Duplex strand joining reactions catalyzed by vaccinia virus DNA polymerase

Multiplex Expression Cassette Assembly: A flexible and versatile method for building complex genetic circuits in conventional vectors

The manipulation of multiple transcription units for simultaneous and coordinated expression is not only key to building complex genetic circuits to accomplish diverse functions in synthetic biology, but is also important in crop breeding for significantly improved productivity and overall performance. However, building constructs with multiple independent transcription units for fine-tuned and coordinated regulation is complicated and time-consuming. Here, we introduce the Multiplex Expression Cassette Assembly (MECA) method, which modifies canonical vectors compatible with Golden Gate Assembly, and then uses them to produce multi-cassette constructs. By embedding the junction syntax in primers that are used to amplify functional elements, MECA is able to make complex constructs using only one intermediate vector and one destination vector via two rounds of one-pot Golden Gate assembly reactions, without the need for dedicated vectors and a coherent library of standardized modules. As a proof-of-concept, we modified eukaryotic and prokaryotic expression vectors to generate constructs for transient expression of green fluorescent protein and β-glucuronidase in Nicotiana benthamiana, genome editing to block monoterpene metabolism in tomato glandular trichomes, production of betanin in tobacco and synthesis of β-carotene in Escherichia coli. Additionally, we engineered the stable production of thymol and carvacrol, bioactive compounds from Lamiaceae family plants, in glandular trichomes of tobacco. These results demonstrate that MECA is a flexible, efficient and versatile method for building complex genetic circuits, which will not only play a critical role in plant synthetic biology, but also facilitate improving agronomic traits and pyramiding traits for the development of next-generation elite crops.

Quick and affordable DNA cloning by reconstitution of Seamless Ligation Cloning Extract using defined factors

The cloning of DNA fragments to plasmid vectors is at the heart of molecular biology. Recent developments have led to various methods utilizing homologous recombination of homology arms. Among them, Seamless Ligation Cloning Extract (SLiCE) is an affordable alternative solution that uses simple Escherichia coli lysates. However, the underlying molecular mechanisms remain unclear and the reconstitution of the extract by defined factors has not yet been reported. We herein show that the key factor in SLiCE is Exonuclease III (ExoIII), a double-strand (ds) DNA-dependent 3'-5' exonuclease, encoded by XthA. SLiCE prepared from the xthAΔ strain is devoid of recombination activity, whereas purified ExoIII alone is sufficient to assemble two blunt-ended dsDNA fragments with homology arms. In contrast to SLiCE, ExoIII is unable to digest (or assemble) fragments with 3' protruding ends; however, the addition of single-strand DNA-targeting Exonuclease T overcomes this issue. Through the combination of commercially available enzymes under optimized conditions, we achieved the efficient, reproducible, and affordable cocktail, "XE cocktail," for seamless DNA cloning. By reducing the cost and time required for DNA cloning, researchers will devote more resources to advanced studies and the careful validation of their own findings.

Membrane-anchored substrate binding proteins are deployed in secondary TAXI transporters

Substrate-binding proteins (SBPs) are part of solute transport systems and serve to increase substrate affinity and uptake rates. In contrast to primary transport systems, the mechanism of SBP-dependent secondary transport is not well understood. Functional studies have thus far focused on Na⁺-coupled Tripartite ATP-independent periplasmic (TRAP) transporters for sialic acid. Herein, we report the in vitro functional characterization of TAXIPm-PQM from the human pathogen Proteus mirabilis. TAXIPm-PQM belongs to a TRAP-subfamily using a different type of SBP, designated TRAP-associated extracytoplasmic immunogenic (TAXI) protein. TAXIPm-PQM catalyzes proton-dependent α-ketoglutarate symport and its SBP is an essential component of the transport mechanism. Importantly, TAXIPm-PQM represents the first functionally characterized SBP-dependent secondary transporter that does not rely on a soluble SBP, but uses a membrane-anchored SBP instead.

Structure of monkeypox virus DNA polymerase holoenzyme

The World Health Organization declared mpox (or monkeypox) a public health emergency of international concern in July 2022, and prophylactic and therapeutic measures are in urgent need. The monkeypox virus (MPXV) has its own DNA polymerase F8, together with the processive cofactors A22 and E4, constituting the polymerase holoenzyme for genome replication. Here, we determined the holoenzyme structure in complex with DNA using cryo-electron microscopy at the global resolution of ~2.8 angstroms. The holoenzyme possesses an architecture that suggests a "forward sliding clamp" processivity mechanism for viral DNA replication. MPXV polymerase has a DNA binding mode similar to that of other B-family DNA polymerases from different species. These findings reveal the mechanism of the MPXV genome replication and may guide the development of anti-poxvirus drugs.

Capripoxviruses, leporipoxviruses, and orthopoxviruses: Occurrences of recombination

Poxviruses are double-stranded DNA viruses with several members displaying restricted host ranges. They are genetically stable with low nucleotide mutation rates compared to other viruses, due to the poxviral high-fidelity DNA polymerase. Despite the low accumulation of mutations per replication cycle, poxvirus genomes can recombine with each other to generate genetically rearranged viruses through recombination, a process directly associated with replication and the aforementioned DNA polymerase. Orthopoxvirus replication is intimately tethered to high frequencies of homologous recombination between co-infecting viruses, duplicated sequences of the same virus, and plasmid DNA transfected into poxvirus-infected cells. Unfortunately, the effect of these genomic alterations on the cellular context for all poxviruses across the family Poxviridae remains elusive. However, emerging sequence data on currently circulating and archived poxviruses, such as the genera orthopoxviruses and capripoxviruses, display a wide degree of divergence. This genetic variability cannot be explained by clonality or genetic drift alone, but are probably a result of significant genomic alterations, such as homologous recombination, gene loss and gain, or gene duplications as the major selection forces acting on viral progeny. The objective of this review is to cross-sectionally overview the currently available findings on natural and laboratory observations of recombination in orthopoxviruses, capripoxviruses, and leporipoxviruses, as well as the possible mechanisms involved. Overall, the reviewed available evidence allows us to conclude that the current state of knowledge is limited in terms of the relevance of genetic variations across even a genus of poxviruses as well as fundamental features governing and precipitating intrinsic gene flow and recombination events.

Poxvirus Recombination

Genetic recombination is used as a tool for modifying the composition of poxvirus genomes in both discovery and applied research. This review documents the history behind the development of these tools as well as what has been learned about the processes that catalyze virus recombination and the links between it and DNA replication and repair. The study of poxvirus recombination extends back to the 1930s with the discovery that one virus can reactivate another by a process later shown to generate recombinants. In the years that followed it was shown that recombinants can be produced in virus-by-virus crosses within a genus (e.g., variola-by-rabbitpox) and efforts were made to produce recombination-based genetic maps with modest success. The marker rescue mapping method proved more useful and led to methods for making genetically engineered viruses. Many further insights into the mechanism of recombination have been provided by transfection studies which have shown that this is a high-frequency process associated with hybrid DNA formation and inextricably linked to replication. The links reflect the fact that poxvirus DNA polymerases, specifically the vaccinia virus E9 enzyme, can catalyze strand transfer in in vivo and in vitro reactions dependent on the 3'-to-5' proofreading exonuclease and enhanced by the I3 replicative single-strand DNA binding protein. These reactions have shaped the composition of virus genomes and are modulated by constraints imposed on virus-virus interactions by viral replication in cytoplasmic factories. As recombination reactions are used for replication fork assembly and repair in many biological systems, further study of these reactions may provide new insights into still poorly understood features of poxvirus DNA replication.

Single 3′-exonuclease-based multifragment DNA assembly method (SENAX)

DNA assembly is a vital process in biotechnology and synthetic biology research, during which DNA plasmids are designed and constructed using bioparts to engineer microorganisms for a wide range of applications. Here, we present an enzymatic homology-based DNA assembly method, SENAX (Stellar ExoNuclease Assembly miX), that can efficiently assemble multiple DNA fragments at ambient temperature from 30 to 37 °C and requires homology overlap as short as 12–18 base pairs. SENAX relies only on a 3′–5′ exonuclease, XthA (ExoIII), followed by Escherichia coli transformation, enabling easy scaling up and optimization. Importantly, SENAX can efficiently assemble short fragments down to 70 bp into a vector, overcoming a key shortcoming of existing commonly used homology-based technologies. To the best of our knowledge, this has not been reported elsewhere using homology-based methods. This advantage leads us to develop a framework to perform DNA assembly in a more modular manner using reusable promoter-RBS short fragments, simplifying the construction process and reducing the cost of DNA synthesis. This approach enables commonly used short bioparts (e.g., promoter, RBS, insulator, terminator) to be reused by the direct assembly of these parts into intermediate constructs. SENAX represents a novel accurate, highly efficient, and automation-friendly DNA assembly method.

Generation of a microRNA-Regulated Oncolytic Coxsackievirus B3

The members of the picornavirus family include various viruses which, due to their impressive oncolytic activity, have the potential to be used for the treatment of cancer. However, the replication of these oncolytic viruses (OV) is not limited to tumor cells but can also take place in various normal tissues. To increase the safety of these OV, target sites (miR-TS) of microRNAs, which are expressed in normal tissues but are absent or only expressed at low levels in cancer cells, can be inserted into the viral genome. Here we describe how miR-TS can easily be inserted into the complementary DNA (cDNA) of coxsackievirus B3 (CVB3) RNA genome using the In-Fusion cloning technology. Here we provide the step-by-step protocol, how miR-TS containing recombinant CVB3 can be generated from these viral cDNA constructs, how the virus is amplified, purified and concentrated, and how the functionality of the miR-TS within the viral genome can be confirmed.

A Guide to Cloning the Products of Polymerase Chain Reactions

This introduction outlines various methods to clone amplified DNAs and to facilitate the construction of complex multicomponent genetic units. Because of the ease with which the termini of amplified DNAs can be tailored by polymerase chain reaction (PCR), many of the methods outlined here use PCR not only to synthesize DNAs but also to link them together into purpose-designed constructs. The most recent refinements however have been the development of modular genetic units that can be harnessed to target DNAs not by PCR but by site-specific recombination enzymes.

Vaccinia Virus Gene Acquisition through Nonhomologous Recombination

Many of the genes encoded by poxviruses are orthologs of cellular genes. These virus genes serve different purposes, but perhaps of most interest is the way some have been repurposed to inhibit the antiviral pathways that their cellular homologs still regulate. What is unclear is how these virus genes were acquired, although it is presumed to have been catalyzed by some form(s) of nonhomologous recombination (NHR). We used transfection assays and substrates encoding a fluorescent and drug-selectable marker to examine the NHR frequency in vaccinia virus (VAC)-infected cells. These studies showed that when cells were transfected with linear duplex DNAs bearing VAC N2L gene homology, it yielded a recombinant frequency (RF) of 6.7 × 10-4. In contrast, DNA lacking any VAC homology reduced the yield of recombinants ∼400-fold (RF = 1.6 × 10-6). DNA-RNA hybrids were also substrates, although homologous molecules yielded fewer recombinants (RF = 2.1 × 10-5), and nonhomologous substrates yielded only rare recombinants (RF ≤ 3 × 10-8). NHR was associated with genome rearrangements ranging from simple insertions with flanking sequence duplications to large-scale indels that produced helper-dependent viruses. The insert was often also partially duplicated and would rapidly rearrange through homologous recombination. Most of the virus-insert junctions exhibited little or no preexiting microhomology, although a few encoded VAC topoisomerase recognition sites (C/T·CCTT). These studies show that VAC can catalyze NHR through a process that may reflect a form of aberrant replication fork repair. Although it is less efficient than classical homologous recombination, the rates of NHR may still be high enough to drive virus evolution. IMPORTANCE Large DNA viruses sometimes interfere in antiviral defenses using repurposed and mutant forms of the cellular proteins that mediate these same reactions. Such virus orthologs of cellular genes were presumably captured through nonhomologous recombination, perhaps in the distant past, but nothing is known about the processes that might promote "gene capture" or even how often these events occur over the course of an infectious cycle. This study shows that nonhomologous recombination in vaccinia virus-infected cells is frequent enough to seed a small but still significant portion of novel recombinants into large populations of newly replicated virus particles. This offers a route by which a pool of virus might survey the host genome for sequences that offer a selective growth advantage and potentially drive discontinuous virus evolution (saltation) through the acquisition of adventitious traits.

Rapid poxvirus engineering using CRISPR/Cas9 as a selection tool

In standard uses of CRISPR/Cas9 technology, the cutting of genomes and their efficient repair are considered to go hand-in-hand to achieve desired genetic changes. This includes the current approach for engineering genomes of large dsDNA viruses. However, for poxviruses we show that Cas9-guide RNA complexes cut viral genomes soon after their entry into cells, but repair of these breaks is inefficient. As a result, Cas9 targeting makes only modest, if any, improvements to basal rates of homologous recombination between repair constructs and poxvirus genomes. Instead, Cas9 cleavage leads to inhibition of poxvirus DNA replication thereby suppressing virus spread in culture. This unexpected outcome allows Cas9 to be used as a powerful tool for selecting conventionally generated poxvirus recombinants, which are otherwise impossible to separate from a large background of parental virus without the use of marker genes. This application of CRISPR/Cas9 greatly speeds up the generation of poxvirus-based vaccines, making this platform considerably more attractive in the context of personalised cancer vaccines and emerging disease outbreaks.

Gowripalan, Smith et al. use CRISPR/Cas9 technology to rapidly select recombinant poxviruses without using selectable marker genes. They find that Cas9 cleavage inhibits poxvirus DNA replication, suppressing virus spread in culture. This application makes poxviruses more attractive vector platforms for fighting cancer and emerging disease outbreaks.

Validation of a site-specific recombination cloning technique for the rapid development of a full-length cDNA clone of a virulent field strain of vesicular stomatitis New Jersey virus

This study reports the use of a site-specific recombination cloning technique for rapid development of a full-length cDNA clone that can produce infectious vesicular stomatitis New Jersey virus (VSNJV). The full-length genome of the epidemic VSNJV NJ0612NME6 strain was amplified in four overlapping cDNA fragments which were linked together and cloned into a vector plasmid by site-specific recombination. Furthermore, to derive infectious virus, three supporting plasmid vectors containing either the nucleoprotein (N), phosphoprotein (P) or polymerase (L) genes were constructed using the same cloning methodology. Recovery of recombinant VSNJV was achieved after transfecting all four vectors on into BSR-T7/5 cells, a BHK-derived cell line stably expressing T7 RNA polymerase (PMID: 9847328). In vitro characterization of recombinant and parental viruses revealed similar growth kinetics and plaque morphologies. Furthermore, experimental infection of pigs with the recombinant virus resulted in severe vesicular stomatitis with clinical signs similar to those previously reported for the parental field strain. These results validate the use of site-directed specific recombination cloning as a useful alternative method for rapid construction of stable full-length cDNA clones from vesicular stomatitis field strains. The approach reported herein contributes to the improvement of previously published methodologies for the development of full-length cDNA clones of this relevant virus.

Identification of Vaccinia Virus Replisome and Transcriptome Proteins by Isolation of Proteins on Nascent DNA Coupled with Mass Spectrometry

Poxviruses replicate within the cytoplasm and encode proteins for DNA and mRNA synthesis. To investigate poxvirus replication and transcription from a new perspective, we incorporated 5-ethynyl-2'-deoxyuridine (EdU) into nascent DNA in cells infected with vaccinia virus (VACV). The EdU-labeled DNA was conjugated to fluor- or biotin-azide and visualized by confocal, superresolution, and transmission electron microscopy. Nuclear labeling decreased dramatically after infection, accompanied by intense labeling of cytoplasmic foci. The nascent DNA colocalized with the VACV single-stranded DNA binding protein I3 in multiple puncta throughout the interior of factories, which were surrounded by endoplasmic reticulum. Complexes containing EdU-biotin-labeled DNA cross-linked to proteins were captured on streptavidin beads. After elution and proteolysis, the peptides were analyzed by mass spectrometry to identify proteins associated with nascent DNA. The known viral replication proteins, a telomere binding protein, and a protein kinase were associated with nascent DNA, as were the DNA-dependent RNA polymerase and intermediate- and late-stage transcription initiation and elongation factors, plus the capping and methylating enzymes. These results suggested that the replicating pool of DNA is transcribed and that few if any additional viral proteins directly engaged in replication and transcription remain to be discovered. Among the host proteins identified by mass spectrometry, topoisomerases IIα and IIβ and PCNA were noteworthy. The association of the topoisomerases with nascent DNA was dependent on expression of the viral DNA ligase, in accord with previous proteomic studies. Further investigations are needed to determine possible roles for PCNA and other host proteins detected.IMPORTANCE Poxviruses, unlike many well-characterized animal DNA viruses, replicate entirely within the cytoplasm of animal cells, raising questions regarding the relative roles of viral and host proteins. We adapted newly developed procedures for click chemistry and iPOND (Isolation of proteins on nascent DNA) to investigate vaccinia virus (VACV), the prototype poxvirus. Nuclear DNA synthesis ceased almost immediately following VACV infection, followed swiftly by the synthesis of viral DNA within discrete cytoplasmic foci. All viral proteins known from genetic and proteomic studies to be required for poxvirus DNA replication were identified in the complexes containing nascent DNA. The additional detection of the viral DNA-dependent RNA polymerase and intermediate and late transcription factors provided evidence for a temporal coupling of replication and transcription. Further studies are needed to assess the potential roles of host proteins, including topoisomerases IIα and IIβ and PCNA, which were found associated with nascent DNA.

Seamless Ligation Cloning Extract (SLiCE) Method Using Cell Lysates from Laboratory Escherichia coli Strains and its Application to SLiP Site-Directed Mutagenesis

Cell lysates from laboratory Escherichia coli strains endogenously exhibit homologous recombination activity, which can be utilized for seamless DNA cloning in vitro. This method, termed Seamless Ligation Cloning Extract (SLiCE) cloning, enables high cloning efficiency with simultaneous integration of two unpurified DNA fragments into a vector. In addition, the SLiCE method is highly cost-effective, as several laboratory E. coli strains may be utilized as sources of SLiCE. Previously, the SLiCE technique has been applied to site-directed mutagenesis to develop a novel technique termed SLiCE-mediated polymerase chain reaction (PCR)-based site-directed mutagenesis (SLiP site-directed mutagenesis). Two DNA fragments containing a mutation site can be simultaneously integrated into a vector while avoiding the introduction of undesirable mutations in the vector. Therefore, SLiP site-directed mutagenesis simplifies multiple procedures involved in PCR-based site-directed mutagenesis such as overlap extension method PCR or the Megaprimer method.

Live-Cell Imaging of Vaccinia Virus Recombination

Recombination between co-infecting poxviruses provides an important mechanism for generating the genetic diversity that underpins evolution. However, poxviruses replicate in membrane-bound cytoplasmic structures known as factories or virosomes. These are enclosed structures that could impede DNA mixing between co-infecting viruses, and mixing would seem to be essential for this process. We hypothesize that virosome fusion events would be a prerequisite for recombination between co-infecting poxviruses, and this requirement could delay or limit viral recombination. We have engineered vaccinia virus (VACV) to express overlapping portions of mCherry fluorescent protein fused to a cro DNA-binding element. In cells also expressing an EGFP-cro fusion protein, this permits live tracking of virus DNA and genetic recombination using confocal microscopy. Our studies show that different types of recombination events exhibit different timing patterns, depending upon the relative locations of the recombining elements. Recombination between partly duplicated sequences is detected soon after post-replicative genes are expressed, as long as the reporter gene sequences are located in cis within an infecting genome. The same kinetics are also observed when the recombining elements are divided between VACV and transfected DNA. In contrast, recombination is delayed when the recombining sequences are located on different co-infecting viruses, and mature recombinants aren’t detected until well after late gene expression is well established. The delay supports the hypothesis that factories impede inter-viral recombination, but even after factories merge there remain further constraints limiting virus DNA mixing and recombinant gene assembly. This delay could be related to the continued presence of ER-derived membranes within the fused virosomes, membranes that may once have wrapped individual factories.

Recombination plays a critical role in DNA repair and also creates the genetic diversity that underpins evolution. This has important implications for viruses, since recombination may create new pathogens with new infectious properties. It has long been known that hybrids can be recovered from cells co-infected with related viruses, some of the first artificial recombinants were produced >50 years ago from variola and rabbitpox viruses. A particular property of poxviruses is that they replicate in membrane-wrapped cytoplasmic structures called “factories”, and each of these factories develops from a single infecting particle. However, if each genome is isolated inside different factories, when and how does the DNA mix to permit recombination? To examine this question, we have developed a fluorescence-based virus recombination assay. Using live cell confocal microscopy, we have timed these reactions and observed that recombinants can be quickly formed when the recombining sequences are located on the same virus genome. However, when the gene fragments are located on different viruses, there is a significant delay (and a reduction) in recombinant gene formation. This delay supports the hypothesis that factories, and the ER-derived cell membranes that surround factories, impede recombination in poxvirus-infected cells.

Zero-Background Helicase-Dependent Amplification and Its Application to Reliable Assay of Telomerase Activity in Cancer Cell by Eliminating Primer-Dimer Artifacts

Primer-dimer artifacts resulting from unintended template-independent primer-primer interactions often hinder the specific amplification of nucleic acids. We demonstrate, for the first time, zero-background helicase-dependent amplification (HDA), with low concentrations of both ATP and dNTPs. This strategy achieved the reliable evaluation of telomerase activity in cancer cells by eliminating primer-dimer artifacts, which have plagued many previous methods with reduced specificity. We found that the performance of the telomerase assay by zero-background HDA was negatively affected by highly concentrated cellular proteins. This inhibitory effect is attributed to the binding of DNA templates to proteins, thus making them unavailable for polymerases. However, gold nanoparticles were demonstrated to highly attenuate such inhibition by abundant proteins, and to enhance the assay sensitivity and reliability when the reaction was performed with concentrated cell extracts.

Biophysical Characterization of a Vaccine Candidate against HIV-1: The Transmembrane and Membrane Proximal Domains of HIV-1 gp41 as a Maltose Binding Protein Fusion

The membrane proximal region (MPR, residues 649-683) and transmembrane domain (TMD, residues 684-705) of the gp41 subunit of HIV-1's envelope protein are highly conserved and are important in viral mucosal transmission, virus attachment and membrane fusion with target cells. Several structures of the trimeric membrane proximal external region (residues 662-683) of MPR have been reported at the atomic level; however, the atomic structure of the TMD still remains unknown. To elucidate the structure of both MPR and TMD, we expressed the region spanning both domains, MPR-TM (residues 649-705), in Escherichia coli as a fusion protein with maltose binding protein (MBP). MPR-TM was initially fused to the C-terminus of MBP via a 42 aa-long linker containing a TEV protease recognition site (MBP-linker-MPR-TM). Biophysical characterization indicated that the purified MBP-linker-MPR-TM protein was a monodisperse and stable candidate for crystallization. However, crystals of the MBP-linker-MPR-TM protein could not be obtained in extensive crystallization screens. It is possible that the 42 residue-long linker between MBP and MPR-TM was interfering with crystal formation. To test this hypothesis, the 42 residue-long linker was replaced with three alanine residues. The fusion protein, MBP-AAA-MPR-TM, was similarly purified and characterized. Significantly, both the MBP-linker-MPR-TM and MBP-AAA-MPR-TM proteins strongly interacted with broadly neutralizing monoclonal antibodies 2F5 and 4E10. With epitopes accessible to the broadly neutralizing antibodies, these MBP/MPR-TM recombinant proteins may be in immunologically relevant conformations that mimic a pre-hairpin intermediate of gp41.

A simple and efficient seamless DNA cloning method using SLiCE from Escherichia coli laboratory strains and its application to SLiP site-directed mutagenesis

Background

Seamless ligation cloning extract (SLiCE) is a simple and efficient method for DNA assembly that uses cell extracts from the Escherichia coli PPY strain, which expresses the components of the λ prophage Red/ET recombination system. This method facilitates restriction endonuclease cleavage site-free DNA cloning by performing recombination between short stretches of homologous DNA (≥15 base pairs).

Results

To extend the versatility of this system, I examined whether, in addition to bacterial extracts from the PPY strain, other E. coli laboratory strains were suitable for the SLiCE protocol. Indeed, carefully prepared cell extracts from several strains exhibited sufficient cloning activity for seamless gene incorporation into vectors with short homology lengths (approximately 15–20 bp). Furthermore, SLiCE was applied to the polymerase chain reaction (PCR)-based site-directed mutagenesis method, in a process termed “SLiCE-mediated PCR-based site-directed mutagenesis (SLiP site-directed mutagenesis)”. SLiP site-directed mutagenesis simplifies the steps of PCR-based site-directed mutagenesis, as it exploits the capability of the SLiCE method to insert multiple fragments.

Conclusions

SLiCE can be performed in the laboratory with no requirement for a special E. coli strain, and the technique is easily established. This method increases the cloning efficiency, shortens the time for DNA manipulation, and greatly reduces the cost of seamless DNA cloning.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-015-0162-8) contains supplementary material, which is available to authorized users.

Site-specific recombinational cloning using gateway and in-fusion cloning schemes

The comprehensive study of protein structure and function, or proteomics, depends on the obtainability of full-length cDNAs in species-specific expression vectors and subsequent functional analysis of the expressed protein. Recombinational cloning is a universal cloning technique based on site-specific recombination that is independent of the insert DNA sequence of interest, which differentiates it from classical restriction enzyme-based cloning methods. Recombinational cloning enables rapid and efficient parallel transfer of DNA inserts into multiple expression systems. This unit summarizes strategies for generating expression-ready clones using the most popular commercial recombinational cloning technologies, Gateway (Life Technologies) and In-Fusion (Clontech).

Subversion of cellular stress responses by poxviruses

Cellular stress responses are powerful mechanisms that prevent and cope with the accumulation of macromolecular damage in the cells and also boost host defenses against pathogens. Cells can initiate either protective or destructive stress responses depending, to a large extent, on the nature and duration of the stressing stimulus as well as the cell type. The productive replication of a virus within a given cell places inordinate stress on the metabolism machinery of the host and, to assure the continuity of its replication, many viruses have developed ways to modulate the cell stress responses. Poxviruses are among the viruses that have evolved a large number of strategies to manipulate host stress responses in order to control cell fate and enhance their replicative success. Remarkably, nearly every step of the stress responses that is mounted during infection can be targeted by virally encoded functions. The fine-tuned interactions between poxviruses and the host stress responses has aided virologists to understand specific aspects of viral replication; has helped cell biologists to evaluate the role of stress signaling in the uninfected cell; and has tipped immunologists on how these signals contribute to alert the cells against pathogen invasion and boost subsequent immune responses. This review discusses the diverse strategies that poxviruses use to subvert host cell stress responses.

Application of In-Fusion™ cloning for the parallel construction of E. coli expression vectors

In-Fusion™ cloning is a flexible DNA ligase-independent cloning technology that has wide-ranging uses in molecular biology. In this chapter we describe the protocols used in the OPPF-UK to design and construct expression vectors using In-Fusion™. Our method for small scale expression screening in Escherichia coli of constructs generated by In-Fusion™ is also outlined.

A novel approach to the generation of seamless constructs for plant transformation

BACKGROUND

When creating plant transformation vectors, full control of nucleotides flanking the insert in the final construct may be desirable. Modern ligase-independent methods for DNA-recombination are based on linearization by classical type II restriction endonucleases (REs) alone or in combination with nicking enzymes leaving residual nucleotides behind in the final construct. We here explore the use of type IIS and type IIB REs for vector linearization that combined with sequence and ligase-independent cloning (SLIC) overcomes this problem and promotes seamless gene-insertion in vectors. Providing the basis for a collection of biolistic plant transformation vectors ready to be cloned with different genes-of-interest, we present two vectors, where promoter and terminator are joined by a spacer. During spacer-removal linearization (SRL), type IIS and type IIB REs remove their own recognition sequences from the vector leaving no undesired, short sequences behind.

RESULTS

We designed two plant transformation vectors prepared for SRL in combination with SLIC, pAUrumII and pAUrumIII, harboring a spacer with recognition sites for a type IIS and IIB RE, respectively. The gene for a green fluorescent protein, gfp, was successfully cloned into both vectors; traces of pAUrumIII, however, contaminated the transformation due to incomplete linearization, an issue not encountered with the type IIS linearized pAUrumII. Both constructs, pAUrumII-gfp and pAUrumIII-gfp, were functional, when tested in vitro on wheat and barley endosperm cells for transient gfp expression.

CONCLUSIONS

All nucleotides flanking an insert in a biolistic plant transformation vector can be customized by means of SRL in combination with SLIC. Especially type IIS REs promote an efficient cloning result. Based on our findings, we believe that the SRL system can be useful in a series of plant transformation vectors, favoring the presence of functional sequences for optimal expression over redundant cloning-site remnants.

Non-enzymatic DNA cleavage reaction induced by 5-ethynyluracil in methylamine aqueous solution and application to DNA concatenation

DNA can be concatenated by hybridization of DNA fragments with protruding single-stranded termini. DNA cleavage occurring at a nucleotide containing a DNA base analogue is a useful method to obtain DNA with designed protruding termini. Here, we report a novel non-enzymatic DNA cleavage reaction for DNA concatenation. We found that DNA is cleaved at a nucleotide containing 5-ethynyluracil in a methylamine aqueous solution to generate 5′-phosphorylated DNA fragment as a cleavage product. We demonstrated that the reaction can be applied to DNA concatenation of PCR-amplified DNA fragments. This novel non-enzymatic DNA cleavage reaction is a simple practical approach for DNA concatenation.

Retroviral vectors for homologous recombination provide efficient cloning and expression in mammalian cells

Homologous recombination technologies enable high-throughput cloning and the seamless insertion of any DNA fragment into expression vectors. Additionally, retroviral vectors offer a fast and efficient method for transducing and expressing genes in mammalian cells, including lymphocytes. However, homologous recombination cannot be used to insert DNA fragments into retroviral vectors; retroviral vectors contain two homologous regions, the 5'- and 3'-long terminal repeats, between which homologous recombination occurs preferentially. In this study, we have modified a retroviral vector to enable the cloning of DNA fragments through homologous recombination. To this end, we inserted a bacterial selection marker in a region adjacent to the gene insertion site. We used the modified retroviral vector and homologous recombination to clone T-cell receptors (TCRs) from single Epstein Barr virus-specific human T cells in a high-throughput and comprehensive manner and to efficiently evaluate their function by transducing the TCRs into a murine T-cell line through retroviral infection. In conclusion, the modified retroviral vectors, in combination with the homologous recombination method, are powerful tools for the high-throughput cloning of cDNAs and their efficient functional analysis.

A simplified positive-sense-RNA virus construction approach that enhances analysis throughput

Here we present an approach that advances the throughput of a genetic analysis of a positive-sense RNA virus by simplifying virus construction. It enabled comprehensive dissection of a complex, multigene phenotype through rapid derivation of a large number of chimeric viruses and construction of a mutant library directly from a virus pool. The versatility of the approach described here expands the applicability of diverse genetic approaches to study these viruses.

Plasmid vectors and molecular building blocks for the development of genetic manipulation tools for Trypanosoma cruzi

The post genomic era revealed the need for developing better performing, easier to use and more sophisticated genetic manipulation tools for the study of Trypanosoma cruzi, the etiological agent of Chagas disease. In this work a series of plasmids that allow genetic manipulation of this protozoan parasite were developed. First of all we focused on useful tools to establish selection strategies for different strains and which can be employed as expression vectors. On the other hand molecular building blocks in the form of diverse selectable markers, modifiable fluorescent protein and epitope-tag coding sequences were produced. Both types of modules were harboured in backbone molecules conceived to offer multiple construction and sub-cloning strategies. These can be used to confer new properties to already available genetic manipulation tools or as starting points for whole novel designs. The performance of each plasmid and building block was determined independently. For illustration purposes, some simple direct practical applications were conducted.

Bridging the past and the future of virology: surface plasmon resonance as a powerful tool to investigate virus/host interactions

Despite decades of antiviral drug research and development, viruses still remain a top global healthcare problem. Compared to eukaryotic cells, viruses are composed by a limited numbers of proteins that, nevertheless, set up multiple interactions with cellular components, allowing the virus to take control of the infected cell. Each virus/host interaction can be considered as a therapeutical target for new antiviral drugs but, unfortunately, the systematic study of a so huge number of interactions is time-consuming and expensive, calling for models overcoming these drawbacks. Surface plasmon resonance (SPR) is a label-free optical technique to study biomolecular interactions in real time by detecting reflected light from a prism-gold film interface. Launched 20 years ago, SPR has become a nearly irreplaceable technology for the study of biomolecular interactions. Accordingly, SPR is increasingly used in the field of virology, spanning from the study of biological interactions to the identification of putative antiviral drugs. From the literature available, SPR emerges as an ideal link between conventional biological experimentation and system biology studies functional to the identification of highly connected viral or host proteins that act as nodal points in virus life cycle and thus considerable as therapeutical targets for the development of innovative antiviral strategies.

Poxvirus DNA replication

Poxviruses are large, enveloped viruses that replicate in the cytoplasm and encode proteins for DNA replication and gene expression. Hairpin ends link the two strands of the linear, double-stranded DNA genome. Viral proteins involved in DNA synthesis include a 117-kDa polymerase, a helicase-primase, a uracil DNA glycosylase, a processivity factor, a single-stranded DNA-binding protein, a protein kinase, and a DNA ligase. A viral FEN1 family protein participates in double-strand break repair. The DNA is replicated as long concatemers that are resolved by a viral Holliday junction endonuclease.

FX cloning: a versatile high-throughput cloning system for characterization of enzyme variants

Methods for the cloning of large numbers of open reading frames (ORFs) into expression vectors are of critical importance for diverse disciplines in biology. Here I describe a system termed FX cloning that facilitates the high-throughput generation of expression constructs. FX cloning combines attractive features of established recombination- and single-strand-annealing-based cloning methods that were thus far not unified in one single method. FX cloning allows the straightforward transfer of a sequence-verified ORF to a variety of expression vectors, and it avoids the common but undesirable feature of significantly extending target ORFs with cloning-related sequences. It leaves a minimal seam of only a single amino acid to either side of the protein. Furthermore, FX cloning is highly efficient and economic in its use. The method is based on a class IIS restriction enzyme and negative selection markers. The full procedure takes place in one pot and does not require intermediate purifications. The method has proven to be very robust and suitable for all common pro- and eukaryotic expression systems.

One-step sequence- and ligation-independent cloning as a rapid and versatile cloning method for functional genomics studies

We developed one-step sequence- and ligation-independent cloning (SLIC) as a simple, cost-effective, time-saving, and versatile cloning method. Highly efficient and directional cloning can be achieved by direct bacterial transformation 2.5 min after mixing any linearized vector, an insert(s) prepared by PCR, and T4 DNA polymerase in a tube at room temperature.

Bulged DNA substrates for identifying poxvirus resolvase inhibitors

Resolvase enzymes that cleave DNA four-way (Holliday) junctions are required for poxvirus replication, but clinically useful inhibitors have not been developed. Here, we report an assay for resolvase cleavage activity based on fluorescence polarization (FP) for high-throughput screening and mechanistic studies. Initial analysis showed that cleavage of a fluorescently labeled Holliday junction substrate did not yield an appreciable change in FP, probably because the cleavage product did not have sufficiently increased mobility to yield a strong FP signal. Iterative optimization yielded a substrate with an off-center DNA bulge, which after cleavage released a labeled short stand and yielded a greatly reduced FP signal. Using this assay, 133,000 compounds were screened, identifying 1-hydroxy-1,8-naphthyridin-2(1H)-one compounds as inhibitors. Structure-activity studies revealed functional parallels to Food and Drug Administration (FDA)-approved drugs targeting the related human immunodeficiency virus integrase enzyme. Some 1-hydroxy-1,8-naphthyridin-2(1H)-one compounds showed anti-poxvirus activity.

Orthopoxvirus targets for the development of new antiviral agents

Investments in the development of new drugs for orthopoxvirus infections have fostered new avenues of research, provided an improved understanding of orthopoxvirus biology and yielded new therapies that are currently progressing through clinical trials. These broad-based efforts have also resulted in the identification of new inhibitors of orthopoxvirus replication that target many different stages of viral replication cycle. This review will discuss progress in the development of new anti-poxvirus drugs and the identification of new molecular targets that can be exploited for the development of new inhibitors. The prototype of the orthopoxvirus group is vaccinia virus and its replication cycle will be discussed in detail noting specific viral functions and their associated gene products that have the potential to serve as new targets for drug development. Progress that has been achieved in recent years should yield new drugs for the treatment of these infections and might also reveal new approaches for antiviral drug development with other viruses.

SLiCE: a novel bacterial cell extract-based DNA cloning method

We describe a novel cloning method termed SLiCE (Seamless Ligation Cloning Extract) that utilizes easy to generate bacterial cell extracts to assemble multiple DNA fragments into recombinant DNA molecules in a single in vitro recombination reaction. SLiCE overcomes the sequence limitations of traditional cloning methods, facilitates seamless cloning by recombining short end homologies (≥15 bp) with or without flanking heterologous sequences and provides an effective strategy for directional subcloning of DNA fragments from Bacteria Artificial Chromosomes (BACs) or other sources. SLiCE is highly cost effective as a number of standard laboratory bacterial strains can serve as sources for SLiCE extract. In addition, the cloning efficiencies and capabilities of these strains can be greatly improved by simple genetic modifications. As an example, we modified the DH10B Escherichia coli strain to express an optimized λ prophage Red recombination system. This strain, termed PPY, facilitates SLiCE with very high efficiencies and demonstrates the versatility of the method.

Recombination-based DNA assembly and mutagenesis methods for metabolic engineering

In recent years there has been a growing interest in the precise and concerted assembly of multiple DNA fragments of diverse sizes, including chromosomes, and the fine tuning of gene expression levels and protein activity. Commercial DNA assembly solutions have not been conceived to support the cloning of very large or very small genetic elements or a combination of both. Here we summarize a series of protocols that allow the seamless, simultaneous, flexible, and highly efficient assembly of DNA elements of a wide range of sizes (up to hundred thousand base pairs). The protocols harness the power of homologous recombination and are performed either in vitro or within the living cells. The DNA fragments may or may not share homology at their ends. An efficient site-directed mutagenesis protocol enhanced by homologous recombination is also described.

Assembling DNA fragments by USER fusion

Recent advances in DNA synthesis technology make it possible to design and synthesize DNA fragments of several kb in size. However, the process of assembling the smaller DNA fragments into a larger DNA segment is still a cumbersome process. In this chapter, we describe the use of the uracil specific excision reaction (USER)-mediated approach for rapid and efficient assembly of multiple DNA fragments both in vitro and in vivo (using Escherichia coli). For USER fusion in vitro assembly, each of the individual building blocks (BBs), 0.75 kb in size (that are to be assembled), was amplified using the appropriate forward and reverse primers containing a single uracil (U) and DNA polymerase. The overlaps between adjoining BBs were 8-13 base pairs. An equimolar of the amplified BBs were mixed together and treated by USER enzymes to generate complementary 3' single-strand overhangs between adjoining BBs, which were then ligated and amplified simultaneously to generate the larger 3-kb segments. The assembled fragments were then cloned into plasmid vectors and sequenced to confirm their identity. For USER fusion in vivo assembly in E. coli, USER treatment of the BBs was performed in the presence of a synthetic plasmid, which had 8-13 base pair overlaps at the 5'-end of the 5' BB and at the 3'-end of the 3' BB in the mixture. The USER treated product was then transformed directly into E. coli to efficiently and correctly reconstitute the recombinant plasmid containing the desired target insert. The latter approach was also used to rapidly assemble three different target genes into a vector to form a new synthetic plasmid construct.

Targeted gene replacement in fungal pathogens via Agrobacterium tumefaciens- mediated transformation

Genome sequence data on fungal pathogens provide the opportunity to carry out a reverse genetics approach to uncover gene function. Efficient methods for targeted genome modifications such as knockout and in locus over-expression are in high demand. Here we describe two efficient single-step cloning strategies for construction of vectors for Agrobacterium tumefaciens-mediated transformation (ATMT). Targeted genome modifications require integration by a homologous double crossover event, which is achieved by placing target sequences on either side of a selection marker gene in the vector. Protocols are given for two single-step vector construction techniques. The In-Fusion cloning technique is independent of compatible restriction enzyme sites in the vector and the fragment to be cloned. The method can be directly applied to any vector of choice and it is possible to carry out four fragment cloning without the need for subcloning. The cloning efficiency is not always as high as desired, but it still presents an efficient alternative to restriction enzyme and ligase-based cloning systems. The USER technology offers a higher four fragment cloning efficiency than In-Fusion, but depends on specific structures in the binary vector. The available fungal binary vectors adapted for the USER system are described and protocols are provided for vector design and construction. A general protocol for verification of the resulting gene replacement events in the recipient fungal cells is also given. The cloning systems described above are relevant for all transformation vector constructs, but here we describe their application for ATMT compatible binary vectors. Protocols are provided for ATMT exemplified by Fusarium graminearum. For large-scale reverse genetic projects, the USER technology is recommended combined with ATMT.

In-fusion® cloning with vaccinia virus DNA polymerase

Vaccinia virus DNA polymerase (VVpol) encodes a 3'-to-5' proofreading exonuclease that can degrade the ends of duplex DNA and expose single-stranded DNA tails. The reaction plays a critical role in promoting virus recombination in vivo because single-strand annealing reactions can then fuse molecules sharing complementary tails into recombinant precursors called joint molecules. We have shown that this reaction can also occur in vitro, providing a simple method for the directional cloning of PCR products into any vector of interest. A commercial form of this recombineering technology called In-Fusion(®) that facilitates high-throughput directional cloning of PCR products has been commercialized by Clontech. To effect the in vitro cloning reaction, PCR products are prepared using primers that add 16-18 bp of sequence to each end of the PCR amplicon that are homologous to the two ends of a linearized vector. The linearized vector and PCR products are coincubated with VVpol, which exposes the complementary ends and promotes joint molecule formation. Vaccinia virus single-stranded DNA binding protein can be added to enhance this reaction, although it is not an essential component. The resulting joint molecules are used to transform E. coli, which convert these noncovalently joined molecules into stable recombinants. We illustrate how this technology works by using, as an example, the cloning of the vaccinia N2L gene into the vector pETBlue-2.

Mimivirus reveals Mre11/Rad50 fusion proteins with a sporadic distribution in eukaryotes, bacteria, viruses and plasmids

BACKGROUND

The Mre11/Rad50 complex and the homologous SbcD/SbcC complex in bacteria play crucial roles in the metabolism of DNA double-strand breaks, including DNA repair, genome replication, homologous recombination and non-homologous end-joining in cellular life forms and viruses. Here we investigated the amino acid sequence of the Mimivirus R555 gene product, originally annotated as a Rad50 homolog, and later shown to have close homologs in marine microbial metagenomes.

RESULTS

Our bioinformatics analysis revealed that R555 protein sequence is constituted from the fusion of an N-terminal Mre11-like domain with a C-terminal Rad50-like domain. A systematic database search revealed twelve additional cases of Mre11/Rad50 (or SbcD/SbcC) fusions in a wide variety of unrelated organisms including unicellular and multicellular eukaryotes, the megaplasmid of a bacterium associated to deep-sea hydrothermal vents (Deferribacter desulfuricans) and the plasmid of Clostridium kluyveri. We also showed that R555 homologs are abundant in the metagenomes from different aquatic environments and that they most likely belong to aquatic viruses. The observed phyletic distribution of these fusion proteins suggests their recurrent creation and lateral gene transfers across organisms.

CONCLUSIONS

The existence of the fused version of protein sequences is consistent with known functional interactions between Mre11 and Rad50, and the gene fusion probably enhanced the opportunity for lateral transfer. The abundance of the Mre11/Rad50 fusion genes in viral metagenomes and their sporadic phyletic distribution in cellular organisms suggest that viruses, plasmids and transposons played a crucial role in the formation of the fusion proteins and their propagation into cellular genomes.

Genetic assembly tools for synthetic biology

With the completion of myriad genome sequencing projects, genetic bioengineering has expanded into many applications including the integrated analysis of complex pathways, the construction of new biological parts and the redesign of existing, natural biological systems. All these areas require the precise and concerted assembly of multiple DNA fragments of various sizes, including chromosomes, and the fine-tuning of gene expression levels and protein activity. Current commercial cloning products are not robust enough to support the assembly of very large or very small genetic elements or a combination of both. In addition, current strategies are not flexible enough to allow further modifications to the original design without having to undergo complicated cloning strategies. Here, we present a set of protocols that allow the seamless, simultaneous, flexible, and highly efficient assembly of genetic material, designed for a wide size dynamic range (10s to 100,000s base pairs). The assembly can be performed either in vitro or within the living cells and the DNA fragments may or may not share homology at their ends. A novel site-directed mutagenesis approach enhanced by in vitro recombineering is also presented.

Target-selective joint polymerase chain reaction: a robust and rapid method for high-throughput production of recombinant monoclonal antibodies from single cells

Background

During the development of a therapeutic antibody, large numbers of monoclonal antibodies are required to screen for those that are best suited for the desired activity. Although the single cell-based immunoglobulin variable gene cloning technique is a powerful tool, the current methods remain an obstacle to the rapid production of large numbers of recombinant antibodies.

Results

We have developed a novel overlap extension polymerase chain reaction, the target-selective joint polymerase chain reaction (TS-jPCR), and applied it to the generation of linear immunoglobulin gene expression constructs. TS-jPCR is conducted using a PCR-amplified immunoglobulin variable gene and an immunoglobulin gene-selective cassette (Ig-cassette) that contains all essential elements for antibody expression and overlapping areas of immunoglobulin gene-specific homology. The TS-jPCR technique is simple and specific; the 3'-random nucleotide-tailed immunoglobulin variable gene fragment and the Ig-cassette are assembled into a linear immunoglobulin expression construct, even in the presence of nonspecifically amplified DNA. We also developed a robotic magnetic beads handling instrument for single cell-based cDNA synthesis to amplify immunoglobulin variable genes by rapid amplification of 5' cDNA ends PCR. Using these methods, we were able to produce recombinant monoclonal antibodies from large numbers of single plasma cells within four days.

Conclusion

Our system reduces the burden of antibody discovery and engineering by rapidly producing large numbers of recombinant monoclonal antibodies in a short period of time.

Recombination-dependent concatemeric viral DNA replication

The initiation of viral double stranded (ds) DNA replication involves proteins that recruit and load the replisome at the replication origin (ori). Any block in replication fork progression or a programmed barrier may act as a factor for ori-independent remodelling and assembly of a new replisome at the stalled fork. Then replication initiation becomes dependent on recombination proteins, a process called recombination-dependent replication (RDR). RDR, which is recognized as being important for replication restart and stability in all living organisms, plays an essential role in the replication cycle of many dsDNA viruses. The SPP1 virus, which infects Bacillus subtilis cells, serves as a paradigm to understand the links between replication and recombination in circular dsDNA viruses. SPP1-encoded initiator and replisome assembly proteins control the onset of viral replication and direct the recruitment of host-encoded replisomal components at viral oriL. SPP1 uses replication fork reactivation to switch from ori-dependent θ-type (circle-to-circle) replication to σ-type RDR. Replication fork arrest leads to a double strand break that is processed by viral-encoded factors to generate a D-loop into which a new replisome is assembled, leading to σ-type viral replication. SPP1 RDR proteins are compared with similar proteins encoded by other viruses and their possible in vivo roles are discussed.

Target-selective homologous recombination cloning for high-throughput generation of monoclonal antibodies from single plasma cells

Background

Molecular cloning of functional immunoglobulin genes from single plasma cells is one of the most promising technologies for the rapid development of monoclonal antibody drugs. However, the proper insertion of PCR-amplified immunoglobulin genes into expression vectors remains an obstacle to the high-throughput production of recombinant monoclonal antibodies.

Results

We developed a single-step cloning method, target-selective homologous recombination (TS-HR), in which PCR-amplified immunoglobulin variable genes were selectively inserted into vectors, even in the presence of nonspecifically amplified DNA. TS-HR utilizes Red/ET-mediated homologous recombination with a target-selective vector (TS-vector) with unique homology arms on its termini. Using TS-HR, immunoglobulin variable genes were cloned directly into expression vectors by co-transforming unpurified PCR products and the TS-vector into E. coli. Furthermore, the high cloning specificity of TS-HR allowed plasmids to be extracted from pools of transformed bacteria without screening single colonies for correct clones. We present a one-week protocol for the production of recombinant mouse monoclonal antibodies from large numbers of single plasma cells.

Conclusion

The time requirements and limitations of traditional cloning procedures for the production of recombinant immunoglobulins have been significantly reduced with the development of the TS-HR cloning technique.

Recent advances in the production of proteins in insect and mammalian cells for structural biology

The production of proteins in sufficient quantity and of appropriate quality is an essential pre-requisite for structural studies. Escherichia coli remains the dominant expression system in structural biology with nearly 90% of the structures in the Protein Data Bank (PDB) derived from proteins produced in this bacterial host. However, many mammalian and eukaryotic viral proteins require post-translation modification for proper folding and/or are part of large multimeric complexes. Therefore expression in higher eukaryotic cell lines from both invertebrate and vertebrate is required to produce these proteins. Although these systems are generally more time-consuming and expensive to use than bacteria, there have been improvements in technology that have streamlined the processes involved. For example, the use of multi-host vectors, i.e., containing promoters for not only E. coli but also mammalian and baculovirus expression in insect cells, enables target genes to be evaluated in both bacterial and higher eukaryotic hosts from a single vector. Culturing cells in micro-plate format allows screening of large numbers of vectors in parallel and is amenable to automation. The development of large-scale transient expression in mammalian cells offers a way of rapidly producing proteins with relatively high throughput. Strategies for selenomethionine-labelling (important for obtaining phase information in crystallography) and controlling glycosylation (important for reducing the chemical heterogeneity of glycoproteins) have also been reported for higher eukaryotic cell expression systems.

The 3'-to-5' exonuclease activity of vaccinia virus DNA polymerase is essential and plays a role in promoting virus genetic recombination

Poxviruses are subjected to extraordinarily high levels of genetic recombination during infection, although the enzymes catalyzing these reactions have never been identified. However, it is clear that virus-encoded DNA polymerases play some unknown yet critical role in virus recombination. Using a novel, antiviral-drug-based strategy to dissect recombination and replication reactions, we now show that the 3'-to-5' proofreading exonuclease activity of the viral DNA polymerase plays a key role in promoting recombination reactions. Linear DNA substrates were prepared containing the dCMP analog cidofovir (CDV) incorporated into the 3' ends of the molecules. The drug blocked the formation of concatemeric recombinant molecules in vitro in a process that was catalyzed by the proofreading activity of vaccinia virus DNA polymerase. Recombinant formation was also blocked when CDV-containing recombination substrates were transfected into cells infected with wild-type vaccinia virus. These inhibitory effects could be overcome if CDV-containing substrates were transfected into cells infected with CDV-resistant (CDV(r)) viruses, but only when resistance was linked to an A314T substitution mutation mapping within the 3'-to-5' exonuclease domain of the viral polymerase. Viruses encoding a CDV(r) mutation in the polymerase domain still exhibited a CDV-induced recombination deficiency. The A314T substitution also enhanced the enzyme's capacity to excise CDV molecules from the 3' ends of duplex DNA and to recombine these DNAs in vitro, as judged from experiments using purified mutant DNA polymerase. The 3'-to-5' exonuclease activity appears to be an essential virus function, and our results suggest that this might be because poxviruses use it to promote genetic exchange.

Identification of polymerase and processivity inhibitors of vaccinia DNA synthesis using a stepwise screening approach

Nearly all DNA polymerases require processivity factors to ensure continuous incorporation of nucleotides. Processivity factors are specific for their cognate DNA polymerases. For this reason, the vaccinia DNA polymerase (E9) and the proteins associated with processivity (A20 and D4) are excellent therapeutic targets. In this study, we show the utility of stepwise rapid plate assays that (i) screen for compounds that block vaccinia DNA synthesis, (ii) eliminate trivial inhibitors, e.g. DNA intercalators, and (iii) distinguish whether inhibitors are specific for blocking DNA polymerase activity or processivity. The sequential plate screening of 2222 compounds from the NCI Diversity Set library yielded a DNA polymerase inhibitor (NSC 55636) and a processivity inhibitor (NSC 123526) that were capable of reducing vaccinia viral plaques with minimal cellular cytotoxicity. These compounds are predicted to block cellular infection by the smallpox virus, variola, based on the very high sequence identity between A20, D4 and E9 of vaccinia and the corresponding proteins of variola.

Improved cloning vectors for bifidobacteria, based on the Bifidobacterium catenulatum pBC1 replicon

This study reports the development of several cloning vectors for bifidobacteria based on the replicon of pBC1, a cryptic plasmid from Bifidobacterium catenulatum L48 thought to replicate via the theta mode. These vectors, in which antibiotic resistance genes encoding either erythromycin or tetracycline resistance acted as selection markers, were able to replicate in a series of eight Bifidobacterium species at frequencies ranging from 4.0 x 10(1) to 1.0 x 10(5) transformants microg(-1) but not in Lactococcus lactis or Lactobacillus casei. They showed a relative copy number of around 30 molecules per chromosome equivalent and a good segregational stability, with more than 95% of the cells retaining the vectors after 80 to 100 generations in the absence of selection. Vectors contain multiple cloning sites of different lengths, and the lacZalpha peptide gene was introduced into one of the molecules, thus allowing the easy selection of colonies harboring recombinant plasmids in Escherichia coli. The functionality of the vectors for engineering Bifidobacterium strains was assessed by cloning and examining the expression of an alpha-l-arabinofuranosidase gene belonging to Bifidobacterium longum. E. coli and Bifidobacterium pseudocatenulatum recombinant clones were stable and showed an increase in alpha-arabinofuranosidase activity of over 100-fold compared to that of the untransformed hosts.