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Viruses Requiring Helper Viruses - Satellite Tobacco Necrosis Virus (STNV), Stallite Tobacco Ringspot Virus (STRSV)

Viruses Requiring Helper Viruses - Satellite Tobacco Necrosis Virus (STNV), Stallite Tobacco Ringspot Virus (STRSV) - A satellite virus (SV) is associated with and dependent upon another virus (activator virus) which allows the SV to multiply in a cell. The virus particle may contain deficient nucleic acid or defective nucleic acid.

Deficient nucleic acid cannot multiply without help from another virus, or may replicate but cannot form a capsid protein. When the deficiency is only regarding the ability to produce the coat protein, the virus may exist in the form of the nucleic acid (unstable form).

Defective nucleic acid can replicate and also produce protein, but the protein is abnormal and cannot be associated with the nucleic acid to form the complete virus particle. Incomplete viruses are empty protein shells without nucleic acid, e. g. the T component of TYMV

Satellite tobacco necrosis virus (STNV)

The sedimentation coefficient of the particle is 50S and the particle size 17 nm. The particle contains 2(1% RNA of which there is only one species. The MW of the RNA is 400,000 daltons. The Mw of the protein is 20 X 103 daltons. STNV multiplication can take place only when there is a concurrent infection with tobacco necrosis virus.

The converse is, however, not true. The RNA lacks a replicase specifying gene, and in this respect is Similar to the tobacco rings pot satellite virus.

It, however, differs from this virus in being slightly larger and in containing a gene for its unique capsid protein, which is different from that of the necrosis virus.

Satellite tobacco ringspot virus (STRSV)
 The sedimentation coefficient of STRSV is 91-126S and the particle size 30 nm. The particle contains 12-26 RNA species encapsidated in tobacco ringspot virus protein. The nucleic acid components are of uniform size.
There is a continuous series of particles of different bouyant densities, depending upon the number of nucleic acid strands. The ringspot satellite RNA appears to be too small to contain structural genetic information.

It is, however, assumed that it contains a recognition site for an enzyme coded by the rings pot virus genome, and that it is replicated by the enzyme.Satellite tobacco ringspot virus can infect and replicate only in the presence of TRSV (helper). STRSV capsid protein is coded by helper TRSV, and the two viruses are therefore indistinguishable is size, shape and serological tests.Different strains of the helper TRSV thus produce corresponding strains of STRSV. The nematode vector Xiphinema americanam transmits both viruses.

 
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