Nucleotide sequence of cauliflower mosaic virus DNA

The complete nucleotide sequence (8024 nucleotides) of the circular double-stranded DNA of cauli-flower mosaic virus has been established. The DNA molecule is known to possess three discrete single-stranded discontinuities, often referred to as “gaps”, two in one strand and one in the other. The sequence data indicate that gap 1, the single discontinuity in the α strand, corresponds to the absence of no more than one or two nucleotides with respect to the complementary β strand. The two discontinuities in the β strand, however, are not authentic gaps since no nucleotides are missing, but are instead regions of sequence overlap: a short sequence (19 residues for gap 2, at least 2 residues for gap 3) at one terminus of each discontinuity, probably the 5′ terminus, is displaced from the double helix by an identical sequence at the other boundary of the discontinuity. Analysis of the distribution of nonsense codons in the DNA sequence is consistent with other evidence that only the α strand is transcribed. The coding region extends around the circular molecule from 4 map units of gap 1, the map origin, to map position 91, and consists of six long open reading frames. Our findings suggest, but do not prove, that the DNA sequence of the open reading frames is colinear with viral protein sequences. The cistron for the viral coat protein, which is probably synthesized in the form of a precursor, has been situated in coding region IV on the basis of its unusual amino acid composition.     

Studies on mutagenesis and cross-protection of cauliflower mosaic virus

Three strains of cauliflower mosaic virus (CaMV) designated NVRS, CM4-184 and PK caused respectively severe, moderate and mild reactions in turnip cv. Just Right plants and severe, mild and symptomless reactions in Brussels sprout cv. Fasolt plants. Chlorotic local lesions formed consistently in the leaves of young turnip plants when inoculated with each of the virus strains. Lesions were suitable for infectivity assay of crude and purified preparations of the virus. Three variants of the NVRS strain were isolated by single-lesion transfer after treatment of the virus with nitrous acid (pH 5.0) and two variants were obtained after treating the virus in acetate buffer at the same pH. One of the variants (designated V3) caused symptomless infection in turnip and Brussels sprout plants. In cross-protection tests, Brussels sprout plants infected symptomlessly with the PK, CM4-184 or the V3 strains, subsequently resisted infection by the severe NVRS strain.     

Nuclei purified from cauliflower mosaic virus-infected turnip leaves contain subgenomic, covalently closed circular cauliflower mosaic virus DNAs

Nuclei isolated from cauliflower mosaic virus (CaMV) infected turnip leaves contain subgenomic CaMV DNA species in addition to the genome length CaMV DNA. These subgenomic CaMV DNA species are present as covalently closed circles (form I), relaxed circles (form II) and linear (form III) molecules. The subgenomic form I DNA species range in size from about 10% of genome length to nearly genome length. These subgenomic DNA species appear in tissue infected with cloned CaMV DNA, indicating that they arise rapidly and have not accumulated in the virus population from serial propagation of CaMV. No specific region of the CaMV genome appears to be preferentially deleted to form the subgenomic CaMV DNA species. At least three distinct subgenomic species appear to accumulate preferentially in nuclei isolated from infected tissue. Two of these abundant subgenomic CaMV DNA species are form I and the other one is form III. Some of the subgenomic CaMV DNA species appear to be minichromosomes.     

Molecular dissection of the cauliflower mosaic virus translation transactivator.

The cauliflower mosaic virus (CaMV) transactivator (TAV) is a complex protein that appears to be involved in many aspects of the virus life cycle. One of its roles is to control translation from the polycistronic CaMV 35S RNA. Here we report a molecular dissection of TAV in relation to its ability to enhance dicistronic translation in transient expression experiments. We have identified a protein domain that is responsible and sufficient for that activity. This 'MiniTAV domain' consists of only 140 of the 520 amino acids in the full-length sequence. A further domain located outside the MiniTAV, and therefore dispensable for transactivation, is probably involved in interactions with other molecules. This was identified by its ability to compete with wild-type TAV and some of its deletion mutants. We found, furthermore, that the TAV protein binds RNA. Two regions needed for RNA-binding properties were defined outside the MiniTAV domain and RNA binding seems not to be directly involved in the transactivation mechanism.     

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.     

Selective allele loss and interference between cauliflower mosaic virus DNAs

Summary Some cauliflower mosaic virus (CaMV) alleles are selectively lost during growth of the virus in mixedly infected turnip plants. Viral DNA from plants co-inoculated with DNA of the cabbage S isolate and infectious cabbage S DNA with an extra EcoRI restriciion site lacked the extra site. The EcoRI allele was also lost in most plants co-inoculated with a non-infectious mutant of cabbage S DNA while little selective allele loss was observed with two other non-infectious mutant DNAs. Plants co-inoculated with DNAs of closely-related isolates (CM4-184 and W) contained both parental viral DNAs and some DNAs with characteristics of both parents. Interference, scored as a reduced frequency of infection or a delay in symptom appearance relative to plants inoculated with wild-type DNA, occurred when plants were inoculated with wild-type and mutant DNAs covalently attached to one another in partial dimer plasmid DNAs. Similarities in the conditions leading to selective allele loss and those leading to interference suggest that both may have been due to active gene conversion between CaMV DNA molecules.     

A new strategy to improve a cauliflower mosaic virus vector

Co-infection of plants with non-overlapping deletion mutants of cauliflower mosaic virus usually leads to the production of the wild-type virus. To prevent this, a pair of mutants with overlapping deletions was constructed. In infected plants both mutant DNAs were stably maintained. Such mutants with overlapping deletions will be used as a vector to overcome the size limitation of genes to be cloned.     

Physical map of DNA from a new cauliflower mosaic virus strain

The restriction enzymes AluI, BamHI, BglII, EcoRI, HindIII, and SalI have been used to characterize and map a new cauliflower mosaic virus strain (Cabb-S). These fragments have been ordered by examining their overlapping regions after double enzymatic digestion. The single SalI cleavage site was chosen as the point of origin. We compare this strain with those already described.     

Pathogenic interactions between variants of cauliflower mosaic virus and Arabidopsis thaliana

Pathogenic interactions between genetic variants of cauliflower mosaic virus (CaMV) and Arabidopsis thaliana were characterized to identify combinations potentially useful in molecular genetic analysis. Infections of a glabrous mutant (gl1) of Arabidopsis ecotype Columbia (Col-0 gl1) by 30 CaMV isolates were assessed by recording symptom character. Thirteen isolates failed to cause symptoms; the remainder induced symptoms that varied between mild and very severe. Some CaMV isolates produced symptoms in Arabidopsis that differed significantly in severity or character from those produced in a standard host Brassica rapa (turnip). A greater variety of symptom types was observed in a single Arabidopsis ecotype infected with a range of CaMV isolates than was found in a range of Arabidopsis ecotypes infected with a single, typical CaMV isolate (Cabb B-JI). One isolate, Bari-1, that was asymptomatic but accumulated virus in Arabidopsis ecotype Col-0 gl1, caused mild symptoms in ecotype Ler gl1. A hybrid virus constructed from CaMV isolates Cabb B-JI and Bari-1 produced symptoms in Arabidopsis variants that were more severe than in either parental isolate. From a screen of EMS-mutagenized Arabidopsis, one mutant (Col-0 dv1) with a pale-green, dark-vein phenotype which had an altered symptom response to CaMV, was isolated. From this, a phenotypically near-normal revertant (Col-0 dv1R) spontaneously arose, but which showed altered responses to CaMV. Infection of Col-0 dv1R by CaMV isolate Bari-1 elicited symptoms unlike the parent Arabidopsis ecotype (Col-0 gl1). Also, Col-0 dv1 and Col-0 dv1R expressed an uncharacteristic necrotic reaction to CaMV.     

Mutation of capsid protein phosphorylation sites abolishes cauliflower mosaic virus infectivity

The cauliflower mosaic virus (CaMV) capsid protein is derived by bidirectional processing of the precapsid protein (CP56). We expressed several derivatives of CP56 in Escherichia coli and used them as substrates for virus-associated kinase and casein kinase II purified from plant cells. Three serine residues located at the N terminus of the mature viral protein CP44 were identified as phosphorylation targets. A mutation of one of them in the viral context had little or no effect on viral infectivity, but a mutation of all three serines abolished infectivity. The mapping of phosphorylation sites in CP44, but not CP39 or CP37, and immunodetection of the Zn finger motif in CP44 and CP39, but not CP37, support the model that CP39 is produced from CP44 by N-terminal processing and CP37 is produced from CP39 by C-terminal processing. We discuss the possible role of phosphorylation in the processing and assembly of CaMV capsid protein.     

Aphid transmission of cauliflower mosaic virus requires the viral PIII protein

The open reading frame (ORF) III product (PIII) of cauliflower mosaic virus is necessary for the infection cycle but its role is poorly understood. We have used in vitro protein binding ('far Western') assays to demonstrate that PIII interacts with the cauliflower mosaic virus (CaMV) ORF II product (PII), a known aphid transmission factor. Aphid transmission of purified virions of the PII-defective strain CM4-184 was dependent upon added PII, but complementation was efficient only in the presence of PIII, demonstrating the requirement of PIII for transmission. Deletion mutagenesis mapped the interaction domains of PIII and PII to the 30 N-terminal and 61 C-terminal residues of PIII and PII, respectively. A model for interaction between PIII and PII is proposed on the basis of secondary structure predictions. Finally, a direct correlation between the ability of PIII and PII to interact and aphid transmissibility of the virus was demonstrated by using mutagenized PIII proteins. Taken together, these data argue strongly that PIII is a second 'helper' factor required for CaMV transmission by aphids.     

Region VI of cauliflower mosaic virus encodes a host range determinant.

A domain of cauliflower mosaic virus (CaMV) which controls systemic spread in two solanaceous hosts (Datura stramonium and Nicotiana bigelovii) was mapped to the first half of open reading frame 6. Whereas ordinary strains of CaMV are unable to infect solanaceous species except to replicate locally in inoculated leaves, a new CaMV strain (D4) induces chlorotic local lesions and systemically infects both D. stramonium and N. bigelovii. To determine which portion of the CaMV genome controls systemic spread of the virus in solanaceous hosts, nine recombinant genomes constructed between D4 and two ordinary strains of the virus were tested for their ability to infect solanaceous hosts. A 496-base-pair DNA segment comprising the first half of open reading frame 6 specified the type of local lesions and systemic spread of the virus in solanaceous hosts. Exchange of this segment of the genome between strains of CaMV converted a compatible host reaction to an incompatible (hypersensitive) one in response to infection. This suggests that the gene VI protein interacts with the plant to suppress hypersensitivity, the normal response of solanaceous hosts to CaMV infection.