Molecular dissection of the cauliflower mosaic virus translation transactivator

Influence of the P6 effector protein of Cauliflower mosaic virus (CaMV) on the sustained expression and subcellular localization of the CaMV movement protein

The P6 protein of cauliflower mosaic virus (CaMV) is a multifunctional protein that forms the electron dense, amorphous inclusion bodies that accumulate in the cytoplasm and has been shown to physically interact with all other CaMV proteins, including the CaMV movement protein (P1). In this study, we have investigated the subcellular localization of the P6 and P1 proteins in transient expression assays in Nicotiana benthamiana, as well as the influence of P6 on the expression and subcellular localization of P1. A version of P6 tagged with RFP was shown to envelop the endoplasmic reticulum (ER), whereas P1 tagged with RFP was shown to induce the fragmentation of the ER. Co-expression of P6 with P1 led to an enhancement of the spatial and temporal expression of P1, with a shift from expression through the plasma membrane and interior of the cell to punctate spots associated with the cell wall.

Cauliflower mosaic virus P6 inclusion body formation: A dynamic and intricate process

During an infection, Cauliflower mosaic virus (CaMV) forms inclusion bodies (IBs) mainly composed of viral protein P6, where viral activities occur. Because viral processes occur in IBs, understanding the mechanisms by which they are formed is crucial. FL-P6 expressed in N. benthamiana leaves formed IBs of a variety of shapes and sizes. Small IBs were dynamic, undergoing fusion/dissociation events. Co-expression of actin-binding polypeptides with FL-P6 altered IB size distribution and inhibited movement. This suggests that IB movement is required for fusion and growth. A P6 deletion mutant was discovered that formed a single large IB per cell, which suggests it exhibited altered fusion/dissociation dynamics. Myosin-inhibiting drugs did not affect small IB movement, while those inhibiting actin polymerization did. Large IBs colocalized with components of the aggresome pathway, while small ones generally did not. This suggests a possible involvement of the aggresome pathway in large IB formation.

Cauliflower mosaic virus transactivator protein (TAV) can suppress nonsense-mediated decay by targeting VARICOSE, a scaffold protein of the decapping complex

During pathogenesis, viruses hijack the host cellular machinery to access molecules and sub-cellular structures needed for infection. We have evidence that the multifunctional viral translation transactivator/viroplasmin (TAV) protein from Cauliflower mosaic virus (CaMV) can function as a suppressor of nonsense-mediated mRNA decay (NMD). TAV interacts specifically with a scaffold protein of the decapping complex VARICOSE (VCS) in the yeast two-hybrid system, and co-localizes with components of the decapping complex in planta. Notably, plants transgenic for TAV accumulate endogenous NMD-elicited mRNAs, while decay of AU-rich instability element (ARE)-signal containing mRNAs are not affected. Using an agroinfiltration-based transient assay we confirmed that TAV specifically stabilizes mRNA containing a premature termination codon (PTC) in a VCS-dependent manner. We have identified a TAV motif consisting of 12 of the 520 amino acids in the full-length sequence that is critical for both VCS binding and the NMD suppression effect. Our data suggest that TAV can intercept NMD by targeting the decapping machinery through the scaffold protein VARICOSE, indicating that 5'-3' mRNA decapping is a late step in NMD-related mRNA degradation in plants.

Strawberry Vein Banding Virus P6 Protein Is a Translation Trans-Activator and Its Activity Can be Suppressed by FveIF3g

The strawberry vein banding virus (SVBV) open reading frame (ORF) VI encodes a P6 protein known as the RNA silencing suppressor. This protein is known to form inclusion like granules of various sizes and accumulate in both the nuclei and the cytoplasm of SVBV-infected plant cells. In this study, we have determined that the P6 protein is the only trans-activator (TAV) encoded by SVBV, and can efficiently trans-activate the translation of downstream gfp mRNA in a bicistron derived from the SVBV. Furthermore, the P6 protein can trans-activate the expression of different bicistrons expressed by different caulimovirus promoters. The P6 protein encoded by SVBV from an infectious clone can also trans-activate the expression of bicistron. Through protein-protein interaction assays, we determined that the P6 protein could interact with the cell translation initiation factor FveIF3g of Fragaria vesca and co-localize with it in the nuclei of Nicotiana benthamiana cells. This interaction reduced the formation of P6 granules in cells and its trans-activation activity on translation.

Joining the Crowd: Integrating Plant Virus Proteins into the Larger World of Pathogen Effectors

The first bacterial and viral avirulence ( avr) genes were cloned in 1984. Although virus and bacterial avr genes were physically isolated in the same year, the questions associated with their characterization after discovery were very different, and these differences had a profound influence on the narrative of host-pathogen interactions for the past 30 years. Bacterial avr proteins were subsequently shown to suppress host defenses, leading to their reclassification as effectors, whereas research on viral avr proteins centered on their role in the viral infection cycle rather than their effect on host defenses. Recent studies that focus on the multifunctional nature of plant virus proteins have shown that some virus proteins are capable of suppression of the same host defenses as bacterial effectors. This is exemplified by the P6 protein of Cauliflower mosaic virus (CaMV), a multifunctional plant virus protein that facilitates several steps in the infection, including modulation of host defenses. This review highlights the modular structure and multifunctional nature of CaMV P6 and illustrates its similarities to other, well-established pathogen effectors.

Ribosome Shunting, Polycistronic Translation, and Evasion of Antiviral Defenses in Plant Pararetroviruses and Beyond

Viruses have compact genomes and usually translate more than one protein from polycistronic RNAs using leaky scanning, frameshifting, stop codon suppression or reinitiation mechanisms. Viral (pre-)genomic RNAs often contain long 5′-leader sequences with short upstream open reading frames (uORFs) and secondary structure elements, which control both translation initiation and replication. In plants, viral RNA and DNA are targeted by RNA interference (RNAi) generating small RNAs that silence viral gene expression, while viral proteins are recognized by innate immunity and autophagy that restrict viral infection. In this review we focus on plant pararetroviruses of the family Caulimoviridae and describe the mechanisms of uORF- and secondary structure-driven ribosome shunting, leaky scanning and reinitiation after translation of short and long uORFs. We discuss conservation of these mechanisms in different genera of Caulimoviridae, including host genome-integrated endogenous viral elements, as well as in other viral families, and highlight a multipurpose use of the highly-structured leader sequence of plant pararetroviruses in regulation of translation, splicing, packaging, and reverse transcription of pregenomic RNA (pgRNA), and in evasion of RNAi. Furthermore, we illustrate how targeting of several host factors by a pararetroviral effector protein can lead to transactivation of viral polycistronic translation and concomitant suppression of antiviral defenses. Thus, activation of the plant protein kinase target of rapamycin (TOR) by the Cauliflower mosaic virus transactivator/viroplasmin (TAV) promotes reinitiation of translation after long ORFs on viral pgRNA and blocks antiviral autophagy and innate immunity responses, while interaction of TAV with the plant RNAi machinery interferes with antiviral silencing.

Setting Up Shop: The Formation and Function of the Viral Factories of Cauliflower mosaic virus

Similar to cells, viruses often compartmentalize specific functions such as genome replication or particle assembly. Viral compartments may contain host organelle membranes or they may be mainly composed of viral proteins. These compartments are often termed: inclusion bodies (IBs), viroplasms or viral factories. The same virus may form more than one type of IB, each with different functions, as illustrated by the plant pararetrovirus, Cauliflower mosaic virus (CaMV). CaMV forms two distinct types of IBs in infected plant cells, those composed mainly of the viral proteins P2 (which are responsible for transmission of CaMV by insect vectors) and P6 (required for viral intra-and inter-cellular infection), respectively. P6 IBs are the major focus of this review. Much of our understanding of the formation and function of P6 IBs comes from the analyses of their major protein component, P6. Over time, the interactions and functions of P6 have been gradually elucidated. Coupled with new technologies, such as fluorescence microscopy with fluorophore-tagged viral proteins, these data complement earlier work and provide a clearer picture of P6 IB formation. As the activities and interactions of the viral proteins have gradually been determined, the functions of P6 IBs have become clearer. This review integrates the current state of knowledge on the formation and function of P6 IBs to produce a coherent model for the activities mediated by these sophisticated virus-manufacturing machines.

Auxin Signaling in Regulation of Plant Translation Reinitiation

The mRNA translation machinery directs protein production, and thus cell growth, according to prevailing cellular and environmental conditions. The target of rapamycin (TOR) signaling pathway-a major growth-related pathway-plays a pivotal role in optimizing protein synthesis in mammals, while its deregulation triggers uncontrolled cell proliferation and the development of severe diseases. In plants, several signaling pathways sensitive to environmental changes, hormones, and pathogens have been implicated in post-transcriptional control, and thus far phytohormones have attracted most attention as TOR upstream regulators in plants. Recent data have suggested that the coordinated actions of the phytohormone auxin, Rho-like small GTPases (ROPs) from plants, and TOR signaling contribute to translation regulation of mRNAs that harbor upstream open reading frames (uORFs) within their 5'-untranslated regions (5'-UTRs). This review will summarize recent advances in translational regulation of a specific set of uORF-containing mRNAs that encode regulatory proteins-transcription factors, protein kinases and other cellular controllers-and how their control can impact plant growth and development.

A model for intracellular movement of Cauliflower mosaic virus: the concept of the mobile virion factory

The genomes of many plant viruses have a coding capacity limited to <10 proteins, yet it is becoming increasingly clear that individual plant virus proteins may interact with several targets in the host for establishment of infection. As new functions are uncovered for individual viral proteins, virologists have realized that the apparent simplicity of the virus genome is an illusion that belies the true impact that plant viruses have on host physiology. In this review, we discuss our evolving understanding of the function of the P6 protein of Cauliflower mosaic virus (CaMV), a process that was initiated nearly 35 years ago when the CaMV P6 protein was first described as the 'major inclusion body protein' (IB) present in infected plants. P6 is now referred to in most articles as the transactivator (TAV)/viroplasmin protein, because the first viral function to be characterized for the Caulimovirus P6 protein beyond its role as an inclusion body protein (the viroplasmin) was its role in translational transactivation (the TAV function). This review will discuss the currently accepted functions for P6 and then present the evidence for an entirely new function for P6 in intracellular movement.

Mutations within A 35 amino acid region of P6 influence self-association, inclusion body formation, and Caulimovirus infectivity

Cauliflower mosaic virus gene VI product (P6) is an essential protein that forms cytoplasmic, inclusion bodies (IBs). P6 contains four regions involved in self-association, termed D1-D4. D3 binds to D1, along with D4 and contains a spacer region (termed D3b) between two RNA-binding domains. Here we show D3b binds full-length P6 along with D1 and D4. Full-length P6s harboring single amino acid substitutions within D3b showed reduced binding to both D1 and D4. Full-length P6s containing D3b mutations and fused with green fluorescent protein formed inclusion-like bodies (IL-Bs) when expressed in Nicotiana benthamiana leaves. However, mutant P6s with reduced binding to D1 and D4, showed smaller IL-Bs, than wild type. Likewise, viruses containing these mutations showed a decrease in inoculated leaf viral DNA levels and reduced efficiency of systemic infection. These data suggest that mutations influencing P6 self-association alter IB formation and reduce virus infection.

Association of the P6 protein of Cauliflower mosaic virus with plasmodesmata and plasmodesmal proteins

The P6 protein of Cauliflower mosaic virus (CaMV) is responsible for the formation of inclusion bodies (IBs), which are the sites for viral gene expression, replication, and virion assembly. Moreover, recent evidence indicates that ectopically expressed P6 inclusion-like bodies (I-LBs) move in association with actin microfilaments. Because CaMV virions accumulate preferentially in P6 IBs, we hypothesized that P6 IBs have a role in delivering CaMV virions to the plasmodesmata. We have determined that the P6 protein interacts with a C2 calcium-dependent membrane-targeting protein (designated Arabidopsis [Arabidopsis thaliana] Soybean Response to Cold [AtSRC2.2]) in a yeast (Saccharomyces cerevisiae) two-hybrid screen and have confirmed this interaction through coimmunoprecipitation and colocalization assays in the CaMV host Nicotiana benthamiana. An AtSRC2.2 protein fused to red fluorescent protein (RFP) was localized to the plasma membrane and specifically associated with plasmodesmata. The AtSRC2.2-RFP fusion also colocalized with two proteins previously shown to associate with plasmodesmata: the host protein Plasmodesmata-Localized Protein1 (PDLP1) and the CaMV movement protein (MP). Because P6 I-LBs colocalized with AtSRC2.2 and the P6 protein had previously been shown to interact with CaMV MP, we investigated whether P6 I-LBs might also be associated with plasmodesmata. We examined the colocalization of P6-RFP I-LBs with PDLP1-green fluorescent protein (GFP) and aniline blue (a stain for callose normally observed at plasmodesmata) and found that P6-RFP I-LBs were associated with each of these markers. Furthermore, P6-RFP coimmunoprecipitated with PDLP1-GFP. Our evidence that a portion of P6-GFP I-LBs associate with AtSRC2.2 and PDLP1 at plasmodesmata supports a model in which P6 IBs function to transfer CaMV virions directly to MP at the plasmodesmata.

A phylogeographical study of the cauliflower mosaic virus population in mid-Eurasia Iran using complete genome analysis

The full-length sequences of 34 Iranian cauliflower mosaic virus (CaMV) isolates were compared with others from public nucleotide sequence databases to provide a comprehensive overview of the genetic variability and patterns of genetic exchange in CaMV isolates from Iran. Based on the severity of symptoms and their ability to infect Brassica oleracea var. capitata, Iranian CaMV isolates were grouped into two distinct biotypes: latent/mild mottle (LI/MMo) and severe (S) infection. Recombination breakpoints were detected between the large intergenic region (LIR) and open reading frame (ORF) V (event 2); between ORF VII and ORF II (event 3), between ORF I and ORF III (event 4), and within ORF VI (event 1). Phylogenetic analysis indicated that Iranian CaMV isolates clustered into two subgroups belonging to group I (GI) that were distinct from North American and European isolates from group II (GII). Northeast Iranian isolates (subgroup B) and CaMV isolates from subgroup A closely corresponded to the S and LI/MMo biological groups, respectively. Genome-wide pairwise identity analysis of the CaMV isolates revealed three regions of pairwise identity representation: 92-94 % for GII and 94-96 % and 98-100 % for subgroups A and B. The within-population diversity was lower than the between-population diversity, suggesting the contribution of a founder effect on diversification of CaMV isolates. Amino acid sequences were conserved, with ω values ranging from 0.074 to 0.717 in different proteins. Thirteen amino acids in the deduced proteins of ORFs I, II, III, VI and VII were under positive selection (ω > 1), whereas purifying selection applied to the proteins encoded by ORFs IV and V. This study suggests that variation in the CaMV population can be explained by host-range differentiation and selection pressure. Moreover, recombination analysis revealed that a genomic exchange is responsible for the emergence of CaMV strains, providing valuable new information for understanding the diversity and evolution of caulimoviruses.

Identification of the domains of cauliflower mosaic virus protein P6 responsible for suppression of RNA silencing and salicylic acid signalling

Cauliflower mosaic virus (CaMV) encodes a 520 aa polypeptide, P6, which participates in several essential activities in the virus life cycle including suppressing RNA silencing and salicylic acid-responsive defence signalling. We infected Arabidopsis with CaMV mutants containing short in-frame deletions within the P6 ORF. A deletion in the distal end of domain D-I (the N-terminal 112 aa) of P6 did not affect virus replication but compromised symptom development and curtailed the ability to restore GFP fluorescence in a GFP-silenced transgenic Arabidopsis line. A deletion in the minimum transactivator domain was defective in virus replication but retained the capacity to suppress RNA silencing locally. Symptom expression in CaMV-infected plants is apparently linked to the ability to suppress RNA silencing. When transiently co-expressed with tomato bushy stunt virus P19, an elicitor of programmed cell death in Nicotiana tabacum, WT P6 suppressed the hypersensitive response, but three mutants, two with deletions within the distal end of domain D-I and one involving the N-terminal nuclear export signal (NES), were unable to do so. Deleting the N-terminal 20 aa also abolished the suppression of pathogen-associated molecular pattern-dependent PR1a expression following agroinfiltration. However, the two other deletions in domain D-I retained this activity, evidence that the mechanisms underlying these functions are not identical. The D-I domain of P6 when expressed alone failed to suppress either cell death or PR1a expression and is therefore necessary but not sufficient for all three defence suppression activities. Consequently, concerns about the biosafety of genetically modified crops carrying truncated ORFVI sequences appear unfounded.

Cauliflower mosaic virus major inclusion body protein interacts with the aphid transmission factor, the virion-associated protein, and gene VII product

The Cauliflower mosaic virus (CaMV) gene VI product (P6) is a multifunctional protein essential for viral infection. In order to perform its various tasks, P6 interacts with both viral and host factors, as well as forming electron-dense cytoplasmic inclusion bodies. Here we investigate the interactions of P6 with three CaMV proteins: P2 (aphid transmission factor), P3 (virion-associated protein), and P7 (protein of unknown function). Based on yeast two-hybrid and maltose-binding protein pull-down experiments, P6 interacted with all three of these CaMV proteins. P2 helps to stabilize P6 inclusion bodies. Although the P2s from two CaMV isolates (W260 and CM1841) differ in the ability to stabilize inclusion bodies, both interacted similarly with P6. This suggests that inclusion body stability may not be dependent on the efficiency of P2-P6 interaction. However, neither P2 nor P3 interacted with P7 in yeast two-hybrid assays.

A plethora of virulence strategies hidden behind nuclear targeting of microbial effectors

Plant immune responses depend on the ability to couple rapid recognition of the invading microbe to an efficient response. During evolution, plant pathogens have acquired the ability to deliver effector molecules inside host cells in order to manipulate cellular and molecular processes and establish pathogenicity. Following translocation into plant cells, microbial effectors may be addressed to different subcellular compartments. Intriguingly, a significant number of effector proteins from different pathogenic microorganisms, including viruses, oomycetes, fungi, nematodes, and bacteria, is targeted to the nucleus of host cells. In agreement with this observation, increasing evidence highlights the crucial role played by nuclear dynamics, and nucleocytoplasmic protein trafficking during a great variety of analyzed plant-pathogen interactions. Once in the nucleus, effector proteins are able to manipulate host transcription or directly subvert essential host components to promote virulence. Along these lines, it has been suggested that some effectors may affect histone packing and, thereby, chromatin configuration. In addition, microbial effectors may either directly activate transcription or target host transcription factors to alter their regular molecular functions. Alternatively, nuclear translocation of effectors may affect subcellular localization of their cognate resistance proteins in a process that is essential for resistance protein-mediated plant immunity. Here, we review recent progress in our field on the identification of microbial effectors that are targeted to the nucleus of host plant cells. In addition, we discuss different virulence strategies deployed by microbes, which have been uncovered through examination of the mechanisms that guide nuclear localization of effector proteins.

Viral factor TAV recruits TOR/S6K1 signalling to activate reinitiation after long ORF translation

The protein kinase TOR (target-of-rapamycin) upregulates translation initiation in eukaryotes, but initiation restart after long ORF translation is restricted by largely unknown pathways. The plant viral reinitiation factor transactivator-viroplasmin (TAV) exceptionally promotes reinitiation through a mechanism involving retention on 80S and reuse of eIF3 and the host factor reinitiation-supporting protein (RISP) to regenerate reinitiation-competent ribosomal complexes. Here, we show that TAV function in reinitiation depends on physical association with TOR, with TAV-TOR binding being critical for both translation reinitiation and viral fitness. Consistently, TOR-deficient plants are resistant to viral infection. TAV triggers TOR hyperactivation and S6K1 phosphorylation in planta. When activated, TOR binds polyribosomes concomitantly with polysomal accumulation of eIF3 and RISP--a novel and specific target of TOR/S6K1--in a TAV-dependent manner, with RISP being phosphorylated. TAV mutants defective in TOR binding fail to recruit TOR, thereby abolishing RISP phosphorylation in polysomes and reinitiation. Thus, activation of reinitiation after long ORF translation is more complex than previously appreciated, with TOR/S6K1 upregulation being the key event in the formation of reinitiation-competent ribosomal complexes.

A new plant protein interacts with eIF3 and 60S to enhance virus-activated translation re-initiation

The plant viral re-initiation factor transactivator viroplasmin (TAV) activates translation of polycistronic mRNA by a re-initiation mechanism involving translation initiation factor 3 (eIF3) and the 60S ribosomal subunit (60S). QJ;Here, we report a new plant factor-re-initiation supporting protein (RISP)-that enhances TAV function in re-initiation. RISP interacts physically with TAV in vitro and in vivo. Mutants defective in interaction are less active, or inactive, in transactivation and viral amplification. RISP alone can serve as a scaffold protein, which is able to interact with eIF3 subunits a/c and 60S, apparently through the C-terminus of ribosomal protein L24. RISP pre-bound to eIF3 binds 40S, suggesting that RISP enters the translational machinery at the 43S formation step. RISP, TAV and 60S co-localize in epidermal cells of infected plants, and eIF3-TAV-RISP-L24 complex formation can be shown in vitro. These results suggest that RISP and TAV bridge interactions between eIF3-bound 40S and L24 of 60S after translation termination to ensure 60S recruitment during repetitive initiation events on polycistronic mRNA; RISP can thus be considered as a new component of the cell translation machinery.

The cauliflower mosaic virus protein P6 forms motile inclusions that traffic along actin microfilaments and stabilize microtubules

The gene VI product (P6) of Cauliflower mosaic virus (CaMV) is a multifunctional protein known to be a major component of cytoplasmic inclusion bodies formed during CaMV infection. Although these inclusions are known to contain virions and are thought to be sites of translation from the CaMV 35S polycistronic RNA intermediate, the precise role of these bodies in the CaMV infection cycle remains unclear. Here, we examine the functionality and intracellular location of a fusion between P6 and GFP (P6-GFP). We initially show that the ability of P6-GFP to transactivate translation is comparable to unmodified P6. Consequently, our work has direct application for the large body of literature in which P6 has been expressed ectopically and its functions characterized. We subsequently found that P6-GFP forms highly motile cytoplasmic inclusion bodies and revealed through fluorescence colocalization studies that these P6-GFP bodies associate with the actin/endoplasmic reticulum network as well as microtubules. We demonstrate that while P6-GFP inclusions traffic along microfilaments, those associated with microtubules appear stationary. Additionally, inhibitor studies reveal that the intracellular movement of P6-GFP inclusions is sensitive to the actin inhibitor, latrunculin B, which also inhibits the formation of local lesions by CaMV in Nicotiana edwardsonii leaves. The motility of P6 along microfilaments represents an entirely new property for this protein, and these results imply a role for P6 in intracellular and cell-to-cell movement of CaMV.

Cauliflower mosaic virus gene VI product N-terminus contains regions involved in resistance-breakage, self-association and interactions with movement protein

Cauliflower mosaic virus (CaMV) gene VI encodes a multifunctional protein (P6) involved in the translation of viral RNA, the formation of inclusion bodies, and the determination of host range. Arabidopsis thaliana ecotype Tsu-0 prevents the systemic spread of most CaMV isolates, including CM1841. However, CaMV isolate W260 overcomes this resistance. In this paper, the N-terminal 110 amino acids of P6 (termed D1) were identified as the resistance-breaking region. D1 also bound full-length P6. Furthermore, binding of W260 D1 to P6 induced higher beta-galactosidase activity and better leucine-independent growth in the yeast two-hybrid system than its CM1841 counterpart. Thus, W260 may evade Tsu-0 resistance by mediating P6 self-association in a manner different from that of CM1841. Because Tsu-0 resistance prevents virus movement, interaction of P6 with P1 (CaMV movement protein) was investigated. Both yeast two-hybrid analyses and maltose-binding protein pull-down experiments show that P6 interacts with P1. Although neither half of P1 interacts with P6, the N-terminus of P6 binds P1. Interestingly, D1 by itself does not interact with P1, indicating that different portions of the P6 N-terminus are involved in different activities. The P1-P6 interactions suggest a role for P6 in virus transport, possibly by regulating P1 tubule formation or the assembly of movement complexes.

Translation reinitiation and leaky scanning in plant viruses

While translation of mRNAs in eukaryotic cells in general follows strict rules, viruses infecting these cells break those rules in various ways. Viruses are under high selection pressure to compete with the host, to economize genome size, and to accommodate signals for replication, virus assembly, etc., on their RNAs as well as using them for translation. The cornucopia of extraordinary translation strategies, such as leaky scanning, internal initiation of translation, ribosome shunt, and virus-controlled reinitiation of translation, evolved by viruses continues to surprise and inform our understanding of general translation mechanisms. While internal initiation is treated in another section of this issue, we concentrate on leaky scanning, shunt and reinitiation, with emphasis on plant pararetroviruses.

Characterisation of pararetrovirus-like sequences in the genome of potato (Solanum tuberosum)

Three families of pararetrovirus-like sequences were isolated from the genome of potato using PCR of a characteristic fragment extending from the end of the transactivator domain. The potato pararetrovirus-like sequences are abundant in the nuclear genome of potato as demonstrated by their hybridisation to high-molecular weight DNA in Southern transfers and by fluorescence in situ hybridisation. Sequencing of cloned PCR products demonstrated that the potato pararetrovirus-like sequences were similar to other pararetroviral sequences and also to some expressed sequences from tobacco and tomato, notably from callus and Agrobacterium-infected tissue. It is possible that the potato pararetroviral sequences defend against viral genes via silencing mechanisms, although, as in Petunia or banana, their transcription and recombination may cause infection under stress conditions.

Suppression of RNA silencing by a geminivirus nuclear protein, AC2, correlates with transactivation of host genes

Bipartite geminiviruses encode a small protein, AC2, that functions as a transactivator of viral transcription and a suppressor of RNA silencing. A relationship between these two functions had not been investigated before. We characterized both of these functions for AC2 from Mungbean yellow mosaic virus-Vigna (MYMV). When transiently expressed in plant protoplasts, MYMV AC2 strongly transactivated the viral promoter; AC2 was detected in the nucleus, and a split nuclear localization signal (NLS) was mapped. In a model Nicotiana benthamiana plant, in which silencing can be triggered biolistically, AC2 reduced local silencing and prevented its systemic spread. Mutations in the AC2 NLS or Zn finger or deletion of its activator domain abolished both these effects, suggesting that suppression of silencing by AC2 requires transactivation of host suppressor(s). In line with this, in Arabidopsis protoplasts, MYMV AC2 or its homologue from African cassava mosaic geminivirus coactivated >30 components of the plant transcriptome, as detected with Affymetrix ATH1 GeneChips. Several corresponding promoters cloned from Arabidopsis were strongly induced by both AC2 proteins. These results suggest that silencing suppression and transcription activation by AC2 are functionally connected and that some of the AC2-inducible host genes discovered here may code for components of an endogenous network that controls silencing.

The open reading frame VI product of Cauliflower mosaic virus is a nucleocytoplasmic protein: its N terminus mediates its nuclear export and formation of electron-dense viroplasms

The Cauliflower mosaic virus (CaMV) open reading frame VI product (P6) is essential for the viral infection cycle. It controls translation reinitiation of the viral polycistronic RNAs and forms cytoplasmic inclusion bodies (viroplasms) where virus replication and assembly occur. In this study, the mechanism involved in viroplasm formation was investigated by in vitro and in vivo experiments. Far protein gel blot assays using a collection of P6 deletion mutants demonstrated that the N-terminal alpha-helix of P6 mediates interaction between P6 molecules. Transient expression in tobacco (Nicotiana tabacum) BY-2 cells of full-length P6 and P6 mutants fused to enhanced green fluorescent protein revealed that viroplasms are formed at the periphery of the nucleus and that the N-terminal domain of P6 is an important determinant in this process. Finally, this study led to the unexpected finding that P6 is a nucleocytoplasmic shuttle protein and that its nuclear export is mediated by a Leu-rich sequence that is part of the alpha-helix domain implicated in viroplasm formation. The discovery that P6 can localize to the nucleus opens new prospects for understanding yet unknown roles of this viral protein in the course of the CaMV infection cycle.

Suppression of RNA silencing by a geminivirus nuclear protein, AC2, correlates with transactivation of host genes

Bipartite geminiviruses encode a small protein, AC2, that functions as a transactivator of viral transcription and a suppressor of RNA silencing. A relationship between these two functions had not been investigated before. We characterized both of these functions for AC2 from Mungbean yellow mosaic virus-Vigna (MYMV). When transiently expressed in plant protoplasts, MYMV AC2 strongly transactivated the viral promoter; AC2 was detected in the nucleus, and a split nuclear localization signal (NLS) was mapped. In a model Nicotiana benthamiana plant, in which silencing can be triggered biolistically, AC2 reduced local silencing and prevented its systemic spread. Mutations in the AC2 NLS or Zn finger or deletion of its activator domain abolished both these effects, suggesting that suppression of silencing by AC2 requires transactivation of host suppressor(s). In line with this, in Arabidopsis protoplasts, MYMV AC2 or its homologue from African cassava mosaic geminivirus coactivated >30 components of the plant transcriptome, as detected with Affymetrix ATH1 GeneChips. Several corresponding promoters cloned from Arabidopsis were strongly induced by both AC2 proteins. These results suggest that silencing suppression and transcription activation by AC2 are functionally connected and that some of the AC2-inducible host genes discovered here may code for components of an endogenous network that controls silencing.

P6 protein of Cauliflower mosaic virus, a translation reinitiator, interacts with ribosomal protein L13 from Arabidopsis thaliana

The P6 protein of Cauliflower mosaic virus (CaMV) transactivates translation of the CaMV 35S polycistronic pregenomic RNA and its spliced versions, and thus allows synthesis of a complete set of viral proteins. Previous studies have shown that P6 interacts with plant L18 and L24 ribosomal proteins and initiation factor eIF3, and it has been proposed that these interactions are involved in the reinitiation of translation of polycistronic viral RNAs. This study characterizes a novel cellular partner of P6, the ribosomal protein L13 from Arabidopsis thaliana. Far-Western assays performed with several P6 deletion mutants have shown that L13 interacts with the miniTAV of P6, which represents the minimal domain for transactivation, suggesting that the P6-L13 interaction might also be involved in this process. L13 and L18 were found to bind to the same region within the miniTAV. Competition assays between L18 and L13 for binding to miniTAV suggest that interactions between P6 and these ribosomal proteins involve separate P6 molecules, and/or occur at different stages of translation or in the context of another function also mediated by P6.

Control of translation reinitiation on the cauliflower mosaic virus (CaMV) polycistronic RNA

Translation of the polycistronic 35S RNA of CaMV (cauliflower mosaic virus) occurs via a reinitiation mechanism, which requires TAV (transactivator/viroplasmin). To allow translation reinitiation of the major open reading frames on the polycistronic RNA, TAV interacts with the host translational machinery via eIF3 (eukaryotic initiation factor 3) and the 60S ribosome. Accumulation of TAV and eIF3 in the polysomal fraction isolated from CaMV-infected cells suggested that TAV prevents loss of eIF3 from the translating ribosomes during the first initiation event. The TAV–eIF3–80S complex could be detected in vitro by sucrose-gradient-sedimentation analysis. The question is whether TAV interacts directly with the 48S preinitiation complex or enters polysomes after the first initiation event. eIF4B, a component of the 48S initiation complex, can preclude formation of the TAV–eIF3 complex via competition with TAV for eIF3 binding; the eIF4B- and TAV-binding sites on eIF3g overlap. eIF4B out-competes TAV for binding to eIF3 and to the eIF3–40S complex. Transient overexpression of eIF4B in plant protoplasts specifically inhibits TAV-mediated transactivation of polycistronic translation. Our results thus indicate that eIF4B precludes TAV–eIF3–40S complex formation during the first initiation event. Consequently, overexpression of TAV in plant protoplasts affects only the second and subsequent initiation events. We propose a model in which TAV enters the host translational machinery at the eIF4B-removal step to stabilize eIF3 within polysomes.

Dissection of cauliflower mosaic virus transactivator/viroplasmin reveals distinct essential functions in basic virus replication

Cauliflower mosaic virus (CaMV) transactivator/viroplasmin (Tav) is an essential multifunctional viral protein. Dissection of Tav by deletion mutagenesis revealed that the central region is essential for CaMV replication in single cells but that the N- and C-terminal parts are not. Strains with mutations in the central region were defective in the translational transactivator function and could be complemented by coexpressing Gag (capsid protein precursor) and Pol (polyprotein with protease, reverse transcriptase, and RNase H activity) from separate monocistronic plasmids. In contrast, total omission of Tav was only partially complemented by Gag and Pol overexpression from separate plasmids. These results indicate that CaMV basic replication requires both Tav-activated polycistronic translation and some posttranslational function(s) of Tav that is not affected by the deletions in the central region of Tav.

Cauliflower mosaic virus: still in the news

SUMMARY Taxonomic relationship: Cauliflower mosaic virus (CaMV) is the type member of the Caulimovirus genus in the Caulimoviridae family, which comprises five other genera. CaMV replicates its DNA genome by reverse transcription of a pregenomic RNA and thus belongs to the pararetrovirus supergroup, which includes the Hepadnaviridae family infecting vertebrates. Physical properties: Virions are non-enveloped isometric particles, 53 nm in diameter (Fig. 1). They are constituted by 420 capsid protein subunits organized following T= 7 icosahedral symmetry (Cheng, R.H., Olson, N.H. and Baker, T.S. (1992) Cauliflower mosaic virus: a 420 subunit (T= 7), multilayer structure. Virology, 16, 655-668). The genome consists of a double-stranded circular DNA of approximately 8000 bp that is embedded in the inner surface of the capsid. Viral proteins: The CaMV genome encodes six proteins, a cell-to-cell movement protein (P1), two aphid transmission factors (P2 and P3), the precursor of the capsid proteins (P4), a polyprotein precursor of proteinase, reverse transcriptase and ribonuclease H (P5) and an inclusion body protein/translation transactivator (P6). Hosts: The host range of CaMV is limited to plants of the Cruciferae family, i.e. Brassicae species and Arabidopsis thaliana, but some viral strains can also infect solanaceous plants. In nature, CaMV is transmitted by aphids in a non-circulative manner.

Multiple domains within the Cauliflower mosaic virus gene VI product interact with the full-length protein

The Cauliflower mosaic virus (CaMV) gene VI product (P6) is a multifunctional protein essential for viral propagation. It is likely that at least some of these functions require P6 self-association. The work described here was performed to confirm that P6 self-associates and to identify domains involved in this interaction. Yeast two-hybrid analyses indicated that full-length P6 self-associates and that this interaction is specific. Additional analyses indicated that at least four independent domains bind to full-length P6. When a central domain (termed domain D3) was removed, these interactions were abolished. However, this deleted P6 was able to bind to the full-length wild-type protein and to isolated domain D3. Viruses lacking domain D3 were incapable of producing a systemic infection. Isolated domain D3 was capable of binding to at least two of the other domains but was unable to self-associate. This suggests that domain D3 facilitates P6 self-association by binding to the other domains but not itself. The presence of multiple domains involved in P6 self-association may help explain the ability of this protein to form the intracellular inclusions characteristic of caulimoviruses.

Third position codon composition suggests two classes of genes within the Cauliflower mosaic virus genome

The translation of viral mRNAs by host ribosomes is essential for infection. Hence, codon usage of virus genes may influence efficiency of infection. In addition, composition of nucleotides in the third position within codons of genes can reflect evolutionary relationships. In this study, third position codon composition was examined for the seven genes of eight Cauliflower mosaic virus isolates. Genes IV-VII had similar codon composition values and were termed Class 1 genes. Genes I-III possessed corresponding codon composition values and were termed Class 2 genes. The codon composition values of Class 1 and genes differed significantly. Neither Class 1 nor Class 2 genes had codon composition values identical to that of the host plant, Arabidopsis thaliana. However, Class 1 genes possessed codon composition values closer to those of the host than Class 2 genes. Examination of the genomes of three Rous sarcoma virus isolates indicated that codon composition values were similar for the gag, pol, and env genes but these genes differed significantly from the src genes. Since codon composition values for Rous sarcoma virus distinguished a "foreign" gene from the rest of the viral genome, it is possible that the Cauliflower mosaic virus genome is composed of genes from two different sources. Others have suggested that Cauliflower mosaic virus evolved in this manner and our data provide support for this hypothesis.

A comparative analysis of the avirulence and translational transactivator functions of gene VI of Cauliflower mosaic virus

The primary function associated at present with the gene VI product of Cauliflower mosaic virus (CaMV) is that of a translational transactivator (TAV). In this capacity, it alters the host translational machinery to allow reinitiation of translation of other CaMV genes on the polycistronic 35S RNA of CaMV. In addition, the gene VI protein can elicit a specific type of plant defense response called the hypersensitive response (HR) in Nicotiana edwardsonii. In this study, we have adapted the agroinfiltration technique to compare the sequences of CaMV gene VI required for TAV function and elicitation of HR. To measure the activity of the TAV, we coagroinfiltrated gene VI of CaMV strain W260 with a bicistronic GUS reporter plasmid. TAV function could be assayed 4 days postinfiltration, before the onset of HR in N. edwardsonii. Through the use of the TAV and HR assays, we could show that the TAV functions of gene VI of CaMV strains W260 and D4 were equivalent, but only W260 gene VI elicited HR. A mutational analysis of W260 gene VI showed that the structural requirements for elicitation of HR were much more stringent than those for TAV function. Small deletions from either the 5' or 3' end of W260 gene VI abolished its ability to elicit HR, although the TAV function was retained in the mutant. The TAV function could also tolerate a small insertion within gene VI; this insertion abolished the elicitor function. This study provides direct evidence that the TAV function of gene VI is separate from its role as an elicitor of HR.

The 5' Third of Cauliflower mosaic virus Gene VI Conditions Resistance Breakage in Arabidopsis Ecotype Tsu-0

ABSTRACT Arabidopsis thaliana ecotypes vary in their responses to viruses. In this study, we analyzed the variation in response of A. thaliana ecotype Tsu-0 to Cauliflower mosaic virus (CaMV). This ecotype was previously reported to be resistant to two CaMV isolates (CM1841 and CM4-184), but susceptible to W260. In this study, we show that Tsu-0 is resistant to four additional CaMV isolates. CaMV propagated within the rosette leaves of Tsu-0 plants, but did not appear to spread systemically into the inflorescence. However, virus viability in rosette leaves of Tsu-0 plants apparently was not compromised because infectious CaMV could be recovered from these organs. W260 overcomes Tsu-0 resistance by a passive mechanism (i.e., this virus avoids activating plant defenses). The portion of the viral genome responsible for W260 resistance breakage was mapped to the 5' third of gene VI, which we have termed RBR-1. This region is also responsible for controlling the ability of CaMV to infect different types of solanaceous plants. Hence, the pathways by which plants of different families interact with CaMV may be conserved through evolution.

Viral strategies of translation initiation: ribosomal shunt and reinitiation

Due to the compactness of their genomes, viruses are well suited to the study of basic expression mechanisms, including details of transcription, RNA processing, transport, and translation. In fact, most basic principles of these processes were first described in viral systems. Furthermore, viruses seem not to respect basic rules, and cases of "abnormal" expression strategies are quiet common, although such strategies are usually also finally observed in rare cases of cellular gene expression. Concerning translation, viruses most often violate Kozak's original rule that eukaryotic translation starts from a capped monocistronic mRNA and involves linear scanning to find the first suitable start codon. Thus, many viral cases have been described where translation is initiated from noncapped RNA, using an internal ribosome entry site. This review centers on other viral translation strategies, namely shunting and virus-controlled reinitiation as first described in plant pararetroviruses (Caulimoviridae). In shunting, major parts of a complex leader are bypassed and not melted by scanning ribosomes. In the Caulimoviridae, this process is coupled to reinitiation after translation of a small open reading frame; in other cases, it is possibly initiated upon pausing of the scanning ribosome. Most of the Caulimoviridae produce polycistronic mRNAs. Two basic mechanisms are used for their translation. Alternative translation of the downstream open reading frames in the bacilliform Caulimoviridae occurs by a leaky scanning mechanism, and reinitiation of polycistronic translation in many of the icosahedral Caulimoviridae is enabled by the action of a viral transactivator. Both of these processes are discussed here in detail and compared to related processes in other viruses and cells.

A plant viral "reinitiation" factor interacts with the host translational machinery

The cauliflower mosaic virus transactivator, TAV, controls translation reinitiation of major open reading frames on polycistronic RNA. We show here that TAV function depends on its association with polysomes and eukaryotic initiation factor eIF3 in vitro and in vivo. TAV physically interacts with eIF3 and the 60S ribosomal subunit. Two proteins mediating these interactions were identified: eIF3g and 60S ribosomal protein L24. Transient expression of eIF3g and L24 in plant protoplasts strongly affects TAV-mediated reinitiation activity. We demonstrate that TAV/eIF3/40S and eIF3/TAV/60S ternary complexes form in vitro, and propose that TAV mediates efficient recruitment of eIF3 to polysomes, allowing translation of polycistronic mRNAs by reinitiation, overcoming the normal cell barriers to this process.

Genes Ia, II, III, IV and V of Soybean chlorotic mottle virus are essential but the gene Ib product is non-essential for systemic infection

Soybean chlorotic mottle virus (SbCMV) is the type species of the genus ‘Soybean chlorotic mottle-like viruses’, within the family Caulimoviridae. The double-stranded DNA genome of SbCMV (8178 bp) contains eight major open reading frames (ORFs). Viral genes essential and non-essential for the replication and movement of SbCMV were investigated by mutational analysis of an infectious 1·3-mer DNA clone. The results indicated that ORFs Ia, II, III, IV and V were essential for systemic infection. The product of ORF Ib was non-essential, although the putative tRNAMet primer-binding site in ORF Ib was proved to be essential. Immunoselection PCR revealed that an ORF Ia deletion mutant was encapsidated in primarily infected cells, indicating that ORF Ia is required for virus movement but not for replication. ORF IV was confirmed to encode a capsid protein by peptide sequencing of the capsid. Analysis of the viral transcripts showed that the SbCMV DNA genome gives rise to a pregenomic RNA and an ORF VI mRNA, as shown in the case of Cauliflower mosaic virus.

Transient expression of a GUS reporter gene from cauliflower mosaic virus replacement vectors in the presence and absence of helper virus

Vectors based upon the genome of cauliflower mosaic virus (CaMV) have only a limited capacity for replicating foreign DNA in plants. A helper virus system has been developed to complement CaMV constructs capable of carrying a large foreign gene (glucuronidase; GUS). GUS replaced part or all of the non-essential CaMV gene II and the essential genes III, IV and V. This construct was co-inoculated mechanically with wild-type CaMV helper virus onto Brassica rapa leaves to promote GUS vector complementation. After 1 week, blue foci of GUS activity were observed in the centres of the local lesions. Leaves inoculated with the GUS construct in the absence of helper virus showed randomly distributed foci of GUS activity that were generally smaller than the lesion-associated GUS foci. Inoculation with a simple non-replicating CaMV 35S promoter-GUS construct also produced small GUS foci. Co-inoculation of helper virus with CaMV gene replacement vectors in which replication was prevented by moving the primer-binding site or by deletion of an essential splice acceptor produced only small, randomly distributed GUS activity foci, demonstrating that the lesion-associated foci were produced by gene expression from replicating constructs. These experiments show that CaMV genes III-V can be complemented by wild-type virus and replacement gene vectors can be used for transient gene expression studies with CaMV constructs that distinguish gene expression associated with a replicating vector from that associated with a non-replicating vector.

Transgene-induced lesion mimic

Lesion mimic, i.e., the spontaneous formation of lesions resembling hypersensitive response (HR) lesions in the absence of a pathogen, is a dramatic phenotype occasionally found to accompany the expression of different, mostly unrelated, transgenes in plants. Recent studies indicated that transgene-induced lesion formation is not a simple case of necrosis, i.e., direct killing of cells by the transgene product, but results from the activation of a programmed cell death (PCD) pathway. Moreover, activation of HR-like cell death by transgene expression is viewed as an important evidence for the existence of a PCD pathway in plants. The study of lesion mimic transgenes is important to our understanding of PCD and the signals that control it in plants. PCD-inducing transgenes may provide clues regarding the different entry points into the cell death pathway, the relationships between the different branches of the pathway (e.g., developmental or environmental), or the different mechanisms involved in its induction or execution. Cell death-inducing transgenes may also be useful in biotechnology. Some lesion mimic transgenes were found to be induced in plants a state of systemic acquired resistance (SAR). These genes can be used in the development of pathogen-resistant crops. Other cell death-inducing transgenes may be used as specific cell ablation tools. Although mainly revealed unintentionally, and at times considered 'an adverse phenotype', lesion mimic transgenes should not be ignored because they may prove valuable for studying PCD as well as developing useful traits in different plants and crops.

The cauliflower mosaic virus translational transactivator interacts with the 60S ribosomal subunit protein L18 of Arabidopsis thaliana

The cauliflower mosaic virus (CaMV) open reading frame VI product (P6) is involved in several aspects of the infectious cycle. P6 specifically controls the synthesis of other CaMV proteins by transactivating their expression from the polycistronic 35S RNA. By far-Western assays, we have demonstrated that P6 interacts with proteins from both healthy and CaMV-infected leaves of Arabidopsis thaliana. These proteins are found in ribosome-enriched extracts, suggesting that they participate in the translation process. One of these proteins, identified by microsequencing, corresponds to the 60S ribosomal subunit protein L18 (RPL18). Its cDNA was cloned and expressed in Escherichia coli, and the resulting RPL18 protein was shown to interact with the minimal region required for translational transactivation, designated the miniTAV domain of P6.

NMR structure of the N-terminal domain of Saccharomyces cerevisiae RNase HI reveals a fold with a strong resemblance to the N-terminal domain of ribosomal protein L9

In addition to the conserved and well-defined RNase H domain, eukaryotic RNases HI possess either one or two copies of a small N-terminal domain. The solution structure of one of the N-terminal domains from Saccharomyces cerevisiae RNase HI, determined using NMR spectroscopy, is presented. The 46 residue motif comprises a three-stranded antiparallel beta-sheet and two short alpha-helices which pack onto opposite faces of the beta-sheet. Conserved residues involved in packing the alpha-helices onto the beta-sheet form the hydrophobic core of the domain. Three highly conserved and solvent exposed residues are implicated in RNA binding, W22, K38 and K39. The beta-beta-alpha-beta-alpha topology of the domain differs from the structures of known RNA binding domains such as the double-stranded RNA binding domain (dsRBD), the hnRNP K homology (KH) domain and the RNP motif. However, structural similarities exist between this domain and the N-terminal domain of ribosomal protein L9 which binds to 23 S ribosomal RNA.

Cloning, expression, and mapping of ribonucleases H of human and mouse related to bacterial RNase HI

We identified two human sequences and one mouse sequence in the database of expressed sequence tags that are highly homologous to the N-terminal sequence of eukaryotic RNases H1. The cDNAs for human RNASEH1 and mouse Rnaseh1 were obtained, their nucleotide sequences determined, and the proteins expressed in Escherichia coli and partially purified. Both proteins have RNase H activity in vitro and they bind to dsRNA and RNA-DNA hybrids through the N-terminal conserved motif present in eukaryotic RNases H1. The RNASEH1 gene is expressed in all human tissues at similar levels, indicating that RNase H1 may be a housekeeping protein. The human RNASEH1 and mouse Rnaseh1 cDNAs were used to isolate BAC genomic clones that were used as probes for fluorescence in situ hybridization. The human gene was localized to chromosome 17p11.2 and the mouse gene to a nonsyntenic region on chromosome 12A3. The chromosomal location and possible disease association of the human RNASEH1 gene are discussed.

A common 40 amino acid motif in eukaryotic RNases H1 and caulimovirus ORF VI proteins binds to duplex RNAs

Eukaryotic RNases H from Saccharomyces cerevisiae , Schizosaccharomyces pombe and Crithidia fasciculata , unlike the related Escherichia coli RNase HI, contain a non-RNase H domain with a common motif. Previously we showed that S.cerevisiae RNase H1 binds to duplex RNAs (either RNA-DNA hybrids or double-stranded RNA) through a region related to the double-stranded RNA binding motif. A very similar amino acid sequence is present in caulimovirus ORF VI proteins. The hallmark of the RNase H/caulimovirus nucleic acid binding motif is a stretch of 40 amino acids with 11 highly conserved residues, seven of which are aromatic. Point mutations, insertions and deletions indicated that integrity of the motif is important for binding. However, additional amino acids are required because a minimal peptide containing the motif was disordered in solution and failed to bind to duplex RNAs, whereas a longer protein bound well. Schizosaccharomyces pombe RNase H1 also bound to duplex RNAs, as did proteins in which the S.cerevisiae RNase H1 binding motif was replaced by either the C.fasciculata or by the cauliflower mosaic virus ORF VI sequence. The similarity between the RNase H and the caulimovirus domain suggest a common interaction with duplex RNAs of these two different groups of proteins.

Multiple widely spaced elements determine the efficiency with which a distal cistron is expressed from the polycistronic pregenomic RNA of figwort mosaic caulimovirus

The polycistronic expression mechanism of the plant pararetrovirus figwort mosaic caulimovirus (FMV) depends upon cis-acting elements present in its pregenomic RNA and a trans-acting protein (P6) which is expressed from a monocistronic subgenomic RNA. Using transient expression of FMV-derived polycistronic reporter constructs in Nicotiana edwardsonii cell suspension protoplasts, we further analyzed the cis-acting elements involved in polycistronic expression. A cis-acting element located within the first 74 nucleotides of the 7,954-nucleotide pregenomic RNA appears to be essential for P6 to transactivate expression of an internal cistron. Expression of this internal cistron, in the presence of P6, is greatly enhanced by the combined presence of two cis-acting elements located at the 3' end of the polycistronic RNA. Surprisingly, deletion of the most upstream of these two 3' cis-acting elements exposed a negative-acting element located internally on the polycistronic RNA, at the 3' end of open reading frame I. The action of both this negative-acting internal element and the positive-acting 3' elements is more pronounced when the large 5' untranslated leader region is present. This indicates that the 5' untranslated leader region is central to regulation of the FMV gene expression mechanism. Although a limited set of elements suffices to direct polycistronic expression in this eukaryotic system, a complex interplay between elements is involved in the spatial regulation of the genes present on the pregenomic RNA of FMV.

Translation in plants--rules and exceptions

Translation processes in plants are very similar to those in other eukaryotic organisms and can in general be explained with the scanning model. Particularly among plant viruses, unconventional mRNAs are frequent, which use modulated translation processes for their expression: leaky scanning, translational stop codon readthrough or frameshifting, and transactivation by virus-encoded proteins are used to translate polycistronic mRNAs; leader and trailer sequences confer (cap-independent) efficient ribosome binding, usually in an end-dependent mechanism, but true internal ribosome entry may occur as well; in a ribosome shunt, sequences within an RNA can be bypassed by scanning ribosomes. Translation in plant cells is regulated under conditions of stress and during development, but the underlying molecular mechanisms have not yet been determined. Only a small number of plant mRNAs, whose structure suggests that they might require some unusual translation mechanisms, have been described.

Roles of the 35S promoter and multiple overlapping domains in the pathogenicity of the pararetrovirus cauliflower mosaic virus

Elements associated with the 35S promoter involved in generating the pregenomic RNA (35S RNA) of the pararetrovirus cauliflower mosaic virus have been extensively studied in heterologous systems, but little is known about their role in viral pathogenicity. To investigate these elements, premature termination codons were progressively inserted into the 3' end of the adjacent gene VI to dissect it from colinear 35S enhancer sequences. The ability to cause a systemic infection in plants was retained with loss of up to 40 amino acids from the gene VI polypeptide, but truncations into a putative zinc finger proved lethal. In the 35S promoter, removal of the TATA box also abolished infectivity. However, upstream deletions encompassing the 35S enhancer showed that the sequence between -207 and -56 from the cap site comprised nonessential elements, although complete removal of this fragment caused loss of infectivity even when domain spacing was restored by linker insertion. Two separate enhancer domains (-207 to - 150 and -95 to -56) were identified, of which either one or the other, but not both, was required for infectivity. Some mutations affected the cellular levels of viral RNAs in unexpected ways, as with removal of the as-1 enhancer element causing an increase in 35S RNA. Others altered the relative abundance of nuclear and cytoplasmic viral DNAs. Mutations in promoter domains thought to be involved in regulating tissue-specific expression did not significantly affect virus accumulation in leaves versus roots, whereas gene VI mutants showed reduced root accumulation. We conclude that elements associated with the cauliflower mosaic virus 35S promoter contain extensive nonessential regions that can behave differently in their proper context than as isolated elements.

Arabidopsis thaliana expressing the cauliflower mosaic virus ORF VI transgene has a late flowering phenotype

Expression of open reading frame (ORF) VI of cauliflower mosaic virus (CaMV) in transgenic Arabidopsis thaliana caused a typical syndrome characterised by leaf chlorosis, vein clearing, plant stunting and reduced fertility. In addition and in comparison to untransformed controls we observed the formation of much larger rosettes of leaves combined with much later flowering and more extensive tillering. In these aspects, the ORF VI transgenic plants resembled late flowering mutants. All these phenotypes correlated with expression of ORF VI in three lines of transgenic plants which were produced independently, with different Ti-plasmid derived vectors and with different selective markers. The late flowering phenotype cosegregated with the transgene.