonstration that a late SFV promoter sequence designated cryptic TRT, which differs substantially from the native TRT in terms of sequence, can support telomere resolution when placed in the correct spatial context. The proposed model for telomere resolution invokes directed unwinding of the TRT double helix by a transcription initiation complex and processing of the resulting secondary structure by viral late-gene products.

Mutational analysis of the resolution sequence of vaccinia virus DNA: essential sequence consists of two separate AT-rich regions highly conserved among poxviruses

In replicative forms of vaccinia virus DNA, the unit genomes are connected by palindromic junction fragments that are resolved into mature viral genomes with hairpin termini. Bacterial plasmids containing the junction fragment for vaccinia virus or Shope fibroma virus were converted into linear minichromosomes of vector sequence flanked by poxvirus hairpin loops after transfection into infected cells. Analysis of a series of symmetrical deletion mutations demonstrated that in vaccinia virus the presence of the DNA sequence ATTTAGTGTCTAGAAAAAAA on both sides of the apical segment of the concatemer junction is crucial for resolution. To determine the precise architecture of the resolution site, a series of site-directed mutations within this tract of nucleotides were made and the relative contribution of each nucleotide to the efficaciousness of resolution was determined. The nucleotide sequence necessary for the resolution of the vaccinia virus concatemer junction, (A/T)TTT(A/G)N7-9AAAAAAA, is highly conserved among poxviruses and found proximal to the hairpin loop in the genomes of members of the Leporipoxvirus, Avipoxvirus, and Capripoxvirus genera.