Replication of the parvovirus MVM. II. Isolation and characterization of intermediates in the replication of the viral deoxyribonucleic acid

Herpes simplex virus co-infection facilitates rolling circle replication of the adeno-associated virus genome

Adeno-associated virus (AAV) genome replication only occurs in the presence of a co-infecting helper virus such as adenovirus type 5 (AdV5) or herpes simplex virus type 1 (HSV-1). AdV5-supported replication of the AAV genome has been described to occur in a strand-displacement rolling hairpin replication (RHR) mechanism initiated at the AAV 3' inverted terminal repeat (ITR) end. It has been assumed that the same mechanism applies to HSV-1-supported AAV genome replication. Using Southern analysis and nanopore sequencing as a novel, high-throughput approach to study viral genome replication we demonstrate the formation of double-stranded head-to-tail concatemers of AAV genomes in the presence of HSV-1, thus providing evidence for an unequivocal rolling circle replication (RCR) mechanism. This stands in contrast to the textbook model of AAV genome replication when HSV-1 is the helper virus.

Maintenance of the flip sequence orientation of the ears in the parvoviral left-end hairpin is a nonessential consequence of the critical asymmetry in the hairpin stem

Parvoviral terminal hairpins are essential for viral DNA amplification but are also implicated in multiple additional steps in the viral life cycle. The palindromes at the two ends of the minute virus of mice (MVM) genome are dissimilar and are processed by different resolution mechanisms that selectively direct encapsidation of predominantly negative-sense progeny genomes and conserve a single Flip sequence orientation at the 3' (left) end of such progeny. The sequence and predicted structure of these 3' hairpins are highly conserved within the genus Parvovirus, exemplified by the 121-nucleotide left-end sequence of MVM, which folds into a Y-shaped hairpin containing small internal palindromes that form the "ears" of the Y. To explore the potential role(s) of this hairpin in the viral life cycle, we constructed infectious clones with the ear sequences either inverted, to give the antiparallel Flop orientation, or with multiple transversions, conserving their base composition but changing their sequence. These were compared with a "bubble" mutant, designed to activate the normally silent origin in the inboard arm of the hairpin, thus potentially rendering symmetric the otherwise asymmetric junction resolution mechanism that drives maintenance of Flip. This mutant exhibited a major defect in viral duplex and single-strand DNA replication, characterized by the accumulation of covalently closed turnaround forms of the left end, and was rapidly supplanted by revertants that restored asymmetry. In contrast, both sequence and orientation changes in the hairpin ears were tolerated, suggesting that maintaining the Flip orientation of these structures is a consequence of, but not the reason for, asymmetric left-end processing.

Genomic features of the human bocaviruses

The human bocavirus (HBoV) was initially discovered in 2005 as the second pathogenic member of the parvovirus family, next to the human parvovirus B19. HBoV has since been shown to be extremely common worldwide and to cause a systemic infection in small children often resulting in respiratory disease. Three more, presumably enteric, human bocaviruses (HBoV2-4) have been identified in stool samples. Parvoviruses are assumed to replicate via their genomic terminal hairpin-like structures in a so-called 'rolling-hairpin model'. These terminal sequences have recently been partially identified in head-to-tail HBoV-PCR amplicons from clinical samples, and are most likely hybrid relics of HBoV's predecessors, namely bovine parvovirus 1 on the left-hand side and minute virus of canines on the right, shown for the first time in this article. Thereby, the replication model postulated for HBoV remains questionable as the occurrence of head-to-tail sequences is not a typical feature of the rolling-hairpin replication model. However, such episomes can also be persistent storage forms of the genome.

Human bocavirus: still more questions than answers

The human bocavirus was first detected in 2005 and since then has been found in both respiratory secretions from patients with airway infections and in stool samples from patients with gastroenteritis. Meanwhile, four different genotypes have been identified that most likely derive from recombination events. Although the modified Koch’s postulates have not yet been fulfilled completely, owing to the lack of an animal model or a simple cell culture system, there is increasing evidence that the human bocaviruses are serious participants in infectious diseases of the respiratory and the GI tracts. This article reviews the current status of the clinical features of human bocaviruses and provides an overview of the latest findings concerning the biology, phylogeny, epidemiology and diagnostic tools related to human bocaviruses. Furthermore, it discusses the potential pathogenicity of human bocavirus, as well as its persistence and reactivation in hosts.

Does human bocavirus infection depend on helper viruses? A challenging case report

A case of severe diarrhoea associated with synergistic human bocavirus type 1 (HBoV) and human herpes virus type 6 (HHV6) is reported. The case supports the hypotheses that HBoV infection under clinical conditions may depend on helper viruses, or that HBoV replicates by a mechanism that is atypical for parvoviruses, or that HBoV infection can be specifically treated with cidofovir.

The parvoviral capsid controls an intracellular phase of infection essential for efficient killing of stepwise-transformed human fibroblasts

Members of the rodent subgroup of the genus Parvovirus exhibit lytic replication and spread in many human tumor cells and are therefore attractive candidates for oncolytic virotherapy. However, the significant variation in tumor tropism observed for these viruses remains largely unexplained. We report here that LuIII kills BJ-ELR 'stepwise-transformed' human fibroblasts efficiently, while MVM does not. Using viral chimeras, we mapped this property to the LuIII capsid gene, VP2, which is necessary and sufficient to confer the killer phenotype on MVM. LuIII VP2 facilitates a post-entry, pre-DNA-amplification step early in the life cycle, suggesting the existence of an intracellular moiety whose efficient interaction with the incoming capsid shell is critical to infection. Thus targeting of human cancers of different tissue-type origins will require use of parvoviruses with capsids that effectively make this critical interaction.

Molecular cloning and expression of key gene encoding hypothetical DNA polymerase from B. mori parvo-like virus

BmPLV-Z is the abbreviation for Bombyx mori parvo-like virus (China isolate). This is a novel virus with two single-stranded linear DNA molecules, viz., VD1 (6543 bp) and VD2 (6022 bp), which are encapsidated respectively into separate virions. Analysis of the deduced amino acid sequence of VD1-ORF4 indicated the existence of a putative DNA-polymerase with exonuclease activity, possibly involved in the replication of BmPLV-Z. In the present study, a recombinant baculovirus was constructed to express the full length of the protein encoded by the VD1-ORF4 gene (3318 bp). In addition, a 2163-bp fragment amplified from the very same gene was cloned into prokaryotic expression vector pET-30a and expressed in E.coli Rosetta 2 (DE3) pLysS. The expressed fusion protein was employed to immunize New Zealand white rabbits for the production of an antiserum, afterwards used for examining the expression of the protein encoded by VD1-ORF4 gene in Sf-9 cells infected with recombinant baculovirus. Western blot analysis of extracts from thus cells infected revealed a specific band of about 120 kDa, thereby indicating that the full length protein encoded by the VD1-ORF4 gene had been successfully and stably expressed in Sf-9 cells.

Exploring the contribution of distal P4 promoter elements to the oncoselectivity of Minute Virus of Mice

Minute Virus of Mice (MVM) shares inherent oncotropic properties with other members of the genus Parvovirus. Two elements responsible, at least in part, for this oncoselectivity have been mapped to an Ets1 binding site adjacent to the P4 TATA box of the initiating promoter, P4, and to a more distal cyclic AMP responsive element (CRE), located within the telomeric hairpin stem. Here the CRE overlaps one half-site for the binding of parvoviral initiation factor (PIF), which is essential for viral DNA replication. We used a degenerate oligonucleotide selection approach to show that CRE binding protein (CREB) selects the sequence ACGTCAC within this context, rather than its more generally accepted palindromic TGACGTCA recognition site. We have developed strategies for manipulating these sequences directly within the left-end palindrome of the MVM infectious clone and used them to clone mutants whose CRE either matches the symmetric consensus sequence or is scrambled, or in which the PIF binding site is incrementally weakened with respect to the CRE. The panel of mutants were tested for fitness relative to wildtype in normal murine fibroblasts A9 or transformed human fibroblasts 324 K, through multiple rounds of growth in co-infected cultures, using a differential real-time quantitative PCR assay. We confirmed that inactivating the CRE substantially abrogates oncoselectivity, but found that improving its fit to the palindromic consensus is somewhat debilitating in either cell type. We also confirmed that reducing the PIF half-site spacing by one basepair enhances oncoselectivity, but found that a further basepair deletion significantly reduces this effect.

Host range mutants of Minute Virus of Mice with a single VP2 amino acid change require additional silent mutations that regulate NS2 accumulation

Two host range switch mutants of the immunosuppressive strain of parvovirus Minute Virus of Mice (MVMi) were isolated from plaques on A9 fibroblasts. Both carried a single coding mutation at residue D399 in VP2, to alanine and glycine in hr105 and hr107, respectively, and a second, non-coding, guanine-to-adenine change at nucleotide 1970 in hr105 and 1967 in hr107. These mutations were recreated in a wild type MVMi infectious plasmid clone, both alone and as pairs, in either the original or switched combinations. All single mutants failed to replicate productively in fibroblasts, but the two pairs of changes were functionally equivalent. Single D399 mutations allowed the viruses to initiate infection in fibroblasts, but NS2 expression was severely restricted and correlated with poor accumulation and release of progeny virus. Mutations at 1967 or 1970 enhanced NS2 accumulation, and allowed efficient progeny production and release. Conversely, the D399 mutations destroyed the viruses' ability to infect EL4 lymphocytes. In all productive EL4 infections, NS2 was expressed at high ratios even in the absence of upstream mutations, and progeny accumulation was efficient. However, EL4 cells lack a mechanism for early progeny release, potentially explaining why virus amplification in these cells is slow.

Genome packaging sense is controlled by the efficiency of the nick site in the right-end replication origin of parvoviruses minute virus of mice and LuIII

The parvovirus minute virus of mice (MVM) packages predominantly negative-sense single strands, while its close relative LuIII encapsidates strands of both polarities with equal efficiency. Using genomic chimeras and mutagenesis, we show that the ability to package positive strands maps not, as originally postulated, to divergent untranslated regions downstream of the capsid gene but to the viral hairpins and predominantly to the nick site of OriR, the right-end replication origin. In MVM, the sequence of this site is 5'-CTAT(black triangle down)TCA-3', while in LuIII a two-base insertion (underlined) changes it to 5'-CTATAT(black triangle down)TCA-3'. Matched LuIII genomes differing only at this position (designated LuIII and LuDelta2) packaged 47 and <8% positive-sense strands, respectively. OriR sequences from these viruses were both able to support NS1-mediated nicking in vitro, but initiation efficiency was consistently two- to threefold higher for LuDelta2 derivatives, suggesting that LuIII's ability to package positive strands is determined by a suboptimal right-end origin rather than by strand-specific packaging sequences. These observations support a mathematical "kinetic hairpin transfer" model, previously described by Chen and colleagues (K. C. Chen, J. J. Tyson, M. Lederman, E. R. Stout, and R. C. Bates, J. Mol. Biol. 208:283-296, 1989), that postulates that preferential excision of particular strands is solely responsible for packaging specificity. By analyzing replicative-form (RF) DNA generated in vivo during LuIII and LuDelta2 infections, we extend this model, showing that positive-sense strands do accumulate in LuDelta2 infections as part of duplex RF DNA, but these do not support packaging. However, replication is biphasic, so that accumulation of positive-sense strands is ultimately suppressed, probably because the onset of packaging removes newly displaced single strands from the replicating pool.

Reverse genetic system for the analysis of parvovirus telomeres reveals interactions between transcription factor binding sites in the hairpin stem

The left-hand or 3'-terminal hairpin of minute virus of mice (MVM) contains sequence elements essential for both viral DNA replication at the left-hand origin (oriL) and for the modulation of the P4 promoter, from which the viral nonstructural gene cassette is transcribed. This hairpin sequence has proven difficult to manipulate in the context of the viral genome. Here we describe a system for generating mutant viruses using synthetic hairpin oligonucleotides and a truncated form of the infectious clone. This allows manipulation of the sequence of the left-hand hairpin and examination of the effects in the context of the viral life cycle. We have confirmed the requirement for a functional parvovirus initiation factor (PIF) binding site and determined that an optimized PIF binding site, with 6 bases between the half-sites, was actually detrimental to viral growth. The distal PIF half-site overlaps a cyclic AMP-responsive element (CRE), which was shown to play an important role in initiating infection, particularly in 324K simian virus 40-transformed human fibroblasts. Interestingly, reducing the spacing of the PIF half-sites, and thus the affinity of the binding site for PIF, increased viral fitness relative to wild type in 324K cells, but not in murine A9 cells. These results indicate that the relative importance of factor binding to the CRE and PIF sites during the establishment of an infection differs markedly between these two host cells and suggest that the suboptimal spacing of PIF half-sites found in wild-type virus represents a necessary reduction in the affinity of the PIF interaction in favor of CRE function.

Initiation of DNA replication at palindromic telomeres is mediated by a duplex-to-hairpin transition induced by the minute virus of mice nonstructural protein NS1

The linear single-stranded DNA genome of the minute virus of mice (MVM) is replicated via a double-stranded replicative form (RF) intermediate. Amplification of this RF is initiated by the folding-back of palindromic sequences serving as primers for strand-displacement synthesis and formation of dimeric RF DNA. Using an in vitro replication assay and a cloned MVM DNA template, we observed hairpin-primed DNA replication at both MVM DNA termini, with a bias toward right-end initiation. Initiation of DNA replication is favored by nuclear components of A9 cell extract and highly stimulated by the MVM nonstructural protein NS1. Hairpin-primed DNA replication is also observed in the presence of NS1 and the Klenow fragment of the Escherichia coli DNA polymerase I. Addition of ATPgammaS (adenosine 5'-O-(thiotriphosphate)) blocks the initiation of DNA replication but not the extension of pre-existing hairpin primers formed in the presence of NS1 only. The NS1-mediated unwinding of the right-end palindrome may account for the recently reported capacity of NS1 for driving dimer RF synthesis in vitro.

Specific initiation of replication at the right-end telomere of the closed species of minute virus of mice replicative-form DNA

We have developed an in vitro system that supports the replication of natural DNA templates of the autonomous parvovirus minute virus of mice (MVM). MVM virion DNA, a single-stranded molecule bracketed by short, terminal, self-complementary sequences, is converted into double-stranded replicative-form (RF) DNA when incubated in mouse A9 fibroblast extract. The 3' end of the newly synthesized complementary strand is ligated to the right-end hairpin of the virion strand, resulting in the formation of a covalently closed RF (cRF) molecule as the major conversion product. cRF DNA is not further replicated in A9 cell extract alone. On addition of purified MVM nonstructural protein NS1 expressed from recombinant baculoviruses or vaccinia viruses, cRF DNA is processed into a right-end (5' end of the virion strand) extended form (5'eRF). This is indicative of NS1-dependent nicking of the right-end hairpin at a distinct position, followed by unfolding of the hairpin and copying of the terminal sequence. In contrast, no resolution of the left-end hairpin can be detected in the presence of NS1. In the course of the right-end nicking reaction, NS1 gets covalently attached to the right-end telomere of the DNA product, as shown by immunoprecipitation with NS1-specific antibodies. The 5'eRF product is the target for additional rounds of NS1-induced nicking and displacement synthesis at the right end, arguing against the requirement of the hairpin structure for recognition of the DNA substrate by NS1. Further processing of the 5'eRF template in vitro leads to the formation of dimeric RF (dRF) DNA in a left-to-left-end configuration, presumably as a result of copying of the whole molecule by displacement synthesis initiated at the right-end telomere. Formation of dRF DNA is highly stimulated by NS1. The experimental results presented in this report support various assumptions of current models of parvovirus DNA replication and provide new insights into the replication functions of the NS1 protein.

Early events in the replication of parvovirus LuIII

Purified infectious LuIII virions characterized for their biological and physicochemical properties were used to analyse early events in the interaction of an autonomous parvovirus with its host cell. It could be shown that adsorption and penetration of LuIII virus are rapid processes which, under optimal conditions, can be completed within 30 to 60 minutes of incubation at 37 degrees C. Uncoating of the viral genome is initiated in the cytoplasm and is continued after the virus has been transported to the nucleus. None of these processes appeared to depend on cell cycle-specific helper function(s). However, the first step in viral replication--formation of viral parental RF-DNA consisting of covalently linked complementary strands--evidently could only be achieved in late S-phase of the cell cycle. No evidence was obtained which would support the hypothesis that viral DNA must be integrated into the cellular genome to allow for the initiation of viral DNA synthesis.

Construction of an infectious molecular clone of the autonomous parvovirus minute virus of mice

The linear single-stranded DNA genome of minute virus of mice, an autonomous parvovirus, was cloned in duplex form into the bacterial plasmid pBR322. The recombinant clones of minute virus of mice were infectious when transfected into monolayers of human 324K cells and produced virus plaques with an efficiency of about 6% that obtained with duplex replicative-form DNA purified from cells infected with minute virus of mice. Southern blot analysis of transfected cells indicated that the cloned minute virus of mice genome requires both termini to be intact for excision and replication as a linear duplex molecule.

Are pyrimidine dimers tolerated during DNA replication of UV-irradiated parvovirus minute-virus-of-mice in mouse fibroblasts?

The replication of the single-stranded DNA of parvovirus Minute-Virus-of-MIce (MVM) was depressed when virus was exposed to UV-light prior to infection of mouse fibroblasts. Most of the viral DNA containing pyrimidine dimers was permanently blocked in its conversion to double-stranded replicative forms (RF). Yet dimers might be tolerated to a low extent, considering that a minor fraction of parental RF molecules was sensitive to the action of the UV-specific endonuclease V of bacteriophage T4, UV-irradiation of the cells prior to infection with UV-damaged MVM increased the levels of both parental RF and total viral DNA synthesized. The sensitivity of parental RF molecules to the UV-specific endonuclease was little enhanced by preirradiation of the cells and did not appear to be sufficient to account for the stimulation of RF formation in those cultures. Since parvoviral single-stranded DNA is not a substrate for nucleotidyl excision repair, one interpretation of these results would be that the process(es) activated in preirradiated cells overcome(s) barriers to viral DNA replication other than elongation blocks at pyrimidine dimers. Alternatively, pyrimidine dimers tolerated in pretreated cultures might become protected from attack by the UV-endonuclease.

Effect of UV-irradiation on DNA replication of the parvovirus minute-virus-of-mice in mouse fibroblasts

The effect of UV-irradiation on the conversion of the single-stranded DNA of the parvovirus Minute-Virus-of-Mice (MVM) to duplex Replicative Forms (RF) was studied after infection of mouse A9 fibroblasts. UV-irradiation of the virus prior to infection of unirradiated cells resulted in a dose-dependent, single-hit, inhibition of RF formation. Restriction fragment analysis indicated that this inhibition could be ascribed to the introduction of absolute blocks which prevent elongation of the newly synthesized complementary strand. Cell exposure to UV-light prior to infection with UV-irradiated MVM enhanced the fraction of input viral DNA which was converted to RF. This enhancement required de novo protein synthesis during the interval between cell irradiation and virus infection. These results suggest that DNA replication constitutes a target in the viral life cycle that leads to the UV-enhanced Reactivation of virus survival, however, they do not permit us to identify the step of RF formation which is enhanced in UV-pretreated cells.

Intracellular DNA of the parvovirus minute virus of mice is organized in a minichromosome structure

Minute virus of mice (MVM) nucleoprotein complexes were leached from infected cell nuclei in the presence of a hypotonic buffer. Detailed biochemical analyses performed on the extracted complexes revealed nucleoprotein complexes sedimenting together with virions at 110S and defective particles sedimenting at 50S. In contrast to the virions, the nucleoprotein complexes were found to be sensitive to treatment with DNase, Sarkosyl, and heparin. They were found to be composed of replicative forms of MVM DNA and cellular histones. After extensive micrococcal nuclease digestion performed on purified nucleoprotein complexes, a viral nucleosomes core containing a DNA segment of about 140 base pairs in length was identified. These complexes when visualized by electron microscopy revealed the existence of beaded structures (minichromosomes) having 26 and 52 beads per monomer and dimer molecules, respectively. We suggest that the organization of the intracellular viral DNA in a minichromosome structure is an essential step in the virus growth cycle.

Levels of cellular DNA polymerases in synchronized bovine paravovirus-infected cells

Infection of synchronized bovine fetal spleen cells with bovine parvovirus results in changes in the levels and patterns of DNA polymerases alpha and gamma during the cell cycle. The pattern of DNA polymerase alpha activity closely paralled viral DNA synthesis and the production of progeny virus, and levels, of this enzyme were threefold greater than in mock-infected cells during the period of maximal viral DNA synthesis. DNA polymerase gamma activity remained slightly elevated during viral DNA replication. Levels and patterns of DNA polymerase beta were similar in mock- and virus-infected cells.

Levels of cellular DNA polymerases in synchronized bovine paravovirus-infected cells

Infection of synchronized bovine fetal spleen cells with bovine parvovirus results in changes in the levels and patterns of DNA polymerases alpha and gamma during the cell cycle. The pattern of DNA polymerase alpha activity closely paralled viral DNA synthesis and the production of progeny virus, and levels, of this enzyme were threefold greater than in mock-infected cells during the period of maximal viral DNA synthesis. DNA polymerase gamma activity remained slightly elevated during viral DNA replication. Levels and patterns of DNA polymerase beta were similar in mock- and virus-infected cells.

Isolation of the viral DNA replication complex from adeno-associated virus type 1-infected cells

The replication complex active in adeno-associated virus type 1 (AAV-1) DNA synthesis in vitro was solubilized, with a nonionic detergent, from the nuclei of human embryonic kidney cells coinfected with AAV-1 and an early temperative-sensitive mutant (ts125) of human adenovirus type 5 at the nonpermissive temperature (40.5 degrees C). The complex sedimented with a mean size of 23S and contained parental AAV-1 DNA. Most of the DNA synthesized with the AAV-1 DNA replication complex in vitro was AAV-1 DNA, as revealed by DNA-DNA hybridization and sedimentation in a neutral sucrose gradient. However, it sedimented in an alkaline sucrose gradient as molecules smaller than AAV-1 DNA (14.4S). The AAV-1 DNA replication complex was not formed in cells infected with AAV-1 alone.

Embryonal carcinoma cells (and their somatic cell hybrids) are resistant to infection by the murine parvovirus MVM, which does infect other teratocarcinoma-derived cell lines

Minute virus of mice (MVM), a non-defective parvovirus, has been shown to infect cultures of non-pluripotent differentiated teratocarcinoma-derived cells, but pluripotent (and "nullipotent") embryonal carcinoma cells derived from the same teratocarcinoma resist MVN infection. Somatic cell hybrids between an embryonal carcinoma line and Friend erythroblastic leukemia cells are also resistant to MVM, even though Friend cells are susceptible. Among three blastocyst-derived lines tested, only one, a parietal yolk sac cell line, resists MVM infection. These results suggest that teratocarcinoma cultures may provide useful systems in which to study the cellular factors which mediate susceptibility to this teratogenic and oncolytic virus.

Replication process of the parvovirus H-1. VI. Characterization of a replication terminus of H-1 replicative-form DNA

The linear duplex replicative form (RF) DNA of the parvovirus H-1 has been characterized with respect to cleavage by the bacterial restriction endonuclease of Escherichia coli, EcoRI. RF DNA has a single cleavage site 0.22 genome length from the left end of the molecule. The molecular weight of H-1 RF DNA determined by gel electrophoresis is 3.26 X 10(6). H-1 RF DNA has been found to dimerize by hydrogen-bounded linkage at the molecular left end, and in some molecules the viral strand is covalently linked to the complementary strand. Some 10% of monomeric RF DNA also has a covalent linkage between the viral and complementary strands at the left end. The EcoRI-B fragment, containing the left end of the RF molecule, appears to be a replication terminus by its labeling characteristics for both RF and progeny DNA synthesis. These findings suggest that the left end of H-1 RF DNA has some type of "turn-around" structure and that this end is not an origin for DNA synthesis.

Three structural polypeptides coded for by minite virus of mice, a parvovirus

Purified full and empty virions of minute virus of mice were separated on CsCl gradients, and their polypeptides were examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The empty particle contains two polypeptides, A (83,300 daltons) and B (64,300 daltons), which are 15 to 18% and 82 to 85%, respectively, of the virion mass. The full particle contains the single-stranded DNA genome, proteins A and B, and a third polypeptide, C (61,400 daltons). Again A is 15 to 18% of the protein mass, but the amounts of B and C vary inversely in different preparations of full particles. These polypeptides comprise greater than 99.6% of the protein in either virion, and their molecular weights and molar ratios are independent of the species of host cell on which the virus is propagated, They are not found in uninfected cells, and no protein component of uninfected cells copurifies with either virion under our conditions. Pulse-chase experiments show that the three proteins are synthesized only after virus infection and are therefore probably virus coded. Sequential harvesting from the nuclei of cells infected under one cycle growth conditions shows an increase in the proportion of C in full particles as infection progresses, suggesting that C is derived from B in a late maturation step.

Isolation and structural characterization of monomeric and dimeric forms of replicative intermediates of Kilham rat virus DNA

Two virus-specific species of newly synthesized DNA were isolated from rat fibroblast cell cultures infected with the Kilham rat virus (RV). These two DNA species were purified; their behavior on hydroxyapatite chromatography and their sedimentation coefficients in sucrose gradients were determined. One of the two species corresponds to the linear double-stranded form of the RV DNA, and the other corresponds to the dimeric duplex form. After denaturation, a fraction of both species showed an intramolecular renaturation; these molecules are composed of viral strand covalently linked to complementary strand. Models for the structure of both species are posposed. Both species may be considered as double-strand replicative intermediates of the single-stranded RV DNA.

DNA of minute virus of mice: self-priming, nonpermuted, single-stranded genome with a 5'-terminal hairpin duplex

The genome of the nondefective parvovirus minute virus of mice (MVM) is a linear DNA molecular weight 1.48 x 10(6), which is single stranded for approximately 94% of its length. In contrast to the genomes from defective parvoviruses MVM DNA does not contain a detectable inverted terminal redundancy. A combination of enzymatic and physical techniques has shown that the molecule contains a stable hairpin duplex of approximately 130 base pairs located at the 5' terminus of the genome. MVM DNA is efficiently utilized as a template-primer by a number of DNA polymerases, including reverse transcriptases. Polymerases lacking 5' to 3' exonuclease activity yield a duplex DNA product with a molecular weight 1.96 times that of the viral genome, in which the newly synthesized complementary strand is covalently attached to the template. This duplex contains an internal "nick" that can be sealed by DNA ligase to produce a self-complementary single-strand circle. The MVM DNA duplex is cleaved twice by EcoR-RI restriction endonuclease to yield three distinct fragments in molar amounts. These results suggest that the initiation of DNA synthesis in vitro occurs at a point within 100 bases of the 3' end of the genome, using the 3' terminus of viral DNA as a primer, and that the sequence of nucleotides in the genome is not permuted.

Rolling hairpin model for replication of parvovirus and linear chromosomal DNA

A novel, quasicircular scheme is proposed for the replication of parvovirus DNA. Daughter strands are initiated after the copying and rearrangement of a terminal palindromic sequence, a process termed 'hairpin transfer'. Such a process may be involved in the replication of other viruses and host cell DNA.

Concatemers of alternating plus and minus strands are intermediates in adenovirus-associated virus DNA synthesis

Replicating DNA molecules of adenovirus-associated virus (AAV) were selectively extracted from KB cells coinfected at 39.5 detrees with a DNA minus, temperature-sensitive mutant of adenovirus 5 (ts125) as helper. Under these conditions AAV DNA replication proceeds normally, but there is little, if any, adenovirus DNA synthesis. An analysis of the replicating molecules in sucrose density gradients reveals that there are AAV DNA intermediates which consist of covalently linked plus and minus DNA strands. Under denaturing conditions, these concatemers are linear single strands whose lengths can reach at least four times the size of the AAV genome. The most abundant concatemeric species is a dimer which presumably exists in vivo as a unit length hairpin. Unit length linear duplexes appear to be immediate precursors of plus and minus progeny strands. These findings are compatible with a self-priming mechanism for the synthesis of AAV DNA.

Multiplication of parvovirus LuIII in a synchronized culture system. III. Replication of viral DNA

The replication of the single-stranded DNA (ssDNA) of parvovirus LuIII was studied in synchronized HeLa cells. After infection of the cells in early S phase, synthesis of a replicative form (RF) DNA became detectable as early as 9 h postinfection, i.e., after display of the cellular helper function(s) indispensable for the replication of LuIII virus. According to digestion with nuclease S1, hybridization studies, and electron microscopy, RF DNA is a linear, double-stranded molecule comparable in length to mature ssDNA. It sedimented around 15S in neutral solution and banded at 1.714 g/ml in CsCl. Moreover, replication of LuIII DNA obviously includes a further replicative intermediate DNA which sedimented in front of RF DNA and bore single-stranded side-chains. Newly synthesized DNA disappeared from pools containing both RF DNA and replicative intermediate DNA within 5 min and reappeared in progeny virions only after 15 min. Intranuclear accumulation of significant amounts of progeny ssDNA could not be detected. It was postulated, therefore, that newly synthesized ssDNA is immediately enclosed in a stable maturation complex and resists extraction by the method of Hirt (1967).

Replication process of the parvovirus H-1 V. Isolation and characterization of temperature-sensitive H-1 mutants

Two temperature-sensitive mutants of the parvovirus H-1 have been isolated and characterized. These mutants are distinguishable by the immunofluorescent staining of cells infected by them and by the thermolability of their hemagglutinins. Under restrictive conditions both mutants synthesize a missense capsid protein defective in hemmaglutination. Synthesis of the viral DNA strand was shown to be diminished for both wild-type virus and these mutants at the restrictive temperature of 39.5 C, but the mutants were more defective than the wild type at the less restrictive temperature of 38 C. Replicative form DNA replication was not decreased in these mutants. It is proposed that H-1 requires one of the capsid proteins for the synthesis of the single-stranded progeny DNA.