Tobacco mosaic, tobamoviruses


Butler, P.J.G., Klug, A. The assembly of a virus. Sci. Amer. Nov. 1978, p. 62
Culver J.N. Tobacco mosaic virus assembly and disassembly: Determinants in pathogenicity and resistance Ann. Rev. Phytopath. 40, 287- 308 (2002)
Dawson, W.O. Tobamoviruses in Encyclopedia of Virology, 2nd edition (1999)
Dawson, W.O. and Lehto, K.M. Regulation of tobamovirus gene expression. Adv. Virus Res. 38, 307-342 (1990)
Dodds, J.A. Satellite tobacco mosaic virus. Ann. Rev. Phytopathol. 36, 295-310 (1998)
Knapp, E., Lewandowski, D.J. Tobacco mosaic virus, not just a single component virus anymore. Mol. Plant Pathology 2, 117 2001
Lartey, R.T., Voss, T.C., and Melcher, U. Tobamovirus evolution: gene overlaps, recombination, and taxonomic implications. Mol Biol Evol 13, 1327-38 (1996)

I- Tobamoviruses - see also, as well as
1- TMV, Tobacco mild green mosaic, Ribgrass mosaic, Sunn-hemp mosaic, Cucumber green mottle mosaic
2- Particle 300nm long, 2100 coat protein subunits (one/3nucleotides)
3- RNA is encapsidated in right handed helical array of capsid proteins
4- Stable particles, transmitted mechanically
1- 6.4 Kb - several tobamoviruses have been sequenced
2- 7-methylguanosine cap at 5'end followed by A-rich "leader"
3- Aminoacyl tRNA ligase can charge 3'end with histidine. The tRNA-like structure improves translational efficiency, stability of mRNAs
III- Translation
1- Host ribosomes strip off most of capsid protein (cotranslational disassembly)
2- 126K product, 183K readthrough protein are only primary products
a- 126K has methyltransferase and helicase domains
b- Readthrough has polymerase motif (GDD)
3- 18K coat protein translated from 3' terminal subgenomic message
a- Subgenomic message transcribed from promoter on (-) strand
b- Capsid gene essential for long distance movement
4- 30K protein (transport function) translated from larger 3' terminal subgenomic RNA
a- Co-localizes with plasmodesmata
b- Increases plasmodesmatal pore size
IV- Genetics
1- Sense codon in readthrough position reverts (i.e. is deleterious)
2- Neither capsid nor 30K proteins required to replicate in protoplasts
3- Mutations in the capsid gene affect virus symptoms
4- Specific regions of TMV genome trigger host necrotic responses
a- Helicase region of 126K gene triggers tobacco N gene response
b- Capsid protein alleles of TMV trigger N' gene of N. sylvestris
5- Tomato mosaic strains which overcome host resistance can be isolated
a- Strains which overcome Tm-1 gene have mutations in 126K
b- Strains which overcome Tm-2 gene have mutations in 30K
6- Plants with transgenic capsid gene are resistant (cross-protection), only capsid proteins which encapsidate RNA cross protect
7- Transgenics with 30K gene show symptoms and complement 30K mutants
a- 30K transgenics have larger plasmodesmata
b- Transgenics for defective 30K have generalized virus resistance
8- Transgenics for defective replicases can have specific resistance
9- Transgenic resistance, other than capsid mediated resistance, often mediated through gene silencing
VI- Satellite tobacco mosaic virus
1- Found in association with tobacco mild green mosaic
2- Coat protein similar in size to TMVcp
3- 0.3kb genome encapsidated in small icosahedral particle
4- Many tobamoviruses will suffice as helpers