In vitro expression of cauliflower mosaic virus genes

Functional differentiation in the leucine-rich repeat domains of closely related plant virus-resistance proteins that recognize common avr proteins

The N' gene of Nicotiana sylvestris and L genes of Capsicum plants confer the resistance response accompanying the hypersensitive response (HR) elicited by tobamovirus coat proteins (CP) but with different viral specificities. Here, we report the identification of the N' gene. We amplified and cloned an N' candidate using polymerase chain reaction primers designed from L gene sequences. The N' candidate gene was a single 4143 base pairs fragment encoding a coiled-coil nucleotide-binding leucine-rich repeat (LRR)-type resistance protein of 1,380 amino acids. The candidate gene induced the HR in response to the coexpression of tobamovirus CP with the identical specificity as reported for N'. Analysis of N'-containing and tobamovirus-susceptible N. tabacum accessions supported the hypothesis that the candidate is the N' gene itself. Chimera analysis between N' and L(3) revealed that their LRR domains determine the spectrum of their tobamovirus CP recognition. Deletion and mutation analyses of N' and L(3) revealed that the conserved sequences in their C-terminal regions have important roles but contribute differentially to the recognition of common avirulence proteins. The results collectively suggest that Nicotiana N' and Capsicum L genes, which most likely evolved from a common ancestor, differentiated in their recognition specificity through changes in the structural requirements for LRR function.

Nuclear targeting of the cauliflower mosaic virus coat protein

The entry of the viral genomic DNA of cauliflower mosaic virus into the nucleus is a critical step of viral infection. We have shown by transient expression in plant protoplasts that the viral coat protein (CP), which is processed from the product of open reading frame IV, contains an N-terminal nuclear localization signal (NLS). The NLS is exposed on the surface of the virion and is thus available for interaction with a putative NLS receptor. Phosphorylation of the matured CP did not influence the nuclear localization of the protein but improved protein stability. Mutation of the NLS completely abolished viral infectivity, thus indicating its importance in the virus life cycle. The NLS seems to be regulated by the N terminus of the precapsid, which inhibits its nuclear targeting. This regulation could be important in allowing virus assembly in the cytoplasm.

Requirement of cauliflower mosaic virus open reading frame VI product for viral gene expression and multiplication in turnip protoplasts

Cauliflower mosaic virus (CaMV) open reading frame (ORF) VI product (P6) has been shown to be the major constituent of viral inclusion body, to function as a post-transcriptional transactivator, and to be essential for infectivity on whole plants. Although these findings suggest that P6 has an important role in viral multiplication, it is unknown whether P6 is required for viral multiplication in a single cell. To address this question, we transfected turnip protoplasts with an ORF VI frame-shift (4 bp deletion) mutant (pCaFS6) of an infectious CaMV DNA clone (pCa122). The mutant was uninfectious. Co-transfection of plasmids expressing P6 complemented the mutant. Overexpression of P6 elevated the infection rate in co-transfection experiments with either pCa122 or pCaFS6. This would have been achieved by elevating the level of pregenomic 35S RNA, a putative polycistronic mRNA for ORFs I, II, III, IV and V, and by enhancing the accumulation of these five viral gene products. When CaMV ORFs I, II, III, IV and V were expressed from monocistronic constructs in which each of the ORFs was placed just downstream of the 35S promoter, the accumulation of ORF III, IV and V products depended on the co-expression of P6. The accumulation of ORF I and II products was not detected, even in the presence of P6. These results suggest that P6 is involved in the stabilization of other viral gene products as well as in the activation of viral gene expression, and thus, is a prerequisite for CaMV multiplication.

Evidence for a dual strategy in the expression of cauliflower mosaic virus open reading frames I and IV

Studies have indicated that cauliflower mosaic virus (CaMV) gene expression is mediated by the translation of polycistronic 35S pregenomic RNA, but the involvement of some minor subgenomic RNA species is also suspected. We examined the involvement of the 35S promoter in the expression of CaMV open reading frames (ORFs) I and IV using both 35S RNA-driven and promoter-less ORF I- and ORF IV-beta-glucuronidase (GUS) fusion constructs. In addition to the 35S promoter-dependent expression of both ORF I- and IV-GUS fusions, we detected the 35S promoter-independent expression of both fusion genes via subgenomic mRNAs, which were detected by Northern blotting in the protoplasts transfected with the 35S promoter-driven constructs as well as in those transfected with the promoter-less constructs. These results suggest the involvement of subgenomic RNAs in the expression of CaMV ORFs I and IV, and the operation of a dual strategy in the expression of two viral genes.

Accumulation kinetics of viral gene products in cauliflower mosaic virus-infected turnip protoplasts

The expression of cauliflower mosaic virus (CaMV) genes was studied in a turnip protoplast system. Six CaMV-encoded gene products were detected in infected turnip protoplasts by means of Western blotting. The infected turnip protoplasts showed different patterns of protein accumulation; e.g. an open reading frame (ORF) I-encoded movement protein, an ORF V-encoded reverse transcriptase and an ORF VI-encoded posttranscriptional transactivator representing the early accumulated proteins, an ORF II-encoded aphid transmission factor and an ORF IV-encoded coat protein the late accumulated proteins and an ORF III-encoded DNA binding protein the intermediate protein. The results suggest that the expression of CaMV genes is differentially regulated.

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.

Molecular biology of rice tungro viruses

Rice tungro, the most important virus disease of rice in South and Southeast Asia, is caused by a complex of two viruses, rice tungro bacilliform virus (RTBV) and rice tungro spherical virus (RTSV). RTBV is a plant pararetrovirus with bacilliform particles, the structure of which is based on T = 3 icosahedral symmetry cut across the threefold axis.The particles encapsidate a circular double-stranded DNA of 8 kbp that encodes four proteins. The current information on the properties, functions, and expression of these proteins is discussed, as is the evidence for replication by reverse transcription. Two major strains of RTBV have been recognized, one from the Indian subcontinent and the other from Southeast Asia. RTSV particles contain a single-stranded RNA genome of 12 kb that encodes a large polyprotein and possibly one or two smaller proteins. The properties and processing of the polyprotein are described and the resemblance to picornaviruses noted.

Characterization of different electrophoretic forms of cauliflower mosaic virus virions (strain Cabb-S)

The electrophoretic forms of purified cauliflower mosaic virus (CaMV), strain Cabb-S, were examined by electrophoresis on agarose gels. Three populations of viral particles were identified: a faster migrating component (the form F) and two slower migrating components (the forms S and S'). When the different forms of virions, after excision from gels, were subjected to analysis in SDS-polyacrylamide gel, the fast component consisted of the 37 and 42 kDa coat proteins whereas the slow components contained mainly the 39 kDa coat protein. However, there was no difference among the nucleic acids associated within the three forms. The biological significance of the different components is discussed.

Cauliflower Mosaic Virus Gene VI Controls Translation from Dicistronic Expression Units in Transgenic Arabidopsis Plants

Transformed Arabidopsis plants were used to study the effect of the cauliflower mosaic virus (CaMV) inclusion body protein on translation of dicistronic RNA. Reporter plants contain a dicistronic transcription unit with CaMV open reading frame VII (ORF VII) as the first and the [beta]-glucuronidase (GUS) reporter ORF as the second cistron. "Transactivator plants" contain CaMV ORF VI under the control of the strong CaMV 35S promoter. The transactivator plants were difficult to regenerate and showed an abnormal phenotype. Expression of GUS activity in the reporter plants was very low, but high GUS activity could be induced by introduction of gene VI, either by crossing with plants containing gene VI as a transgene or by infection with CaMV. Histological GUS assays showed that transactivation occurred in all types of tissue and at all developmental stages. The practical implications of the induction of GUS expression from the dicistronic unit by virus infection are discussed.

Processing of the minor capsid protein of the cauliflower mosaic virus requires a cysteine proteinase

The major capsid protein of the cauliflower mosaic virus (CaMV) is processed in vivo. The viral aspartic proteinase that catalyses this maturation has been characterized previously and is coded by the CaMV gene V. This virus has a second capsid protein, a minor component, encoded by gene III. This protein, P3, is also processed at its C-terminus in vivo. To determine whether P3 is matured by the CaMV proteinase P5, we expressed, in Saccharomyces cerevisiae, P3, P5 and a fusion protein P7-P4, containing potential sites of cleavage. P5 was found to be involved in maturation of P7-P4 but did not cleave P3. The latter result was confirmed by experiments carried out with an in vitro translation system (the reticulocyte lysate) and with preparations of replication complexes purified from infected plants. Moreover, [N-(L-3-trans-carboxyoxiran-2-carbonyl)-L-leu cyl]-amido(4-guanido)butane, a specific inhibitor of cysteine proteinases, inhibited the maturation of P3, suggesting that the two CaMV capsid proteins are not processed by the same proteolytic event.

Cauliflower mosaic virus: a 420 subunit (T = 7), multilayer structure

The structures of the Cabb-B and CM1841 strains of cauliflower mosaic virus (CaMV) have been solved to about 3 nm resolution from unstained, frozen-hydrated samples that were examined with low-irradiation cryo-electron microscopy and three-dimensional image reconstruction procedures. CaMV is highly susceptible to distortions. Spherical particles, with a maximum diameter of 53.8 nm, are composed of three concentric layers (I-III) of solvent-excluded density that surround a large, solvent-filled cavity (approximately 27 nm dia). The outermost layer (I) contains 72 capsomeric morphological units, with 12 pentavalent pentamers and 60 hexavalent hexamers for a total of 420 subunits (37-42 kDa each) arranged with T = 7 icosahedral symmetry. CaMV is the first example of a T = 7 virus that obeys the rules of stoichiometry proposed for isometric viruses by Caspar and Klug (1962, Cold Spring Harb. Symp. Quant. Biol. 27, 1-24), although the hexameric capsomers exhibit marked departure from the regular sixfold symmetry expected for a structure in which the capsid protein subunits are quasi-equivalently related. The double-stranded DNA genome is distributed in layers II and III along with a portion of the viral protein. The CaMV reconstructions are consistent with the model based on neutron diffraction studies (Kruse et al., 1987, Virology 159, 166-168) and, together, these structural models are discussed in relation to a replication-assembly model (Hull et al., 1987, J. Cell Sci. (Suppl.) 7, 213-229). Remarkable agreement between the reconstructions of CaMV Cabb-B and CM1841 suggests that other strains of CaMV adopt the same basic structure.

Characterization of the genome of rice tungro bacilliform virus: comparison with Commelina yellow mottle virus and caulimoviruses

Rice tungro disease is caused by an infection of two different viruses, rice tungro spherical virus (a (+) sense RNA virus) and rice tungro bacilliform virus (RTBV) with a genome of circular double-stranded DNA. The genome of an RTBV isolate from the Philippines was cloned, sequenced, and found to be 8000 bp in length. It contains four open reading frames (ORFs) on a single strand, with ORF 1 having an internal termination codon (TAA). The 5' and 3' ends of a polyadenylated viral RNA transcript, of genome length, were mapped by primer extension and cDNA sequence analysis, respectively. The transcript is terminally redundant by 265-268 nucleotides. Purified virus particles contain two major proteins with molecular masses of 37 and 33 kDa, although only the 37-kDa protein was detected in the infected rice tissues. The N-terminal amino acid sequence of the 33-kDa protein was determined and its coding region was identified on the RTBV genome. The identity of the coat protein gene was further confirmed by expressing a region of the genome in Escherichia coli, the products of which reacted with anti-RTBV antibody. The unusually long ORF 3 of RTBV is predicted to encode a polyprotein of 194.1 kDa that includes: the coat protein(s), viral proteinase, reverse transcriptase, and ribonuclease H. The sections of the polyprotein show varying degrees of similarity to the counterparts of Commelina yellow mottle virus (a member of the proposed badnavirus group) and caulimoviruses. The functions of the other three ORFs are unknown.

How do viral reverse transcriptases recognize their RNA genome?

Reverse transcription is not solely a retroviral mechanism. Hepadnaviruses and caulimoviruses have RNA intermediates that are reverse transcribed into DNA. Moreover non-viral retroelements, retrotransposons, use reverse transcription in their transposition. All these retroelements encode reverse transcriptase but each group developed their own expression modes capable of assuring a specific and efficient replication of their genomes.

Occurrence of chloroplast ribosome recognition sites within conserved elements of the RNA genomes of carlaviruses

The nucleotide sequences upstream from the carlavirus open reading frames were examined for direct sequence homology. Blocks of homology were evident upstream from the 25 K ORFs of potato virus S (PVS), potato virus M (PVM) and lily symptomless virus (LSV), and upstream from the coat protein initiation codons of PVS, PVM, LSV, carnation latent virus and Helenium virus S. These blocks, which correspond to the 5'-terminal regions of the subgenomic RNAs, were shown to contain potential ribosome recognition sequences. The distances between the binding sites and initiation codons ranged from 20 to 40 nucleotides on the viral RNAs. Whilst the majority of chloroplasts mRNAs have a distance of 8 nucleotides between binding site and initiation codon, the remaining have a distance of 23 nucleotides which is similar to that reported here for the carlaviruses.

The cauliflower mosaic virus reverse transcriptase is not produced by the mechanism of ribosomal frameshifting in Saccharomyces cerevisiae

The capsid protein and the reverse transcriptase of cauliflower mosaic virus (CaMV) are encoded by two genes (ORF IV and ORF V) that lie in different translation reading frames. A comparison can be drawn between the synthesis of both CaMV proteins and the fusion protein in a yeast retrotransposon, Ty, resulting from a +1 frameshifting event which fuses two out-of-phase ORFs encoding the structural protein and the reverse transcriptase of Ty. For this reason, we constructed a yeast expression vector containing CaMV ORF VII fused to CaMV ORF III by a fragment of 452 bp including the overlapping region of ORF IV and ORF V, ORF VII and ORF III being used as reporter genes. We characterized two proteins (22 and 50 kDa) synthesized from this plasmid in the yeast expression system. We demonstrated that the 50-kDa polypeptide is not synthesized from a +1 frameshifting event but is probably a dimeric form of the 22-kDa protein. From this result we conclude that the CaMV reverse transcriptase is not produced by a mechanism of ribosomal frameshifting.

Crystallization of cauliflower mosaic virus

Cauliflower mosaic virus has been crystallized in hanging and sitting drops. The hexagonal and octahedrally shaped crystals are up to 0.5 mm in mean diameter. The octahedrally shaped crystals diffract to about 27 A resolution. The results are discussed in relation to the lability and aggregation of the virions.

The cauliflower mosaic virus open reading frame VII product can be expressed in Saccharomyces cerevisiae but is not detected in infected plants

Antiserum was prepared against a synthetic peptide corresponding to the N-terminal 20 amino acids of the protein encoded by cauliflower mosaic virus (CaMV) open reading frame VII (ORF VII). This antiserum was used to detect the expression of CaMV ORF VII either in Saccharomyces cerevisiae transformed by an expression vector containing CaMV ORF VII or in CaMV-infected plants. Only in S. cerevisiae has a 14-kilodalton protein been detected.

Differential inhibition of downstream gene expression by the cauliflower mosaic virus 35S RNA leader

The effect of the 600 nucleotide-long CaMV 35S RNA 5' leader sequence on the expression of downstream genes was analyzed both in plant protoplasts and in vitro. For transient expression studies in protoplasts derived from host and nonhost plants, the bacterial chloramphenicol acetyl transferase (CAT) gene was fused to the initiation codon of ORF VII. The leader sequence reduced CAT expression two- to four-fold in protoplasts derived from three host species, but 10- to 50-fold in protoplasts derived from three different nonhost species. For in-vitro studies the 35S promoter was replaced by the SP6 promoter. The leader reduced in-vitro translation of SP6 transcripts approximately six-fold, indicating that at least part of the inhibition observed in protoplasts is directly due to the interference of the leader sequence with translation. Other steps in gene expression that may also be affected are discussed.

The pattern of accumulation of cauliflower mosaic virus-specific products in infected turnips

The concentrations of cauliflower mosaic virus (CaMV) DNA and protein products in the developing leaves of a host, turnip, have been measured and the results have been correlated with symptom production. Virus-specific products were limited to the symptomatic leaves. CaMV DNA was detected in the youngest foliar tissues showing full systemic symptoms and continued to accumulate as the leaf expanded, indicating that virus multiplication was not restricted to meristematic tissues of the host plant and that virus concentration was not a primary determinant for symptom production. Using specific antisera for Western blot analysis, the distribution of CaMV-specific proteins (P1-P6) in a range of subcellular fractions of infected tissue was determined. The protein products (P2-P6) of genes II-VI were all detected in fractions enriched for virus inclusion bodies, although P5 was present only at low levels. A high-speed pellet fraction enriched for virus replication complexes revealed P5 in higher concentrations, and also contained P4 and small amounts of P6 in proportions which indicated that replication complexes had been released from inclusion bodies. In the different leaves of the host, P2, 3, 4, 5, and 6 all increased in concentration in parallel with viral DNA, although there appeared to be a bias toward protein rather than DNA synthesis in the very young leaves. P1 showed a different pattern of accumulation; it was most concentrated in the very young and the oldest infected tissues, and showed a different spectrum of products between leaves. The experiments described provide a more complete picture of the relationship between CaMV multiplication and expression, and leaf development, and an increased understanding of how the disease syndrome is established.

Cauliflower mosaic virus 35 S RNA leader region inhibits translation of downstream genes

The cauliflower mosaic virus (CaMV) 35 S RNA is a full-length transcript of the viral genome. It encodes the genes VII and I-V, arranged in tandem along the RNA, preceded by a long leader region (600 bases) containing many short open reading frames. We have examined the effects of the leader and the first gene (gene VII) on downstream gene I translation in vitro and in an in vivo transient expression system (carrot protoplasts). RNAs from constructs containing the intact leader, and from various deletion constructs, were translated in a rabbit reticulocyte system. Gene I was translated efficiently only when the long leader region and the upstream gene VII were deleted. Translational fusions of gene VII or I to the firefly luciferase reporter gene were also constructed, and a similar series of leader sequence deletion mutants were examined in vivo and in vitro. The 600-base leader region was found to repress translation of gene VII 8- to 30-fold as compared to the truncated gene lacking the leader region. Gene I expression as compared to that of gene VII was reduced an additional 7- to 20-fold by the presence of the upstream leader region including gene VII. This represented an overall reduction in gene I expression of greater than 100-fold as compared to expression in the absence of any leader sequence. The reduced translation of gene I in the context of the 35 S RNA leader region was not due to the action of the gene VII protein product but may result from efficient blocking of scanning 40 S ribosomes by translation of upstream open reading frames.

The instability of a recombinant plasmid, caused by a prokaryotic-like promoter within the eukaryotic insert, can be alleviated by expression of antisense RNA

A region of the cauliflower mosaic virus genome was found to direct the expression of a nucleic-acid-binding protein in Escherichia coli. This protein is apparently toxic for the bacteria and leads to a destabilization of plasmids containing that region. Antisense RNA was used to diminish the unwanted expression and to stabilize the respective recombinant plasmids. The approach described may prove useful in other cases where problems with cloning of eukaryotic DNA arise.

Expression of CaMV ORF IV in Escherichia coli

A CaMV DNA fragment corresponding to nucleotides 2200-3992 and including the coding sequence (2200-3670) of open reading frame IV was inserted in the pTG908 prokaryotic expression vector. In the recombinant pTG-IV plasmid, ORF IV, which codes for the coat protein precursor, was fused to the N-terminal coding sequence of the lambda CII gene, which is under transcriptional control of the lambda PL promoter. The expected fusion protein CII-ORF IV had a calculated molecular weight of 58.4 Kd. Nevertheless, temperature induction of the PL promoter resulted in synthesis of a major 76-Kd fusion protein: the coat protein precursor migrated abnormally on SDS polyacrylamide gel.

Expression of the cauliflower mosaic virus capsid gene in vivo

Antisera against the N-terminal and C-terminal parts of the potential ORF IV product were used to analyse extracts from CaMV-infected turnip leaves by immunoblotting. Polypeptides of 87, 83, 82, 60 and 57 kDa were detected. The origin of these proteins is discussed.