Schematic model of TMV: 1. nucleic acid (RNA), 2. capsomer (protomer), 3. capsid
A monomeric unit of the tobacco mosaic virus coat protein♦
Tobacco mosaic virus has a rod-like appearance. Its capsid is made from 2130 molecules of coat protein (see image above) and one molecule of genomic RNA 6400 bases long. The coat protein self-assembles into the rod like helical structure (16.3 proteins per helix turn) around the RNA which forms a hairpin loop structure (see the electron micrograph below). The protein monomer consists of 158 amino acids which are assembled into four main alpha-helices, which are joined by a prominent loop proximal to the axis of the virion. Virions are ~300 nm in length and ~18 nm in diameter. Negatively stained electron microphotographs show a distinct inner channel of ~4 nm. The RNA is located at a radius of ~6 nm and is protected from the action of cellular enzymes by the coat protein. There are three RNA nucleotides per protein monomer. A molecular model of the intact virus based on an electron density map at 3.6 angstroms resolution was derived from a fiber diffraction study.
Photoacclimation was studied in tobacco leaves (Nicotiana tabacum cv Xanthi) infected with two strains of tobacco mosaic virus (TMV) and grown under different light and nitrogen nutrition regimes. Photosynthetic acclimation measured by the quantum yield and the maximum rate in saturating light of CO2-saturated photosynthesis was impaired to a greater extent in tobacco leaves infected with TMV strain PV230 than in those infected with TMV strain PV42. Infection with TMV strain PV230 severely impaired photosynthetic acclimation at high light/low nitrogen and during transfer from low to high light. Expanding leaves showing chlorotic-mosaic symptoms had greatly reduced capacity to acclimate to high light compared with controls and with developed leaves without visible symptoms. We conclude that the failure of expanding leaves to acclimate was...