Nucleotide sequence and in vitro expression of the capsid protein gene of tobacco ringspot virus

AlphaFold modeling of nepovirus 3C-like proteinases provides new insights into their diverse substrate specificities

The majority of picornaviral 3C proteinases (3Cpro) cleavage sites possess glutamine at the P1 position. Plant nepovirus 3C-like proteinases (3CLpro) show however much broader specificity, cleaving not only after glutamine, but also after several basic and hydrophobic residues. To investigate this difference, we employed AlphaFold to generate structural models of twelve selected 3CLpro, representing six substrate specificities. Generally, we observed favorable correlations between the architecture and charge of nepovirus proteinase S1 subsites and their ability to accept or restrict larger residues. The models identified a conserved aspartate residue close to the P1 residue in the S1 subsites of all nepovirus proteinases examined, consistent with the observed strong bias against negatively-charged residues at the P1 position of nepovirus cleavage sites. Finally, a cramped S4 subsite along with the presence of two unique histidine and serine residues explains the strict requirement of the grapevine fanleaf virus proteinase for serine at the P4 position.

Re-examination of nepovirus polyprotein cleavage sites highlights the diverse specificities and evolutionary relationships of nepovirus 3C-like proteases

Plant-infecting viruses of the genus Nepovirus (subfamily Comovirinae, family Secoviridae, order Picornavirales) are bipartite positive-strand RNA viruses with each genomic RNA encoding a single large polyprotein. The RNA1-encoded 3C-like protease cleaves the RNA1 polyprotein at five sites and the RNA2 polyprotein at two or three sites, depending on the nepovirus. The specificity of nepovirus 3C-like proteases is notoriously diverse, making the prediction of cleavage sites difficult. In this study, the position of nepovirus cleavage sites was systematically re-evaluated using alignments of the RNA1 and RNA2 polyproteins, phylogenetic relationships of the proteases, and sequence logos to examine specific preferences for the P6 to P1' positions of the cleavage sites. Based on these analyses, the positions of previously elusive cleavage sites, notably the 2a-MP cleavage sites of subgroup B nepoviruses, are now proposed. Distinct nepovirus protease clades were identified, each with different cleavage site specificities, mostly determined by the nature of the amino acid at the P1 and P1' positions of the cleavage sites, as well as the P2 and P4 positions. The results will assist the prediction of cleavage sites for new nepoviruses and help refine the taxonomy of nepoviruses. An improved understanding of the specificity of nepovirus 3C-like proteases can also be used to investigate the cleavage of plant proteins by nepovirus proteases and to understand their adaptation to a broad range of hosts.

Frequent asymptomatic infection with tobacco ringspot virus on melon fruit

Melon is one of the most popular fruits worldwide and has been bred into various cultivars. RNA-sequencing using healthy melon fruit was performed to determine differences in gene expression among cultivars. Unexpected RNA-seq results revealed that viruses asymptomatically infected fruits at a high frequency (16 of 21 fruits examined were infected) and that viral transcripts highly accumulated in comparison with host transcripts (15 %-75 % of total reads). Their nucleotide sequences and phylogenetic analyses indicated that more than 10 novel isolates of tobacco ringspot virus (TRSV) were found in melon fruits. Asymptomatic infection with TRSV on melon fruits was confirmed by both immunoblot and RT-PCR analyses. Numerous isolates of TRSV generated and maintained in melon fields, and this is likely due to their asymptomatic infections. This TRSV melon isolate infected Nicotiana benthamiana plants with stunting and yellowing symptoms. This is the first report of frequent and asymptomatic infection of TRSV in consumable melon fruits.

Development of tobacco ringspot virus-based vectors for foreign gene expression and virus-induced gene silencing in a variety of plants

We report here the development of tobacco ringspot virus (TRSV)-based vectors for the transient expression of foreign genes and for the analysis of endogenous gene function in plants using virus-induced gene silencing. The jellyfish green fluorescent protein (GFP) gene was inserted between the TRSV movement protein (MP) and coat protein (CP) regions, resulting in high in-frame expression of the RNA2-encoded viral polyprotein. GFP was released from the polyprotein via an N-terminal homologous MP-CP cleavage site and a C-terminal foot-and-mouth disease virus (FMDV) 2 A catalytic peptide in Nicotiana benthamiana. The VIGS target gene was introduced in the sense and antisense orientations into a SnaBI site, which was created by mutating the sequence following the CP stop codon. VIGS of phytoene desaturase (PDS) in N. benthamiana, Arabidopsis ecotype Col-0, cucurbits and legumes led to obvious photo-bleaching phenotypes. A significant reduction in PDS mRNA levels in silenced plants was confirmed by semi-quantitative RT-PCR.

Characterization of a putative novel nepovirus from Aeonium sp

A virus was isolated from potted plants of an unidentified species of Aeonium, a succulent ornamental very common in Southern Italy, showing chlorotic spots and rings on both leaf surfaces. It was successfully transmitted by sap inoculation to a limited range of hosts, including Nicotiana benthamiana which was used for ultrastructural observations and virus purification. Virus particles are isometric, ca. 30nm in diameter, have a single type of coat protein (CP) subunits 54kDa in size, that encapsidate single-stranded positive-sense RNA species of 7549 (RNA1) and 4010 (RNA2) nucleotides. A third RNA molecule 3472 nts in size entirely derived from RNA2 was also found. The structural organization of both genomic RNAs and the cytopathological features were comparable to those of nepoviruses. In addition, amino acid sequence comparisons of CP and the Pro-Pol region (a sequence containing parts of the proteinase and polymerase) with those of other nepoviruses showed that the Aeonium virus belongs to the subgroup A of the genus Nepovirus and is phylogenetically close to, but serologically distinct from tobacco ringspot virus (TRSV). Based on the species demarcation criteria for the family Secoviridae, the virus under study appears to be a novel member of the genus Nepovirus for which the name of Aeonium ringspot virus (AeRSV) is proposed.

Viruses and Virus Diseases of Rubus

Blackberry and raspberry are members of the family Rosaceae. They are classified in the genus Rubus, which comprises hundreds of species and has a center of origin in the Far East. Rubus is divided into 15 subgenera with blackberries classified in the Rubus (formerly Eubatus) and raspberries in the Idaeobatus subgenera. Rubus species are propagated vegetatively and are subject to infection by viruses during development, propagation, and fruit production stages. Reports of initial detection and symptoms of more than 30 viruses, virus-like diseases, and phytoplasmas affecting Rubus spp. were reviewed more than 20 years ago. Since the last review on Rubus viruses, significant progress has been made in the molecular characterization of many of the viruses that infect Rubus spp. Currently, reverse transcription-polymerase chain reaction detection methods are available for most of the viruses known to infect Rubus. The goals of this article are to update the knowledge on previously characterized viruses of Rubus, highlight recently described viruses, review the virus-induced symptoms, describe the advances made in their detection, and discuss our knowledge about several virus complexes that cause serious diseases in Rubus. Virus complexes have been identified recently as the major cause of diseases in blackberries and raspberries.

A Stochastic Model of Icosahedral Capsid Growth

A model of growth of icosahedral capsids is proposed. It is similar to the stochastic model of fullerene growth, but takes into account variations in the composition of pentamers and hexamers that are responsible for the final capsid size. We show that the observed high yield of capsid production implies a high level of self-organization of the elementary building blocks.

Cloning and sequencing of peach rosette mosaic virus RNA1

The complete nucleotide sequence of peach rosette mosaic nepovirus (PRMV) RNA1 has been determined. A grapevine isolate of PRMV from Michigan was propagated and purified and cDNA clones representing 99. 5% of the RNA1 were constructed. The cDNA and direct RNA sequence analysis revealed a RNA species of 8004 nucleotides, excluding a 3' polyadenylated tail. The 5'- and 3'-untranslated regions were 52 and 1474 nucleotides, respectively. Computer analysis of the PRMV RNA1 nucleotide sequence unveiled a single long open reading frame of 6477 nucleotides, which is capable of encoding a 240 kDa polyprotein. Analysis of the predicted amino acid sequence of RNA1 revealed amino acid motifs characteristic of a replicase, proteinase, NTP-binding protein and a proteinase cofactor. The order and identity of these putative proteins are consistent with other nepoviruses. Analysis of PRMV RNA1 further distinguishes the taxonomic subdivisions within the nepovirus group, confirms the subgroup three status of PRMV and lays the groundwork for a replicase-mediated resistance strategy.

Mutagenesis of amino acids at two tomato ringspot nepovirus cleavage sites: effect on proteolytic processing in cis and in trans by the 3C-like protease

Tomato ringspot nepovirus (ToRSV) encodes two polyproteins that are processed by a 3C-like protease at specific cleavage sites. Analysis of ToRSV cleavage sites identified previously and in this study revealed that cleavage occurs at conserved Q/(G or S) dipeptides. In addition, a Cys or Val is found in the -2 position. Amino acid substitutions were introduced in the -6 to +1 positions of two ToRSV cleavage sites: the cleavage site between the protease and putative RNA-dependent RNA polymerase, which is processed in cis, and the cleavage site at the N-terminus of the movement protein, which is cleaved in trans. The effect of the mutations on proteolytic processing at these sites was tested using in vitro translation systems. Substitution of conserved amino acids at the -2, -1, and +1 positions resulted in a significant reduction in proteolytic processing at both cleavage sites. The effects of individual substitutions were stronger on the cleavage site processed in trans than on the one processed in cis. The cleavage site specificity of the ToRSV protease is discussed in comparison to that of related proteases.

The structure of tobacco ringspot virus: a link in the evolution of icosahedral capsids in the picornavirus superfamily

BACKGROUND

Tobacco ringspot virus (TRSV) is a member of the nepovirus genus of icosahedral RNA plant viruses that cause disease in fruit crops. Nepoviruses, comoviruses and picornaviruses are classified in the picornavirus superfamily. Crystal structures of comoviruses and picornaviruses and the molecular mass of the TRSV subunit (sufficient to accommodate three beta-barrel domains) suggested that nepoviruses may represent a link in the evolution of the picornavirus capsids from a T = 3 icosahedral virus. This evolutionary process is thought to involve triplication of the capsid protein gene, to encode a three-domain polyprotein, followed by development of cleavage sites in the interdomain linking regions. Structural studies on TRSV were initiated to determine if the TRSV subunit corresponds to the proposed uncleaved three-domain polyprotein.

RESULTS

The 3.5 A resolution structure of TRSV shows that the capsid protein consists of three beta-barrel domains covalently linked by extended polypeptides. The order of connectivity of the domains in TRSV confirms the proposed connectivity for the precleaved comovirus and picornavirus capsid polyprotein. Structural differences between equivalent domains in TRSV and comoviruses are confined to the external surface loops, interdomain connecting polypeptides and N termini. The three different domains within TRSV and comoviruses are more closely related at the structural level than the three individual domains within picornaviruses.

CONCLUSIONS

The structural results confirm the notion of divergent evolution of the capsid polyproteins of nepoviruses, comoviruses and picornaviruses from a common ancestor. A number of residues were found to be conserved among various nepoviruses, some of which stabilize the quaternary structure of the three domains in the TRSV capsid protein subunit. Two conserved regions were identified on the external surface of TRSV, however, mutational studies will be needed to understand their functional significance. Nepoviruses transmitted by the same nematode species do not share regions with similar amino acid composition on the viral surface.

Characterization of the coat protein gene of mite-transmitted blackcurrant reversion associated nepovirus

The nucleotide sequence of the 3' terminal 3105 nucleotides (nt) of RNA2 of blackcurrant reversion associated virus (BRAV), the first mite-transmitted member of the nepovirus group, has been determined. The sequence contains an open reading frame of 1744 nt in the virus-sense strand, a 3' untranslated region of 1360 nt and a 3' poly(A) tail. Analysis of the amino-terminal residues of purified coat protein (CP) suggests that the CP gene is located between nts 1361 and 2959 (from the 3' terminus) in the RNA2, and that Asp/Ser is the proteolytic cleavage site of CP in the RNA2 encoded polyprotein. The predicted translation product from the CP gene is a polypeptide of 533 amino acids with a calculated Mr of 57 561. The amino acid sequence of BRAV CP showed highest similarity to blueberry leaf mottle virus (BLMV), and tomato ringspot virus (ToRSV), two members of the proposed sub-group three of nepoviruses possessing large RNA2 components. Nucleic and amino acid sequence comparisons between BRAV CP and the CPs of other nepoviruses indicate that specific conserved nepovirus CP domains occur in the BRAV CP thus confirming that BRAV is a member of the subgroup three of nepoviruses. reserved.

Nucleotide sequence of the coat protein genes of citrus mosaic virus

The sequence of the 3'-terminal 2,201 nucleotides of RNA2 of citrus mosaic virus (CiMV) was determined. The sequence contains a long open reading frame (ORF) of 1989 nucleotides in the virus-sense strand and at 3' untranslated region of 212 nucleotides upstream of a poly(A) tail. The N-terminal amino acid sequence of the two coat proteins was determined by Edman degradation and the corresponding coding regions were identified in the polyprotein. The larger coat protein with Mr 48,122 is encoded upstream of the smaller one with Mr 24,172. The coat proteins are apparently cleaved from the polyprotein at an Arg-Gly and a Thr-Asn bond. Although CiMV has properties in common with the comoviruses and nepoviruses, there is no significant sequence homology between the coat proteins of CiMV and those of either group. Furthermore, the coat proteins of CiMV lack homology with those of strawberry latent ringspot virus (SLRSV), which has been reported to be more distantly related to the como- and nepoviruses. This lack of homology distinguishes CiMV from the como- and nepoviruses, SLRSV and other viruses.