T.J. Morris

Turnip crinkle virus genes required for RNA replication and virus movement

We have used infectious in vitro transcripts from mutagenized turnip crinkle virus (TCV) cDNA clones to identify the gene products required for viral RNA replication, virion assembly, and intercellular movement. Previous sequence analysis of the TCV genome revealed the presence of five open reading frames which had the potential to encode gene products of 88, 38, 28, 9, and 8 kDa. Inoculation of protoplasts with infectious RNA revealed that only the p28 and p88 gene products are required for viral RNA synthesis. Although the p8 and p9 gene products were dispensable for RNA replication and virion assembly in protoplasts, mutations in the p8 and p9 genes prevented the production of systemic infections in plants. No viral RNA or protein was observed in the inoculated or systemic leaves of plants inoculated with transcripts synthesized from p8 or p9 mutant cDNAs. In contrast to these results, viral RNA was recovered from the inoculated, but not the systemic leaves, of plants inoculated with an RNA lacking the coat protein (CP) gene. With the CP mutant, no symptoms were observed on normally systemic hosts, but small local lesions were induced on Chenopodium amaranticolor. These results indicate that p8, p9, and CP are required for viral movement.

Defective-interfering RNAs and elevated temperatures inhibit replication of tomato bushy stunt virus in inoculated protoplasts

Tomato bushy stunt virus (TBSV) genomic RNA and one of its defective interfering (DI) RNAs were inoculated in various combinations to protoplasts of Nicotiana benthamiana. Ethidium bromide staining of electrophoretically separated RNAs from infected protoplasts, incorporation of [3H]uridine into TBSV and DI RNAs, and Northern hybridization at different times after inoculation clearly demonstrated reduced accumulation of genomic RNA in the presence of DI RNA. Accumulation of genomic RNA was very rapid between 3 and 9 hr postinfection. The presence of equimolar amounts of genomic and DI RNA in the inoculum resulted in a 65% suppression of genomic RNA accumulation. Suppression of genomic RNA was mediated by a reduction in the rate at which genomic RNA accumulated. Analysis of protoplasts inoculated with increasing ratios of DI:genomic RNA suggested that DI RNA-mediated suppression of genomic RNA synthesis results from competition for factors essential for viral replication. Incubation of protoplasts at different temperatures also had a profound effect on replication of both genomic and DI RNAs. Both replicated well at 27 degrees but were barely detectable at 32 degrees. Suppression of genomic RNA synthesis by DI RNA was similar at all temperatures tested. Thus, this study suggests that DI suppression of TBSV symptoms in whole plants and symptom attenuation at elevated temperatures are primarily the result of reduced viral replication.

De novo generation of defective interfering RNAs of tomato bushy stunt virus by high multiplicity passage.

Abstract Defective interfering (DI) RNAs were generated de novo in each of 12 independent isolates of tomato bushy stunt virus (TBSV) upon serial passage at high multiplicities of infection (m.o.i.) in plants, but not in any of 4 additional isolates after 11 serial passages at low m.o.i. The DI RNAs were detected in RNA isolated from virus particles and in 2.3 M LiCl-soluble RNA fractions isolated from inoculated leaves. Symptom attenuation leading to persistent infections was closely correlated with the passage in which DIs first developed. Comparisons of nucleotide sequences of 10 cDNA clones from 2 DI RNA populations and with a previously characterized TBSV DI RNA revealed the same four regions of sequence from the TBSV genome were strictly conserved in each of the DI RNAs: the virus 5′ leader sequence of 168 bases; a region of approximately 200–250 bases from the viral polymerase gene; approximately 70 bases from the 3′ terminus of the viral pl9 and p22 genes; and approximately 130 bases from the 3′terminal noncoding region. Conservation of the sequence motif present in all of the DIs suggests that there might be a common mechanism of DI formation as well as selection pressure to maintain sequences essential for replication and encapsidation.

Point mutations in the turnip crinkle virus capsid protein affect the symptoms expressed by Nicotiana benthamiana

In an effort to determine the biological function(s) of the capsid protein protruding domains unique to the plant carmo- and tombusviruses, we constructed turnip crinkle virus (TCV) mutants in which tandem, in-frame translation terminators replaced the first two codons of the five-amino acid hinge between the shell and the protruding domains of the TCV capsid protein. One of the mutants replicated in inoculated leaves and protoplasts without detectable accumulation of capsid protein. The mutant lacked the capacity to move systemically in Brassica campestris and Nicotiana benthamiana. After 8 weeks, revertant virions that had regained the capacity to move systemically were purified and found to have sense codons at the positions of the introduced translation terminators. One of the revertants, with amino acid substitutions in the hinge, elicited milder symptoms than those elicited by the wild-type virus, and another elicited more severe symptoms. Oligonucleotide-directed mutagenesis was used to show that the hinge mutations were sufficient to elicit the milder, but not the more severe, symptom syndrome. Single amino acid substitutions were also shown to be sufficient to elicit the milder, but not the more severe, symptoms.

Structure and assembly of turnip crinkle virus: VI. Identification of coat protein binding sites on the RNA

Structural studies of turnip crinkle virus have been extended to include the identification of high-affinity coat protein binding sites on the RNA genome. Virus was dissociated at elevated pH and ionic strength, and a ribonucleoprotein complex (rp-complex) was isolated by chromatography on Sephacryl S-200. Genomic RNA fragments in the rp-complex, resistant to RNase A and RNase T1 digestion and associated with tightly bound coat protein subunits, were isolated using coat-protein-specific antibodies. The identity of the protected fragments was determined by direct RNA sequencing. These approaches allowed us to study the specific RNA-protein interactions in the rp-complex obtained from dissociated virus particles. The location of one protected fragment downstream from the amber terminator codon in the first and largest of the three viral open reading frames suggests that the coat protein may play a role in the regulation of the expression of the polymerase gene. We have also identified an additional cluster of T1-protected fragments in the region of the coat protein gene that may represent further high-affinity sites involved in assembly recognition.

Characterization of the cell-free translation products of carnation mottle virus genomic and subgenomic RNAs

The in vitro translation products of carnation mottle virus (CarMV) genomic and subgenomic RNAs were analysed using a rabbit reticulocyte lysate system. Viral RNAs directed synthesis of three main polypeptides, p80, p40, and p34. p40, which was the predominant product using unfractionated virion RNA as template, was identified as coat protein based on electrophoretic mobility through SDS-polyacrylamide gels and immunoprecipitation with anti-CarMV serum. Upon size-fractionation of virion RNAs by sucrose gradient centrifugation and subsequent translational analysis, p40 was found to be encoded by subgenomic RNA, whereas p80 and p34 were synthesized from templates of genome length. p80 and p34 were found to contain common or overlapping amino acid sequences by analysis of cleavage peptides formed during proteolysis with alpha-chymotrypsin. These data support CarMV translational mechanisms other than those proposed in a previous study (R. Saloman, M. Bar-Joseph, H. Soreq, I. Gozes, and U. Z. Littauer, 1978,Virology 90, 288-298).

Complementary DNA Cloning and Characterization of Cymbidium Ringspot Virus RNA

Summary DNA complementary to the 4.7 kb cymbidium ringspot virus (CyRSV) genome was cloned in Escherichia coli. Recombinant plasmids were screened with a cDNA probe synthesized from randomly primed CyRSV genomic RNA, and two clones were chosen for further analysis. Two additional subclones were prepared after deletion of two sequences in the 3′ region and one clone containing 5′ virus genome sequences was constructed by primer extension. Three different RNA species associated with CyRSV were detected in purified virions and virus-infected tissue. The largest RNA was genomic RNA and the other two subgenomic. Northern blot hybridization indicated that the two subgenomic RNAs had in common a region corresponding to the 3′ region of genomic RNA. All three RNAs were found in tissue in both single- and double-stranded forms. In vitro translation experiments with fractionated virus RNA indicated that the coat protein gene is internally located and that it is followed by a 3′-terminal gene coding for a 22K protein.

Physicochemical Characterization of a Small RNA Virus Associated with Baculovirus Infection in Trichoplusia ni

An RNA virus was isolated from Trichoplusia ni larvae infected with inoculum of the baculovirus of Autographa californica. Physicochemical characterization has revealed a 35-nm virus with a density in CsCl of 1.3 g/cm3 and a sedimentation coefficient of 200S. The virus consisted of 10--15% single-stranded RNA with an apparent molecular weight of 1.9 x 10(6) and coat protein with a subunit size of 67,000. The virus is similar to the Nudaurelia capensis beta virus and also possesses some interesting similarities to the mammalian caliciviruses. The proposed cryptogram of the virus is R/1:1.9/12:S/S:I/O.

An Invertebrate Calici-like Virus: Evidence for Partial Virion Disintegration in Host Excreta

SUMMARY A virus with morphological and physicochemical properties similar to those of the vertebrate caliciviruses was isolated from navel orangeworms, Amyelois transitella (Walker). Infected larvae contained two types of virus particles: a 185S, 38 nm cupped particle (ACSVi) with a single major polypeptide of 70000 mol. wt. and a 165S, 28 nm smooth particle (ACSVii) with a single major polypeptide of 29000 tool. wt. Larval frass contained a heterogeneous population of virus particles. Evidence is presented which suggests that the 38 nm particle degrades by proteolytic digestion to produce predominantly 28 nm particles in frass. Virus particles contained a single-stranded RNA of 36S (about 2.5 x 106 mol. wt.).

Unstable infectivity and sedimentable ds-RNA associated with lettuce speckles mottle virus.

Infectivity associated with extracts of lettuce speckles mottle virus (LSMV)-infected tissues is unstable and is susceptible to short periods of aging in vitro or low concentrations of RNase. Infected leaves extracted using phenol yielded greater infectivity than comparable tissue extracted with buffer. No typical virus particles were observed in infected tissues or clarified preparations, but 50- to 70-nm spherical membranous particles were observed associated with the tonoplast in vacuoles of infected cells. Polyacrylamide gel electrophoresis of nucleic acids from infected tissue and LSMV-clarified preparations revealed abundant double-stranded RNA (ds-RNA) in both preparations. A species of LSMV-specific single-stranded RNA (ss-RNA) was also present in LSMV bentonite-clarified preparations. Infectivity was associated only with the ss-RNA fraction. Quantitative isolation of ds-RNA from whole tissue and clarified preparations showed the majority of the ds-RNA was recovered from the bentonite-clarified fractions, and this RNA appeared to be associated with a relatively small structure of a low buoyant density. These data suggest that LSMV lacks a functional coat protein and the sedimentable dsRNA fraction from LSMV-infected tissues more likely contains a viral-specific replication site and not true virus particles.