Satellite tobacco necrosis virus RNA

Kaempfer, R., Van Emmelo, J., and Piers, W., 1981, Specific binding of eukaryotic initiation factor 2 to satellite tobacco necrosis virus RNA at a 5 -terminal sequence comprising the ribosome binding site, Proc, Natl, Acad. Sci. USA 78 1542. 

Klein, W. H., Nolan, C., Lazar, J. M., and Clark, J. M., Jr., 1972, Translation of satellite tobacco necrosis virus RNA Characterization of in vitro procaryotic and eucaryotic translation products. Biochemistry 11 2009. 

Ysebaert, M., van Emmelo, J., and Fiers, W., 1980, Total nucleotide sequence of a nearly full-size DNA copy of satellite tobacco necrosis virus RNA, J. Mol. Biol. 143 273. 

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. 

The size of this viral particle is of course larger than that of a virus with only 60 subunits. The diameter of tomato bushy stunt virus is 330 A compared with 180 A for satellite tobacco necrosis virus. The increase in volume of the capsid means that a roughly four times larger RNA molecule can be accommodated. 

One of the most striking results that has emerged from the high-resolution crystallographic studies of these icosahedral viruses is that their coat proteins have the same basic core structure, that of a jelly roll barrel, which was discussed in Chapter 5. This is true of plant, insect, and mammalian viruses. In the case of the picornaviruses, VPl, VP2, and VP3 all have the same jelly roll structure as the subunits of satellite tobacco necrosis virus, tomato bushy stunt virus, and the other T = 3 plant viruses. Not every spherical virus has subunit structures of the jelly roll type. As we will see, the subunits of the RNA bacteriophage, MS2, and those of alphavirus cores have quite different structures, although they do form regular icosahedral shells. 

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. 

Bendey, G. A., Lewit-Bentley, A., Liljas, L., Skoglund, U., Roth, M., and Unge, T. (1987). Structure of RNA in satellite tobacco necrosis virus A low resolution neutron diffraction study using H20/ H20 solvent contrast variation. J. Mol. Biol. 194, 129-141. 

RNA structure and, 49-50 satellite tobacco necrosis virus (STNV) and, 49-50... 

These experiments have been dissapointing in the sense that the only features which seem to be universal are the AUG initiation codon and the 5 cap structure (7I, 72), and even the latter is missing from some viral RNAs, notably picornavirus RNAs (both the virion RNA (75) and. the viral mRNA (74)), Cowpea Mosaic Virus (CPMV) RNA (75)f and Satellite Tobacco Necrosis (STNV) RNA (76). The role of the cap has been studied by examining the properties of mRNA... 

Of all the viruses at present known and which have been studied biochemically, the simplest is a special virus described by Reichman (1964) as satellite of the tobacco necrosis virus. This virus is a ribonucleoprotein with the smallest RNA molecule of any virus (mol. wt. 395,000). This RNA is composed of 1200 nucleotides. Given the triplet nucleotide character of the genetic code, such an RNA cannot code more than 400 amino acid residues. Since each protein molecule of the virus coat contains about 370-380 amino acid residues, clearly the RNA of this virus can code only one protein. 

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). 

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