Nuclear export of plant pararetrovirus mRNAs involves the TREX complex, two viral proteins and the highly structured 5' leader region.

In eukaryotes, the major nuclear export pathway for mature mRNAs uses the dimeric receptor TAP/p15, which is recruited to mRNAs via the multisubunit TREX complex, comprising the THO core and different export adaptors. Viruses that replicate in the nucleus adopt different strategies to hijack cellular export factors and achieve cytoplasmic translation of their mRNAs. No export receptors are known in plants, but Arabidopsis TREX resembles the mammalian complex, with a conserved hexameric THO core associated with ALY and UIEF proteins, as well as UAP56 and MOS11.

Strength in Diversity: Nuclear Export of Viral RNAs.

The nuclear export of cellular mRNAs is a complex process that requires the orchestrated participation of many proteins that are recruited during the early steps of mRNA synthesis and processing. This strategy allows the cell to guarantee the conformity of the messengers accessing the cytoplasm and the translation machinery. Most transcripts are exported by the exportin dimer Nuclear RNA export factor 1 (NXF1)-NTF2-related export protein 1 (NXT1) and the transcription-export complex 1 (TREX1).

[Nuclear export of viral RNAs in metazoan].

The nuclear export of mRNAs is a complex process, involving the participaton of numerous proteins, the recruitement of which starts during the early steps of mRNAs biosynthesis and maturation. This strategy allows the cell to export only mature and non-defective transcripts to the cytoplasm where they are directed to the translational machinery. The vast majority of mRNAs is exported by the dimeric transport receptor TAP-p15, which is mainly recruited by the large multiprotein complex TREX-1.

Transfection of Small Noncoding RNAs into Arabidopsis thaliana Protoplasts.

Polyethylene glycol transfection of plant protoplasts represents an efficient method to incorporate foreign DNA and study transient gene expression. Here, we describe an optimized protocol to deliver small noncoding RNAs into Arabidopsis thaliana protoplasts. An example of application is provided by demonstrating the incorporation of a 20 nt long small noncoding RNA deriving from the 5' extremity of an A.

Formation of large viroplasms and virulence of Cauliflower mosaic virus in turnip plants depend on the N-terminal EKI sequence of viral protein TAV.

Cauliflower mosaic virus (CaMV) TAV protein (TransActivator/Viroplasmin) plays a pivotal role during the infection cycle since it activates translation reinitiation of viral polycistronic RNAs and suppresses RNA silencing. It is also the major component of cytoplasmic electron-dense inclusion bodies (EDIBs) called viroplasms that are particularly evident in cells infected by the virulent CaMV Cabb B-JI isolate. These EDIBs are considered as virion factories, vehicles for CaMV intracellular movement and reservoirs for CaMV transmission by aphids.

GTPase ROP2 binds and promotes activation of target of rapamycin, TOR, in response to auxin.

Target of rapamycin (TOR) promotes reinitiation at upstream ORFs (uORFs) in genes that play important roles in stem cell regulation and organogenesis in plants. Here, we report that the small GTPase ROP2, if activated by the phytohormone auxin, promotes activation of TOR, and thus translation reinitiation of uORF-containing mRNAs. Plants with high levels of active ROP2, including those expressing constitutively active ROP2 (CA-ROP2), contain high levels of active TOR ROP2 physically interacts with and, when GTP-bound, activates TOR TOR activation in response to auxin is abolished in ROP-deficient plants.

Cauliflower mosaic virus Transcriptome Reveals a Complex Alternative Splicing Pattern.

The plant pararetrovirus Cauliflower mosaic virus (CaMV) uses alternative splicing to generate several isoforms from its polycistronic pregenomic 35S RNA. This pro-cess has been shown to be essential for infectivity. Previous works have identified four splice donor sites and a single splice acceptor site in the 35S RNA 5' region and suggested that the main role of CaMV splicing is to downregulate expression of open reading frames (ORFs) I and II.

Le virus de la mosaïque du chou-fleur (CaMV), encore et toujours.

As a pararetrovirus and because of the non-canonical translation of its polycistronic pregenomic 35S RNA, Cauliflower mosaic virus (CaMV) is an original model system that has been extensively studied. Recent advances have improved our understanding of CaMV aphid transmission, cell-to-cell movement, protein expression and virus counter-defense strategy against host plant defense. Since P6/TAV is involved in many aspects of viral pathogenesis as well as in some replication steps, it is considered as the key player of CaMV infectious cycle.

TOR and S6K1 promote translation reinitiation of uORF-containing mRNAs via phosphorylation of eIF3h.

Mammalian target-of-rapamycin (mTOR) triggers S6 kinase (S6K) activation to phosphorylate targets linked to translation in response to energy, nutrients, and hormones. Pathways of TOR activation in plants remain unknown. Here, we uncover the role of the phytohormone auxin in TOR signalling activation and reinitiation after upstream open reading frame (uORF) translation, which in plants is dependent on translation initiation factor eIF3h.

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.

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.

Nuclear import of CaMV P6 is required for infection and suppression of the RNA silencing factor DRB4.

Replication of Cauliflower mosaic virus (CaMV), a plant double-stranded DNA virus, requires the viral translational transactivator protein P6. Although P6 is known to form cytoplasmic inclusion bodies (viroplasms) so far considered essential for virus biology, a fraction of the protein is also present in the nucleus. Here, we report that monomeric P6 is imported into the nucleus through two importin-alpha-dependent nuclear localization signals, and show that this process is mandatory for CaMV infectivity and is independent of translational transactivation and viroplasm formation.

Functional characterization of the Beet necrotic yellow vein virus RNA-5-encoded p26 protein: evidence for structural pathogenicity determinants.

A Beet necrotic yellow vein virus isolate containing a fifth RNA is present in the Pithiviers area of France. A full-length cDNA clone of RNA-5 was obtained and placed under the control of a T(7)-RNA-pol promoter that allowed the production of infectious transcripts. "Pithiviers" isolate-specific necrotic symptoms were obtained on Chenopodium quinoa when RNA-5-encoded p26 was expressed either from RNA-5 or from an RNA-3-derived replicon.

Quantification of the 35S promoter in DNA extracts from genetically modified organisms using real-time polymerase chain reaction and specificity assessment on various genetically modified organisms, part I: operating procedure.

A highly sensitive quantitative real-time assay targeted on the 35S promoter of a commercial genetically modified organism (GMO) was characterized (sF/sR primers) and developed for an ABI Prism 7700 Sequence Detection System and TaqMan chemistry. The specificity assessment and performance criteria of sF/sR assay were compared to other P35S-targeted published assays. sF/sR primers amplified a 79 base pair DNA sequence located in a part of P35S that is highly conserved among many caulimovirus strains, i.

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.

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.

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.