The Evolution of Viruses - Zhdanov, V. M. 1990

Special Section
RNA-Containing Rod-Shaped and Filamentous Viruses

The groups of rod-shaped and filamentous Viruses discussed in this chapter are exclusively plant viruses (Matthews R., 1982]. The reason for this host restriction to the plant kingdom remains unclear, given that rod-shaped DNA-containing (rather than RNA-containing) viruses are found in both animals (Baculoviruses) and Bacteria (inoviruses). The Molecular Weight of their genomes ranges from 2 x 10⁶ to 4.7 x 10⁶. There are 5 monopartite, one bipartite, and two tripartite viruses of this type. We do not discuss the tobacco mosaic, alfalfa mosaic, and brome mosaic virus groups here, as they will be considered alongside Togaviruses. Consequently, we will focus on the 6 groups of viruses with helical Symmetry.

Three groups of viruses lack cap structures at the 5' ends of their genomes.

Viruses of the beet yellows group — genus Closterovirus (15 members) — have a genome with a molecular weight of 2.2 x 10⁶–4.7 x 106, which accounts for 5% of the virion mass. Virions appear as long, flexible filaments measuring 12x600–2000 nm, in which RNA molecules are packaged within protein molecules (23,000–27,000). Replication mechanisms remain poorly understood. These viruses infect a wide range of hosts, and several show serological relationships. They are transmitted by aphids.

The carnation latent virus group — genus Carlavirus (35 members) — has a genome with a molecular weight of 2.7 x 10⁶, representing 6% of the virion mass. RNA molecules are packaged into protein molecules (32,000) to form virions structured as long, flexible filaments measuring 13x600–700 nm. Many of these viruses are serologically related. Their replication mechanism is poorly understood, they infect a broad host range, and they are transmitted by aphids.

The potato virus Y group — genus Potyvirus — comprises 115 viruses. The molecular weight of their genome ranges from 3.0 x 10⁶ to 3.5 x 10⁶, making up 5% of the virion mass. RNA molecules are packaged in protein molecules (32,000–36,000), forming long, flexible filamentous virions measuring 11 x 680–900 nm. Some viruses are immunologically related. They infect a wide host range, though individual viruses have narrow specificities. They are transmitted by aphids, mites, and Fungi.

Studies on The Genome of the tobacco vein mottling virus, a member of the potyvirus group, have demonstrated that its genome contains 4 primary Cleavage sites — p75, p120, p52, and p75 — which serve as the basis for generating final cleavage products, namely structural and nonstructural Proteins.

The genome of papaya ringspot virus, a filamentous positive-sense plant potyvirus featuring a VPg Structure at the 5' end and a poly(A) tail at the 3' end, is translated as a polyprotein (300,000) that undergoes proteolytic cleavage. This process yields the capsid protein (36,000), cylindrical inclusion proteins (70,000), amorphous inclusion proteins (51,000), and several other proteins [Yeh S., Consalves D., 1985].

The potato virus X group — genus Potexvirus — includes approximately 40 members. The genomic RNA (2.1 x 10⁶) accounts for 5% of the virion mass and is packaged into protein molecules (18,000–23,000) to form long, flexible filaments measuring 13x470–580 nm. Several viruses are serologically related. The replication mechanism is poorly understood. These viruses infect a broad range of hosts, although individual viruses have narrow host ranges. They are transmitted mechanically without vectors.

The translational genome strategy of potexviruses has been elucidated through studies on in vitro Protein Synthesis of papaya mosaic virus. Under these conditions, three proteins are synthesized — pA (155,000), pB (73,000), and pC (22,000). The latter is the capsid protein and apparently plays a crucial regulatory role in synthesizing different proteins: upon encapsulation of the viral RNA, the synthesis of pA and pB proteins drops sharply while the synthesis rate of the pC protein remains high.

Potexviruses possess a genome of 6 kb, which is uniform across potato virus X, papaya mosaic virus, and narcissus mosaic virus. In vitro experiments translate two proteins with molecular weights of 180,000 (corresponding to the full-length genome) and 145,000 (subgenomic). In infected Cells, alongside genomic RNA, 5 subgenomic RNAs (4.9, 4, 2.1, 1.4, and 0.8 kb) are detected, along with their corresponding double-stranded RNAs. Single-stranded RNAs contain poly(A) tails. The capsid protein has a molecular weight of 26,000.

A small group of tobacco rattle viruses, comprising 3 bipartite viruses (genus Tobravirus), has a genome consisting of two RNA fragments (2.4 x 106 and 0.6 x 10⁶) with a cap structure at the 5' end of the second fragment. Both RNA strands are encapsidated in protein molecules (22,000); RNA1 is infectious, whereas RNA2 encodes the capsid protein. Consequently, the virus forms long L rods (21–22x180–215 nm) and short S rods (21–22x46–114 nm). Individual isolates of the same virus are immunologically heterogeneous. The host range is broad, and transmission occurs mechanically and via nematodes.

The small group (3 members) of barley stripe mosaic viruses (genus Hordeivirus) possesses a multipartite genome consisting of two RNA fragments with molecular weights of 1 x 10⁶–1.5 x 10⁶, featuring a cap structure at the 5' end and a short poly(A) sequence (15–20 NUCLEOTIDES) at the 3' end. RNA molecules are packaged into protein molecules (21,000), forming rigid rods (20 x 10–150 nm). These viruses infect cereals and are transmitted mechanically or through seeds.

Mention should also be made of the 4th group of viruses, represented by soil-borne wheat mosaic virus (SBWMV). This virus is fungus-transmitted. It has been proposed to assign this group to a separate genus, Furovirus. Particles with lengths of 281, 138, and 92 nm contain RNA with molecular weights of 2.28 x 106 (6,500 bases), 1.23 x 106 (3,500 bases),

0.97 x 106 (2,800 bases), as well as a fourth component of 0.86 x 106 (2,450 bases). The molecular weight of the capsid protein is 19,000. Fungi are involved in the transmission of the virus.

As this brief Overview shows, rod-shaped and filamentous viruses exhibit evolutionary pathways similar to those of the previously discussed isometric virus groups: progression from continuous to fragmented genomes, from uncapped to capped RNA, and The formation of 3' poly(A) tails. In other groups not covered here, a tRNA molecule—even retaining its amino acid-accepting capacity—may fulfill The Role of a cap structure. This indicates that viruses have captured and incorporated preexisting cellular structures.

Our limited understanding of these virus groups, particularly the specifics of their replication mechanisms, precludes definitive Conclusions regarding their ORIGIN AND EVOLUTION. They may have evolved from cellular RNAs, an event that could have occurred repeatedly as primitive ribonucleoprotein structures with helical symmetry emerged. These structures provided more or less reliable protection for the genetic material against cellular Nucleases, which are less active in plant cells (compared to animal cells). Could this be why filamentous and rod-shaped viruses survived exclusively in plants, whereas in animal cells, where metabolic processes are considerably more intense, their evolution progressed further, leaving helical ribonucleoproteins preserved only as internal components of more complex virions possessing outer envelopes? Helical symmetry offered yet another advantage, at least in the Cytology/cytology/16.html">Early stages of evolution: it did not strictly limit the length of the RNA strand, a constraint unavoidable when packaging RNA into an icosahedral capsid. As for the proteins of rod-shaped viruses, they may have initially served as products of newly autonomous genes and were only later modified to ensure more specific RNA packaging. From this perspective, it is noteworthy that the molecular weights of capsid proteins show minimal variation (20,000–30,000) across all compared viruses, whereas in isometric plant viruses these variations span a much broader range—from 20,000 in tymoviruses to 60,000 in nepoviruses, with some groups, like comoviruses, possessing more than one such protein. It is also characteristic that the diameter of spherical capsids in RNA-containing plant viruses falls within a rather narrow range: 26 nm (bromoviruses), 28 nm (nepoviruses, comoviruses, necromoviruses), 30 nm (tymoviruses, tombusviruses, sobemoviruses), and so on. In contrast, the length of rods and filaments varies from 50 nm (tobraviruses) to 2,000 nm (closteroviruses), and even within the same virus can show substantial fluctuations, from 600 to 2,000 nm in the aforementioned closteroviruses. All these data indicate that the mode of genetic material packaging largely dictates the evolutionary pathways of viruses.