A replication-competent chimera of plant and animal viruses

Cruzain Inhibitory Activity of Leaf Essential Oils of Neotropical Lauraceae and Essential Oil Components

The leaf essential oils of twenty-three species of Lauraceae from Monteverde, Costa Rica, have been screened for inhibition of the cysteine protease cruzain. Of these, nine showed promising cruzain inhibitory activity (IC50 < 100 μg/mL), six showed marginal activity (IC50, 100–500 μg/mL), and eight were inactive (IC50 > 500 μg/mL). The cruzain inhibitory activities of the essential oils can be attributed to active sesquiterpenoid components as well as synergistic effects between two or more components. The sesquiterpenes α-copaene, β-caryophyllene, α-humulene, and germacrene D are active (IC50 ~5–30 μg/mL) alone, but also show increased activity in combination with other essential oil components.

The closterovirus-derived gene expression and RNA interference vectors as tools for research and plant biotechnology

Important progress in understanding replication, interactions with host plants, and evolution of closteroviruses enabled engineering of several vectors for gene expression and virus-induced gene silencing. Due to the broad host range of closteroviruses, these vectors expanded vector applicability to include important woody plants such as citrus and grapevine. Furthermore, large closterovirus genomes offer genetic capacity and stability unrivaled by other plant viral vectors. These features provided immense opportunities for using closterovirus vectors for the functional genomics studies and pathogen control in economically valuable crops. This review briefly summarizes advances in closterovirus research during the last decade, explores the relationships between virus biology and vector design, and outlines the most promising directions for future application of closterovirus vectors.

Agroinoculation of Citrus tristeza virus causes systemic infection and symptoms in the presumed nonhost Nicotiana benthamiana

Citrus tristeza virus (CTV) naturally infects only some citrus species and relatives and within these it only invades phloem tissues. Failure to agroinfect citrus plants and the lack of an experimental herbaceous host hindered development of a workable genetic system. A full-genome cDNA of CTV isolate T36 was cloned in binary plasmids and was used to agroinfiltrate Nicotiana benthamiana leaves, with or without coinfiltration with plasmids expressing different silencing-suppressor proteins. A time course analysis in agroinfiltrated leaves indicated that CTV accumulates and moves cell-to-cell for at least three weeks postinoculation (wpi), and then, it moves systemically and infects the upper leaves with symptom expression. Silencing suppressors expedited systemic infection and often increased infectivity. In systemically infected Nicotiana benthamiana plants, CTV invaded first the phloem, but after 7 wpi, it was also found in other tissues and reached a high viral titer in upper leaves, thus allowing efficient transmission to citrus by stem-slash inoculation. Infected citrus plants showed the symptoms, virion morphology, and phloem restriction characteristic of the wild T36 isolate. Therefore, agroinfiltration of Nicotiana benthamiana provided the first experimental herbaceous host for CTV and an easy and efficient genetic system for this closterovirus.

Tandem leader proteases of Grapevine leafroll-associated virus-2: host-specific functions in the infection cycle

Several viruses in the genus Closterovirus including Grapevine leafroll-associated virus-2 (GLRaV-2), encode a tandem of papain-like leader proteases (L1 and L2) whose functional profiles remained largely uncharacterized. We generated a series of the full-length, reporter-tagged, clones of GLRaV-2 and demonstrated that they are systemically infectious upon agroinfection of an experimental host plant Nicotiana benthamiana. These clones and corresponding minireplicon derivatives were used to address L1 and L2 functions in GLRaV-2 infection cycle. It was found that the deletion of genome region encoding the entire L1-L2 tandem resulted in a ~100-fold reduction in minireplicon RNA accumulation. Five-fold reduction in RNA level was observed upon deletion of L1 coding region. In contrast, deletion of L2 coding region did not affect RNA accumulation. It was also found that the autocatalytic cleavage by L2 but not by L1 is essential for genome replication. Analysis of the corresponding mutants in the context of N. benthamiana infection launched by the full-length GLRaV-2 clone revealed that L1 or its coding region is essential for virus ability to establish infection, while L2 plays an accessory role in the viral systemic transport. Strikingly, when tagged minireplicon variants were used for the leaf agroinfiltration of the GLRaV-2 natural host, Vitis vinifera, deletion of either L1 or L2 resulted in a dramatic reduction of minireplicon ability to establish infection attesting to a host-specific requirement for tandem proteases in the virus infection cycle.

Cruzain Inhibition by Terpenoids: A Molecular Docking Analysis

A molecular docking analysis has been carried out using monoterpene and sesquiterpene hydrocarbons and triterpenoids that have shown enzyme inhibitory activity as ligands for the cysteine protease cruzain. The binding energies of the docked ligands roughly correlate with their inhibitory activities. The orientations of the docked ligands are consistent with a mechanism whereby these hydrophobic compounds dock into a hydrophobic pocket near the active site, thereby blocking binding of the protein target to the protease.

Nidovirales: evolving the largest RNA virus genome

This review focuses on the monophyletic group of animal RNA viruses united in the order Nidovirales. The order includes the distantly related coronaviruses, toroviruses, and roniviruses, which possess the largest known RNA genomes (from 26 to 32kb) and will therefore be called "large" nidoviruses in this review. They are compared with their arterivirus cousins, which also belong to the Nidovirales despite having a much smaller genome (13-16kb). Common and unique features that have been identified for either large or all nidoviruses are outlined. These include the nidovirus genetic plan and genome diversity, the composition of the replicase machinery and virus particles, virus-specific accessory genes, the mechanisms of RNA and protein synthesis, and the origin and evolution of nidoviruses with small and large genomes. Nidoviruses employ single-stranded, polycistronic RNA genomes of positive polarity that direct the synthesis of the subunits of the replicative complex, including the RNA-dependent RNA polymerase and helicase. Replicase gene expression is under the principal control of a ribosomal frameshifting signal and a chymotrypsin-like protease, which is assisted by one or more papain-like proteases. A nested set of subgenomic RNAs is synthesized to express the 3'-proximal ORFs that encode most conserved structural proteins and, in some large nidoviruses, also diverse accessory proteins that may promote virus adaptation to specific hosts. The replicase machinery includes a set of RNA-processing enzymes some of which are unique for either all or large nidoviruses. The acquisition of these enzymes may have improved the low fidelity of RNA replication to allow genome expansion and give rise to the ancestors of small and, subsequently, large nidoviruses.

Comparative and functional genomics of closteroviruses

The largest extant RNA genomes are found in two diverse families of positive-strand RNA viruses, the animal Coronaviridae and the plant Closteroviridae. Comparative analysis of the viruses from the latter family reveals three levels of gene conservation. The most conserved gene module defines RNA replication and is shared with plant and animal viruses in the alphavirus-like superfamily. A module of five genes that function in particle assembly and transport is a hallmark of the family Closteroviridae and was likely present in the ancestor of all three closterovirus genera. This module includes a homologue of Hsp70 molecular chaperones and three diverged copies of the capsid protein gene. The remaining genes show dramatic variation in their numbers, functions, and origins among closteroviruses within and between the genera. Proteins encoded by these genes include suppressors of RNA silencing, RNAse III, papain-like proteases, the AlkB domain implicated in RNA repair, Zn-ribbon-containing protein, and a variety of proteins with no detectable homologues in the current databases. The evolutionary processes that have shaped the complex and fluid genomes of the large RNA viruses might be similar to those that have been involved in evolution of genomic complexity in other divisions of life.

Diverse suppressors of RNA silencing enhance agroinfection by a viral replicon

Launching the Beet yellows virus (BYV) minireplicon by agrobacterial delivery resulted in an unexpectedly low number of infected cells per inoculated leaf. This effect was due to a strong RNA silencing response in the agroinfiltrated leaves. Strikingly, ectopic co-expression of p21, a BYV RNA silencing suppressor, increased minireplicon infectivity by three orders of magnitude. Mutational analysis demonstrated that this effect correlates with suppressor activity of p21. Five diverse, heterologous viral suppressors were also active in this system, providing a useful approach for a dramatic, up to 10,000-fold, increase of the efficiency of agroinfection. The minireplicon agroinfection assay was also used to identify a new suppressor, a homolog of BYV p21, derived from Grapevine leafroll-associated virus-2. In addition, we report preliminary data on the suppressor activity of the p10 protein of Grapevine virus A and show that this protein belongs to a family of Zn-ribbon-containing proteins encoded by filamentous plant RNA viruses from three genera. The members of this family are predicted to have RNA silencing suppressor activity.

Triple gene block: modular design of a multifunctional machine for plant virus movement

Many plant virus genera encode a 'triple gene block' (TGB), a specialized evolutionarily conserved gene module involved in the cell-to-cell and long-distance movement of viruses. The TGB-based transport system exploits the co-ordinated action of three polypeptides to deliver viral genomes to plasmodesmata and to accomplish virus entry into neighbouring cells. Although data obtained on both the TGB and well-studied single protein transport systems clearly demonstrate that plant viruses employ host cell pathways for intra- and intercellular trafficking of genomic nucleic acids and proteins, there is no integral picture of the details of molecular events during TGB-mediated virus movement. Undoubtedly, understanding the molecular basis of the concerted action of TGB-encoded proteins in transporting viral genomes from cell to cell should provide new insights into the general principles of movement protein function. This review describes the structure, phylogeny and expression of TGB proteins, their roles in virus cell-to-cell movement and potential influence on host antiviral defences.

Beet yellows virus: the importance of being different

SUMMARY Taxonomic relationship: Type member of the genus Closterovirus, family Closteroviridae. A member of the alphavirus-like supergroup of positive-strand RNA viruses. Physical properties: Virions are flexuous filaments of approximately 1300 nm in length and approximately 12 nm in diameter that are made up of a approximately 15.5 kb RNA and five proteins. The major capsid protein forms virion body of helical symmetry that constitutes approximately 95% of the virion length. The short virion tail is assembled by the minor capsid protein, Hsp70-homologue, approximately 64-kDa protein, and approximately 20-kDa protein. Viral proteins: The 5'-most ORFs 1a and 1b encode leader proteinase and RNA replicase. The remaining ORFs 2-8 are expressed by subgenomic mRNAs that encode 6-kDa membrane protein, Hsp70 homologue, approximately 64-kDa protein, minor and major capsid proteins, approximately 20-kDa protein, and approximately 21-kDa protein, respectively. Hosts: The principal crop plants affected by Beet yellows virus (BYV) are sugar beet (Beta vulgaris) and spinach (Spinacea oleracea). In addition, BYV was reported to infect approximately 120 species in 15 families. Most suitable propagation species are Nicotiana benthamiana, Tetragonia expansa, and Claytonia perfoliata.

Leader proteinase of beet yellows virus functions in long-distance transport

The 66-kDa leader proteinase (L-Pro) of the Beet yellows virus (BYV) possesses a nonconserved N-terminal domain and a conserved, papain-like C-terminal domain. Previous work revealed that the N-terminal domain functions in RNA amplification, whereas the C-terminal domain is required for autoproteolysis. Alanine-scanning mutagenesis was applied to complete the functional analysis of L-Pro throughout the virus life cycle. This analysis indicated that the C-terminal domain of L-Pro, in addition to being required for proteolysis, also functions in RNA amplification and that these two functions are genetically separable. Examination of the role of L-Pro in BYV cell-to-cell movement revealed that none of the 20 examined replication-competent mutants was movement defective. In contrast, six of the L-Pro mutations affected the long-distance transport of BYV to various degrees, whereas three mutations completely abolished the transport. Because these mutations were located throughout the protein molecule, both domains of L-Pro function in virus transport. We conclude that in addition to previously identified functions of L-Pro, it also serves as the BYV long-distance transport factor.

Suppressor of RNA silencing encoded by Beet yellows virus

Using an Agrobacterium-mediated transient assay, we screened the 15.5-kb genome of the Beet yellows virus for proteins with RNA silencing suppressor activity. Among eight proteins tested, only a 21-kDa protein (p21) was able to suppress double-stranded (ds) RNA-induced silencing of the green fluorescent protein (GFP) mRNA. Restoration of GFP expression by p21 under these conditions had no apparent effect on accumulation of the small interfering RNAs. In addition, p21 elevated the transient expression level of the GFP mRNA in the absence of dsRNA inducer. Similar activities were detected using homologs of p21 encoded by other members of the genus Closterovirus. Computer analysis indicated that p21-like proteins constitute a novel protein family that is unrelated to other recognized suppressors of RNA silencing. Examination of the subcellular distribution in BYV-infected plants revealed that p21 is partitioned between soluble cytoplasmic form and proteinaceous inclusion bodies at the cell periphery.