In vivo removal of protein from tobacco mosaic virus after inoculation of tobacco leaves. II. Some characteristics of the reaction

Tobacco mosaic virus and the study of early events in virus infections

In order to establish infections, viruses must be delivered to the cells of potential hosts and must then engage in activities that enable their genomes to be expressed and replicated. With most viruses, the events that precede the onset of production of progeny virus particles are referred to as the early events and, in the case of positive-strand RNA viruses, they include the initial interaction with and entry of host cells and the release (uncoating) of the genome from the virus particles. Though the early events remain one of the more poorly understood areas of plant virology, the virus with which most of the relevant research has been performed is tobacco mosaic virus (TMV). In spite of this effort, there remains much uncertainty about the form or constituent of the virus that actually enters the initially invaded cell in a plant and about the mechanism(s) that trigger the subsequent uncoating (virion disassembly) reactions. A variety of approaches have been used in attempts to determine the fate of TMV particles that are involved in the establishment of an infection and these are briefly described in this review. In some recent work, it has been proposed that the uncoating process involves the bidirectional release of coat protein subunits from the viral RNA and that these activities may be mediated by cotranslational and coreplicational disassembly mechanisms.

Uncoating-like modification of poliovirus capsid resulting from the cooperative effects of subfreezing temperature and submolar concentrations of urea

Inactivation of poliovirus at subfreezing temperature in the presence of unusually low concentrations of urea (less than or equal to 0.5 M) was investigated. Whereas serotypes 1 and 2 are very sensitive, type 3 is resistant. Inactivation cannot be attributed to concentration of solutes since temperature must be reduced below -13 degrees C for loss of infectivity. Characteristics of the inactivated virion are similar to those of virions in the early stages of uncoating in HeLa cells, viz., loss of infectivity, sensitivity to proteases and detergents, change in isoelectric point, retention of intact genome, and in some instances, loss of VP4. The molecular basis for inactivation is considered to be dissociation of water bound to capsid proteins thereby causing irreversible denaturation of native tertiary structure. The results of this study are discussed in terms of their relevance to the early stages of uncoating in vivo.

Fate of tobacco mosaic virus after entering the host cell. III. Partial uncoating

Diminutive viral RNAs recovered from tobacco leaves inoculated with 32P-TMV were investigated. At 3.5 hr after inoculation, most of the viral RNA without coat protein revealed two peaks after sucrose density gradient analysis of SDS-extract from 12,000 X g leaf pellet. The first peak appeared between bacterial ribosomal RNA of 16 S and 5 S and the second peak was around 5 S. These two peaks were digestible with RNase and they appeared as early as 5 min after inoculation. These diminutive RNAs seemed to be derived from partially uncoated parental virus by abscission of their naked RNA tails. The active formation of these diminutive RNAs and their early appearance after inoculation seemed to indicate that most of the inoculated TMV received incomplete uncoating.

Fate of tobacco mosaic virus after entering the host cell

Tobacco leaves were inoculated with tobacco mosaic virus labeled with 32P or 35S. After various intervals, extracts of the leaves were prepared. In extracts from leaves infected for 5 to 360 min, about 40 to 60% of the virus retained on leaves was recovered in the pellet of the homogenate centrifuged at 12 000 × g. The virus associated with the 12 000 × g pellet was dissociable by treatment with pancreatic RNase, alkali or sodium dodecyl sulfate (SDS). The parental virus extracted by SDS from the pellet at 12 000 × g had a large amount of partially uncoated virus possessing naked RNA. Analysis by density gradient centrifugation suggested that, in addition to partially uncoated virus, some fragmented RNA was also associated with the 12 000 × g pellet. This fragmented RNA seemed to be derived from partially uncoated virus. Density gradient analysis of SDS extracts from the 12 000 × g pellet suggested that some of the virus underwent uncoating at the internal regions of the virus particle.

Mechanism determining the host specificity of tobacco mosaic virus. I. Formation of polysomes containing infecting viral genome in various plants

In order to know the mechanism controling the host specificity of tobacco mosaic virus (TMV), three species of plants showing various degrees of resistance to TMV or TMV‐RNA infection were selected and the fate of infecting viral genome was studied. Extract was obtained from leaves 0.5–6 hr after inoculation of 32P‐TMV or 32P‐TMV‐RNA and analyzed by sucrose density gradient centrifugation. It was found that polysomes containing infecting 32P‐RNA were formed in plants resistant to TMV to the same extent as in susceptible tobacco plants, suggesting that the host specificity of TMV is determined at a stage of viral multiplication later than the step of translation of infecting viral genome.

Degradation of RNA of partially uncoated tobacco mosaic virus during extraction from tobacco leaves

Extracts of tobacco leaves, 3 to 22 hr after infection with 32P‐labeled tobacco mosaic virus (TMV), were analysed by sucrose density‐gradient centrifugation. As reported previously, a shoulder appeared on the lighter side of the main peak representing intact parent viruses. The shoulder was shown to contain partially uncoated virus particles with sedimentation constants of about 140–150 S, which withstood dialysis against a phosphate buffer. RNA from the uncoated particles was isolated on the sucrose density‐gradient, and its sedimentation constant was estimated at 16–18 S. Intact TMV‐RNA and RNA having 16–18 S were also obtained directly from homogenates of tobacco leaves infected with 32P‐TMV. 32P‐TMV was partially stripped by SDS and was added to tobacco leaf sap to test the RNase activity in tobacco leaves. A part of RNA which was exposed by SDS was digested and the 32P‐radioactivity in the top zone of the centrifugation tube increased. This finding supported the idea that the partially uncoated virus particles in infected leaves lost a portion of their RNA during extraction by the action of the RNase contained in the homogenate.