Coevolution and Hierarchical Interactions of Tomato mosaic virus and the Resistance Gene Tm-1

During antagonistic coevolution between viruses and their hosts, viruses have a major advantage by evolving more rapidly. Nevertheless, viruses and their hosts coexist and have coevolved, although the processes remain largely unknown. We previously identified Tm-1 that confers resistance to Tomato mosaic virus (ToMV), and revealed that it encodes a protein that binds ToMV replication proteins and inhibits RNA replication. Tm-1 was introgressed from a wild tomato species Solanum habrochaites into the cultivated tomato species Solanum lycopersicum. In this study, we analyzed Tm-1 alleles in S. habrochaites. Although most part of this gene was under purifying selection, a cluster of nonsynonymous substitutions in a small region important for inhibitory activity was identified, suggesting that the region is under positive selection. We then examined the resistance of S. habrochaites plants to ToMV. Approximately 60% of 149 individuals from 24 accessions were resistant to ToMV, while the others accumulated detectable levels of coat protein after inoculation. Unexpectedly, many S. habrochaites plants were observed in which even multiplication of the Tm-1-resistance-breaking ToMV mutant LT1 was inhibited. An amino acid change in the positively selected region of the Tm-1 protein was responsible for the inhibition of LT1 multiplication. This amino acid change allowed Tm-1 to bind LT1 replication proteins without losing the ability to bind replication proteins of wild-type ToMV. The antiviral spectra and biochemical properties suggest that Tm-1 has evolved by changing the strengths of its inhibitory activity rather than diversifying the recognition spectra. In the LT1-resistant S. habrochaites plants inoculated with LT1, mutant viruses emerged whose multiplication was not inhibited by the Tm-1 allele that confers resistance to LT1. However, the resistance-breaking mutants were less competitive than the parental strains in the absence of Tm-1. Based on these results, we discuss possible coevolutionary processes of ToMV and Tm-1.     

Gaining replicability in a nonhost compromises the silencing suppression activity of Tobacco mild green mosaic virus in a host

Natural isolates of Tobacco mild green mosaic virus (TMGMV) fail to infect tomato because the tomato tm-1 protein binds to the replication proteins of TMGMV and prevents RNA replication. Previously, we isolated a TMGMV mutant that overcomes tm-1-mediated resistance and multiplies in tomato plants. Here, we show that the causal mutations in the replication protein gene that abolish the interaction with tm-1 reduce its ability to suppress RNA silencing in host plant Nicotiana benthamiana. The results suggest that the multifunctionality of the replication proteins is an evolutionary constraint of tobamoviruses that restricts their host ranges.     

Expression of tobacco mosaic virus RNA in transgenic plants

Summary Tobacco mosaic virus (TMV) is a message-sense, single-stranded RNA virus that infects many Solanaceae plants. A full-length cDNA copy of TMV genomic RNA was constructed and introduced into the genomic DNA of tobacco plants using a disarmed Ti plasmid vector. Transformed plants showed typical symptoms of TMV infection, and their leaves contained infectious TMV particles. This is the first example of the expression of RNA virus genomic RNAs in planta.     

Molecular basis of plant viral virulence; the complete nucleotide sequence of an attenuated strain of tobacco mosaic virus.

The total genome sequence of L11A, an attenuated strain of tobacco mosaic virus (TMV), has been determined. This strain is able to multiply in tomato plants without inducing any remarkable symptoms, but to protect them from later infection with virulent TMV strains. When compared with the recently published total genome sequence of TMV L (the virulent ancestral strain of L11A) ten base substitutions were found in the L11A genome. Seven of these occurred in the third letters of in-phase codons and did not influence amino acids. Only three, which were in the common reading frame for both the 130K and 180K proteins, resulted in amino acid changes. Together with the result of the partial sequence of RNA of L11, an intermediate strain in sequential isolation from L to L11A, it is observed that one base at the nucleotide position 1117 is changed from L to L11 and two bases at the positions 2349 and 2754 are changed from L11 to L A11.     

Structural characteristics of two wheat histone H2A genes encoding distinct types of variants and functional differences in their promoter activity

To investigate the regulation of plant histone H2A gene expression, we isolated two H2A genes (TH254 and TH274) from wheat, which encode two variants of H2A. Both genes had an intron in the coding region. In the promoters, some characteristic sequences, such as Oct and Nona motifs, which are conserved among plant histone genes, were located in a short region (about 120 bp) upstream from the putative TATA box. Transient expression analyses of promoter activity with H2A–GUS fusion genes using tobacco protoplasts revealed novel types of positive cis/-acting sequences in the TH254 promoter: a direct repeat of a 13 bp sequence (AGTTACATTATTG) and a stretch composed of an AT-rich sequence (ATATAGAAAATTAAAA) and a G-box (CACGTG). Quantitative S1 assay of the mRNA amounts from the TH254/GUS and TH274/GUS chimeric genes in stably transformed and cell cycle-synchronized tobacco cell lines showed that the promoters of both genes contained at least one cis/-acting element responsible for S phase-specific expression. Histochemical analysis of transgenic tobacco plants carrying the chimeric genes showed that the promoters of the two H2A genes were active in developing seedlings and flower organs but were regulated in a different manner.     

Cooperation of two distinct cis-acting elements is necessary for the S phase-specific activation of the wheat histone H3 promoter

We have previously shown that the proximal promoter region (−185 to +57) of the wheat histone H3 gene ( TH012 ) is sufficient for regulating S phase-specific expression of a reporter GUS gene. To define the cis -acting element(s) responsible for S phase-specific expression, GUS fusion genes under the control of wild-type or variously mutated H3 promoters were stably introduced into cultured rice Oc cells and their temporal expression was analyzed during the cell cycle by quantitative S1 analysis. The S phase-specific expression of the full-sized promoter (−1716 to +52) was significantly impaired by short internal deletions disrupting the type I element from −175 to −158 (CCACGTCACCaATCCGCG), composed of the Hex (CCACG-TCA) and reverse-oriented Oct (GATCCGCG) motifs. Moreover, the H3 proximal promoters (−184 to +52) harboring base-substitution mutations in either or both of the Hex and Oct motifs could no longer activate gene expression during the S phase. These results indicate that the type I element is the first cis-acting element identified responsible for the S phase-specific expression of plant histone genes. Results also suggested the presence of a redundant cis -acting element(s) responsible for S phase-specific expression in the H3 far-upstream region (−1716 to −185).     

Preparation of antibodies that react with the large non-structural proteins of tobacco mosaic virus by usingEscherichia coli expressed fragments

Summary We have establishedEscherichia coli strains that overproduce two regions of the large non-structural proteins of tobacco mosaic virus, the 126K and 183K proteins, as fusion proteins with β-galactosidase. The two fusion proteins included respectively 514 amino acids common to both the 126K and 183K proteins, and 432 amino acids specific to the 183K protein. The synthesis of the fusion proteins inE. coli was controlled by the lipoprotein promoter andlac promoter-operator systems. After induction, the fusion proteins that were synthesized aggregated and formed inclusion bodies. Antisera raised against the purified fusion proteins reacted specifically with both the 126K and 183K proteins or with only the 183K protein in TMV-infected tobacco protoplasts. The 54K protein corresponding to the C-terminus of the 183K protein, which has been suggested to be synthesized from a third subgenomic mRNA, could not be detected by this method.