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Pathogen Biology

CMV is the type species of the genus Cucumovirus in the family Bromoviridae. It consists of three spherical particles, each approximately 28 nm in diameter. The CMV genome consists of three single-stranded, messenger-sense RNA molecules, designated RNA 1 (~3,350 nucleotides), RNA 2 (~3,050 nucleotides) and RNA 3 (~2,200 nucleotides) (Figure 6). Each RNA molecule is enclosed within a protective protein coat with each being a distinct single spherical-shaped particle. Thus, a mature CMV consists of three spherical particles, one particle containing RNA 1, another containing RNA 2 and the third containing RNA 3. The RNA 3 particle may contain a fourth RNA strand, referred to as RNA 4 (~1,030 nucleotides), which encodes the coat protein gene and from which the CMV coat protein is produced. This type of translational strategy, referred to as subgenomic RNA, consists of a separate strand of RNA produced during replication. While CMV RNA 3 contains the coat protein gene, that gene is only translated to produce the coat protein from its subgenomic strand. The general replication/translation scheme for RNAs 1-3 is shown in Figure 7 A-C.

Figure 6. The three genomic RNAs of CMV

RNA 1 encodes a single protein, referred to as 1a, shown to be necessary for replication of the viral genome (Figure 7A). The 1a protein has two functional domains: the N-terminal and C-terminal domains. The N-terminal domain has methyltransferase activity for adding a cap structure to the 5’-terminus of genomic and subgenomic RNAs, while the C-terminal domain of the 1a protein is a putative helicase which functions to “unwind” the double stranded RNA that develops during viral replication.

Figure 7A. Replication of RNA 1

RNA 2 encodes two proteins, referred to as 2a and 2b (Figure 7B). The 2a protein is N-proximal to the 2b protein and is involved in viral genome replication. The 2a protein is required for CMV replication and has characteristic RNA-dependent RNA polymerase motifs, i.e., it uses a RNA strand as a template to produce a complimentary RNA strand. The 2a protein was also shown to interact with the 1a protein (also a replication protein) in yeast two-hybrid tests. The 2b protein is translated from a separate (subgenomic) RNA strand, referred to as RNA 4A. The 2b protein counters the host’s posttranscriptional gene silencing mechanism. When CMV infects a plant, the plant may respond by activation of a resistance mechanism that essentially stops or significantly inhibits the virus from continuing its movement into, and infection of, new tissues. This particular type of resistance is referred to as gene silencing. The CMV 2b protein inhibits the ability of the plant’s gene silencing signal to initiate gene silencing in distant tissues, thereby allowing CMV to continue to invade and infect young developing tissues.

Figure 7B. Replication of RNA 2

RNA 3 encodes two proteins, referred to as 3a and coat protein (Figure 7C). The 3a protein (MP or movement protein in Figure 6) is essential for virus cell-to-cell movement and mutations in this protein identified host-specific differences in movement efficiency. The 3a protein was also shown to be associated with intraplant long distance movement. The coat protein is expressed from a subgenomic RNA species referred to as RNA 4. The coat protein is the only protein associated with virus particles and is the sole determinant for transmission by aphid vectors. The coat protein is involved in virus cell-to-cell movement, although suggested to have an indirect role in this movement process, and is directly involved in intraplant long distance movement. It should be noted that mutations in 1a, 2a and 2b genes affected virus movement in some hosts.

Figure 7C. Replication of RNA 3

In addition to the CMV genomic and subgenomic RNA species, some strains of CMV support a satellite RNA (designated RNA 5 or satRNA). The satRNA is a single-stranded molecule of approximately 332 to 342 nucleotides in length and is completely dependent on CMV for its replication. Moreover, satRNA is encapsidated in CMV particles, which allows spread from plant to plant along with CMV, by aphid vectors. The satRNA, however, does not provide CMV (i.e., the helper virus) with any essential function. The occurrence of satRNA with CMV may have no effect on CMV-induced symptoms or may intensify symptoms by production of systemic chlorosis or necrosis or may actually attenuate symptoms typically induced by CMV. The effect on disease expression is dependent on interaction of the satRNA species, the isolate or strain of CMV and the host plant species and variety. For example, a severe lethal necrosis in tomato occurred due to one CMV-satRNA interaction that affected thousands of acres of processing tomatoes in southern Italy (Figure 8).

CMV replicates in the cytoplasm following a characteristic strategy used by single-stranded messenger-sense viral genomes, whereby viral genomic RNA molecules serve as templates for generation of non-translated minus-sense RNA molecules complementary in sequence to the respective genomic RNA species. Genomic and subgenomic RNAs are generated from the respective complementary minus-sense RNA species. Proteins are translated from specific viral genes in the cytoplasm. Cell-to-cell movement occurs through plasmodesmata and intraplant long distance movement occurs through the phloem.

CMV strains are divided into two subgroups, designated subgroups I and II, that are distinguished by serological relationships and sequence analysis. Subgroup I strains are further divided into IA and IB, based on differences in pathogenicity in cowpea (Vigna unguiculata), whereby IA strains induce systemic mosaic symptoms and IB strains induce necrotic local lesions on inoculated leaves. Some CMV strains are host specific, infecting certain hosts in the same family like the legume strain of CMV. CMV is serologically related to Tomato aspermy virus and Peanut stunt virus.

Figure 8. CMV-satRNA interaction

Figure 9. “Empty” virus crystal

'Empty' virus crystals lacking virions located in a vacuole may appear as a neat hexagonal outline (Figure 9), but more often appear as 'empty' circles. Such 'empty' crystals are diagnostic for CMV, but not for cucumoviruses in general.

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