Satellite viruses

Figure 16.1 Viruses vary in size and shape from the simplest satellite viruses (a) that need another virus for their replication to the T-even bacteriophages (d) that have developed sophisticated mechanisms for injecting DNA into bacteria. Four different virus particles are shown to scale.

Very few self-sufficient viruses have only 60 protein chains in their shells. The satellite viruses do not themselves encode all of the functions required for their replication and are therefore not self-sufficient. The first satellite virus to be discovered, satellite tobacco necrosis virus, which is also one of the smallest known with a diameter of 180 A, has a protein shell of 60 subunits. This virus cannot replicate on its own inside a tobacco cell but needs a helper virus, tobacco necrosis virus, to supply the functions it does not encode. The RNA genome of the satellite virus has only 1120 nucleotides, which code for the viral coat protein of 195 amino acids but no other protein. With this minimal genome the satellite viruses are obligate parasites of the viruses that parasitize cells. 

We have seen in the structure of this simple satellite virus that 60 subunits are sufficient to form a shell around an RNA molecule that codes for the subunit protein, but there is little room for additional genetic information. 

One of the smallest of the encapsulated RNA-containing viruses is the satellite tobacco necrosis virus. It replicates only when the plant is also infected with the larger tobacco necrosis virus. The satellite virus, whose three-dimensional structure is known from X-ray diffraction studies,485 contains a 1200-nucleotide strand of RNA which encodes a 195-residue protein. 

Taxonomy of Known Viral Structure Satellite Viruses"... 

Satellite viruses are those that are dependent for their own replication on some (catalytic) activity encoded in another helper virus that coinfects the host cell. The structures of three plant ssRNA satellite viruses represent some of the highest resolutions known and have been comparatively reviewed (Ban et al., 1995). The structures of satellite tobacco mosaic virus (STMV) (Larson et al., 1993a,b), satellite tobacco necrosis virus (STNV) (Jones and LUjas, 1984 Liljas et al., 1982), and satellite panicum mosaic virus (SPMV) (Ban and McPherson, 1995) have T=1 capsids composed of 60 identical copies of unembellished jelly-roll j3 barrels constructed of only 155 to 195 amino acids (Fig. la see Color Insert). What is remarkable is how little the assembly context of these domains is conserved. The same end always points toward the 5-fold axis, but the domains are rotated to different extents around the 5-fold axis. Furthermore, between STNV and the others, there is a 70° rotation of the barrel about its long axis. Contacts across the dimer interface are... 

The satellite ssRNA viruses are a group of ssRNA icosahedral viruses, which are the satellites to certain plant viruses (Pritsch and Mayo, 1989). Satellite tobacco necrosis virus (STNV), a satellite virus to tobacco necrosis virus, was in fact one of the first icosahedral virus structures to be determined by X-ray crystallography (Liljas et al, 1982). In more recent years, two other satellite plant viruses, satellite tobacco mosaic virus (STMV) (Larson et al, 1993, 1998) and satellite panicum mosaic virus (SPMV) (Ban and McPherson, 1995), have been determined. These are perhaps the simplest and smallest icosahedral viruses whose structures have been determined by X-ray crystallography. Of relevance to our discussion is the structure of STMV determined to 1.8-A resolution, which shows extraordinary details about genome organization (Larson et al, 1998 Larson and McPherson, 2001). 

Satellite tobacco mosaic vims (STMV), 153, 222-225 encapsidated genome of, 223 Satellite tobacco necrosis vims (STNV), neutron scattering and, 49-50, 222 Satellite viruses, viral stmcture taxonomy and, 139-141, 139... 

RNA genome single positive-sense strand, 133-138, 139, 152 RNA genome single stranded, 143—144 satellite viruses and, 139-141, 139 Virion stabilization, 411 Virus (es), 37... 

Hepatitis delta virus RNA. Hepatitis delta virus (HDV) is a satellite virus... 

Since 1982, it is recognized that nucleic acids and particularly RNA may catalyze a broad range of reactions such as formation, cleavage, and rearrangements of some chemical bonds. Catalytic RNAs are known as ribozymes and they can be found in organelles of plants and lower eukaryotes, in amphibians, in prokaryotes, in bacteriophagues, and in viroids or satellite viruses. Arguably they may not... 

However, the volume of information inscribed in 1200 nucleotides is insufficient for spontaneous propagation of the virus in plant cells, and it can reproduce only as the satellite of another virus—virus of tobacco necrosis, possessing RNA with a molecular wei t of 2 X 10. Proteins of these viruses are serologically different, but the basic virus evidently enables certain factors to be synthesized which are equally essential for the formation of both viruses. RNA synthesis in the host cells takes place on DNA templates and is catalyzed by DNA-dependent RNA-polymerase. It may be assumed that a special factor—the enzyme RNA-replicase— which is not used for reproduction of the satellite virus is essential for synthesis of virus RNA on RNA templates (autoreplication of RNA). 

Boucher, D. W., Melnick, J. L., Mayor, H. D. Nonencapsidated infectious DNA of adeno-satellite virus in cells coinfected with herpes virus. Science 173, 1243-1245 (1971). 

Mayor, H. D., Houlditch, G. S., Mumford, D. M. Influence of adeno-associated satellite virus on adenovirus-induced tumours in hamsters. Nature (Lond.) New Biol. 241, 44-46 (1973). 

Parks, W. P., Green, M., Pina, M., Melnick, J. L. Physiochemical characterization of adeno-associated satellite virus type 4 and its nucleic acid. J. Virol. 1, 98O-987 (1967). 

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