Highly purified cucumber mosaic virus-induced RNA-dependent RNA polymerase does not contain any of the full length translation products of the genomic RNAs

Defense signaling pathways in resistance to plant viruses: Crosstalk and finger pointing

Resistance to infection by plant viruses involves proteins encoded by plant resistance (R) genes, viz., nucleotide-binding leucine-rich repeats (NLRs), immune receptors. These sensor NLRs are activated either directly or indirectly by viral protein effectors, in effector-triggered immunity, leading to induction of defense signaling pathways, resulting in the synthesis of numerous downstream plant effector molecules that inhibit different stages of the infection cycle, as well as the induction of cell death responses mediated by helper NLRs. Early events in this process involve recognition of the activation of the R gene response by various chaperones and the transport of these complexes to the sites of subsequent events. These events include activation of several kinase cascade pathways, and the syntheses of two master transcriptional regulators, EDS1 and NPR1, as well as the phytohormones salicylic acid, jasmonic acid, and ethylene. The phytohormones, which transit from a primed, resting states to active states, regulate the remainder of the defense signaling pathways, both directly and by crosstalk with each other. This regulation results in the turnover of various suppressors of downstream events and the synthesis of various transcription factors that cooperate and/or compete to induce or suppress transcription of either other regulatory proteins, or plant effector molecules. This network of interactions results in the production of defense effectors acting alone or together with cell death in the infected region, with or without the further activation of non-specific, long-distance resistance. Here, we review the current state of knowledge regarding these processes and the components of the local responses, their interactions, regulation, and crosstalk.

Characterization of the virus encoded subunit of turnip yellow mosaic virus RNA replicase

An antiserum raised against TYMV-RNA encoded protein P115 partially inhibits TYMV RNA replicase activity, demonstrating that this protein is involved in TYMV RNA synthesis. The detection of protein P115 by an antibody linked polymerase assay demonstrates that protein P115 is indeed a subunit of the TYMV RNA replicase, the enzyme known to synthesize viral RNA in infected Chinese cabbage. The use of translation products of other tymoviruses indicates that the serological relationship between the virus-encoded replicase subunits of these viruses and protein P115 is very weak at the best.

Viral RNA synthesis by a particulate fraction from cucumber seedlings infected with cucumber mosaic virus

Particulate preparations from cucumber mosaic virus (CMV)-infected cucumber cotyledons incorporated [alpha-32P]GTP into products which, after phenol extraction, appeared to be double-stranded forms of the four CMV RNAs. In preparations isolated 3 to 5 days after inoculation, the label was incorporated mostly into plus RNA as shown by dot-blot hybridization using single-strand recombinant DNA clones of CMV RNA. After a short pulse the labeled material consisted mostly of small heterogeneous products, but part of the label could be chased into full-length RNAs, demonstrating that the synthetic process was continuous strand elongation rather than terminal addition. CMV RNAs added to the particulate fraction did not give rise to high molecular weight transcripts. In addition to the high molecular weight defined products which remained in the particulate fraction, small heterogeneous products, complementary to plant RNA and to viral RNA of both plus and minus polarity, were released into the incubation medium. They appeared to be products of the virus-induced Mr 100,000 (100K) protein which is solubilized from the particulate fraction during incubation. However, when the particulate fraction was first subjected to an extensive solubilization and washing treatment in order to remove the M(r) 100K protein [D. S. Gill, R. Kumarasamy, and R. H. Symons (1981) Virology 113, 1-8], and then used for product synthesis, a large amount of the product was still of small size, suggesting that the synthesis of high and low molecular weight RNA was intrinsically connected, and that the M(r) 100K protein was not merely contaminating the particulate fraction.

Subgenomic RNAs with nucleotide sequences derived from RNAs 1 and 2 of cucumber mosaic virus can act as messenger RNAs in vitro

Encapsidated RNAs of cucumber mosaic virus (CMV) were analyzed by hybridization to specific probes after gel electrophoresis. [32P]-complementary DNA (cDNA) probes were prepared by transcription of genomic RNA 1 and RNA 2 nucleotide sequences that had been cloned in a bacteriophage M13 vector. Probes that correspond to unique sequences near the 3' ends of RNA 1 and RNA 2 revealed over 20 smaller RNAs. The subgenomic RNAs derived from each genomic RNA were analyzed more definitively by hybrid selection from total encapsidated RNA, using minus DNA clones derived from sequences in either RNA 1 or RNA 2, and a cDNA probe for the 3' sequence conserved among all the genomic RNAs. Different patterns of over 20 minor RNA species, which were 3'-coterminal with RNAs 1 and 2, were detected, and they were reproducible irrespective of the host, cucumber or Nicotiana clevelandii, from which the virus was isolated. The same RNA patterns were found in RNA extracted from the particulate fraction of CMV-infected cucumber orN. clevelandii. In order to determine whether the subgenomic RNAs could function as messenger RNAs, hybrid-selected RNAs were tested by in vitro translation, using the rabbit reticulocyte lysate. The subgenomic RNAs from RNA 1 produced over 10 major polypeptides from Mr 27,000 to Mr 90,000 all of which could be translated from a few RNA species over about 2,300 nucleotides long. The 3'-coterminal subgenomic RNAs derived from RNA 2 gave less than 10 products from Mr, 17,000 toM(r) 85,000. The smallest product (Mr 17,000) was produced by an RNA about 880 nucleotides long, whereas longer RNAs from 1400 to 2500 nucleotides were efficient mRNAs for polypeptides from Mr 30,000 up to the largest translation products consistent with the size of the RNA.

Cucumoviruses

Research on the molecular biology of cucumoviruses and their plant-virus interactions has been very extensive in the last decade. Cucumovirus genome structures have been analyzed, giving new insights into their genetic variability, evolution, and taxonomy. A new viral gene has been discovered, and its role in promoting virus infection has been delineated. The localization and various functions of each viral-encoded gene product have been established. The particle structures of Cucumber mosaic virus (CMV) and Tomato aspermy virus have been determined. Pathogenicity domains have been mapped, and barriers to virus infection have been localized. The movement pathways of the viruses in some hosts have been discerned, and viral mutants affecting the movement processes have been identified. Host responses to viral infection have been characterized, both temporally and spatially. Progress has been made in determining the mechanisms of replication, gene expression, and transmission of CMV. The pathogenicity determinants of various satellite RNAs have been characterized, and the importance of secondary structure in satellite RNA-mediated interactions has been recognized. Novel plant genes specifying resistance to infection by CMV have been identified. In some cases, these genes have been mapped, and one resistance gene to CMV has been isolated and characterized. Pathogen-derived resistance has been demonstrated against CMV using various segments of the CMV genome, and the mechanisms of some of these forms of resistances have been analyzed. Finally, the nature of synergistic interactions between CMV and other viruses has been characterized. This review highlights these various achievements in the context of the previous work on the biology of cucumoviruses and their interactions with plants.

Complete replication of a eukaryotic virus RNA in vitro by a purified RNA-dependent RNA polymerase

A soluble RNA-dependent RNA polymerase was isolated from Nicotiana tabacum plants infected with cucumber mosaic virus (CMV), which has a genome of three positive-strand RNA components, 1, 2, and 3. The purified polymerase contained two virus-encoded polypeptides and one host polypeptide. Polymerase activity was completely dependent on addition of CMV RNA as template, and the products of reaction were single-stranded (ss) RNA and double-stranded (ds) RNA, corresponding to RNAs 1, 2, and 3, and a subgenomic RNA (RNA 4) derived from RNA 3. The ratio of ssRNA to dsRNA was about 5:1, and the ssRNA was shown to be predominantly the positive strand. This demonstrates the complete replication of a eukaryotic virus RNA in vitro by a template-dependent RNA polymerase.

Nucleotide sequence of cucumber mosaic virus RNA. 1. Presence of a sequence complementary to part of the viral satellite RNA and homologies with other viral RNAs

The nucleotide sequence of the 3389 residues of RNA 1 (Mr 1.15 X 10(6) of the Q strain of cucumber mosaic virus (CMV) was determined, completing the primary structure of the CMV genome (8617 nucleotides). CMV RNA 1 was sequenced by the dideoxy-chain-termination method using M13 clones carrying RNA 1 sequences as well as synthetic oligonucleotide primers on RNA 1 as a template. At the 5' end of the RNA there are 97 noncoding residues between the cap structure and the first AUG (98-100), which is the start of a single long open-reading frame. This reading frame encodes a translation product of 991 amino acid residues (Mr 110791) and stops 319 nucleotide residues from the 3' end of RNA 1. In addition to the conserved 3' region present in all CMV RNAs (307 residues in RNA 1), RNAs 1 and 2 have highly homologous 5' leader sequences, a 12-nucleotide segment of which is also conserved in the corresponding RNAs of brome mosaic virus (BMV). CMV satellite RNA can form stable base pairs with a region of CMV RNAs 1 and 2 including this 12-nucleotide sequence, implying a regulatory function. This conserved sequence is part of a hairpin structure in RNAs 1 and 2 of CMV and BMV and in CMV satellite RNA. The entire translation products of RNA 1 of CMV and BMV could be aligned with significant homology. Less prominent homologies were found with alfalfa mosaic virus RNA 1 translation product and with tobacco mosaic virus Mr-126000 protein.

Nucleotide sequence of cucumber-mosaic-virus RNA 2 reveals a translation product significantly homologous to corresponding proteins of other viruses

The nucleotide sequence of the 3035 residues of RNA 2 (Mr 1.03 X 10(6) ) of the Q strain of cucumber mosaic virus (CMV) was determined by sequencing M13 clones of the RNA 2 cDNA and by dideoxy sequencing using primers prepared either from M13 clones or by chemical synthesis. A single long open reading frame starts at the second AUG from the 5' end of RNA 2 and encodes 839 amino acids (Mr 94333). This frame has flanking regions of 92 nucleotides at the 5' terminus and 423 nucleotides at the 3' terminus. Computer analysis of the nucleotide sequence showed that CMV RNA 2 has a significant homology with RNA 2 of brome mosaic virus (BMV) and alfalfa mosaic virus (AMV) and also with a region for tobacco mosaic virus (TMV) RNA encoding the read-through part of the 183-kDa protein. About 400 amino acids in the central region of the CMV RNA 2 translation product have a striking homology with the corresponding proteins encoded by BMV and AMV and with the read-through part of the TMV 183-kDa protein. Hydrophobicity plots of CMV and BMV RNA translation products also had apparent similarities. It is concluded that CMV is related to BMV, AMV and TMV in order of increasing evolutionary divergence.