In vivo removal of protein from tobacco mosaic virus after inoculation of tobacco leaves

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.

Disassembly of tobacco mosaic virus by membrane lipid isolated from tobacco leaves and polyornithine

In vitro disassembly of tobacco mosaic virus (TMV) virions occurred in the presence of both polyornithine and a lipid fraction isolated from tobacco leaf membrane. The latter could be replaced by lecithine. Disassembly of 10 microgram of TMV virions was attained in the presence of a 500-mg leaf equivalent of membrane lipid and 20 microgram of polyornithine in 1 ml of 0.01 M Tris-HCl buffer, pH 7.4 at 30 C. Similarity and dissimilarity between the in vitro disassembly and the in vivo uncoating mechanisms are discussed.

Infectivity suppressing and virus-binding activities of a membrane material isolated from tobacco leaves

TMV binding substance (R) was isolated from a tobacco leaf membrane fraction and was purified by extraction with organic solvents and by column chromatography. Experimental results suggest that the binding of R with TMV results in inactivation of TMV. When tobacco leaves were inoculated with the R-TMV complex, it was found that the formation of polysome containing infecting viral RNA was inhibited. Model experiments showed that the mode of R-TMV adsorption to the membrane is different from that of TMV adsorption and that stripping of coat protein from TMV by SDS was inhibited by R. A possible explanation for the mechanism of this inhibition by R is that the R-TMV complex follows a pathway which does not lead to establishment of infection. Although less efficient, R was still active when it was applied after virus inoculation. Due to its affinity to coat protein, R might also interfere with a later process of viral multiplication.

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.

Polysomes containing infecting viral genome in tobacco leaves infected with tobacco mosaic virus

This study concerns the interaction between parental virus RNA and host cell components in tobacco leaves infected with tobacco mosaic virus (TMV). By using a gentle method for disruption, extracts were obtained from tobacco leaves infected with 32P‐labelled TMV and the fate of infecting parental 32P‐TMV‐RNA was studied. Sucrose gradient centrifugation analysis showed that the parental 32P‐RNA distributed into four regions: free RNA, partially uncoated TMV, TMV and a structure heavier than TMV. The heavier structure was considered to be polysomes carrying the parental TMV‐RNA as messenger RNA based on its size, sensitivity to RNase, dependence of its formation on protein synthesis and the kinetics of its appearance after infection. Control experiments were done to exclude the possibility that the structure is not an artifact aggregate produced by an interaction between partially uncoated virus or its RNA and host cell components. Polysomes containing TMV‐RNA were found as membrane bound forms. Based on these data, it is suggested that uncoating of TMV and formation of polysomes are closely related and evidence has been obtained which suggests that uncovering of the viral genome and its translation takes place hand‐in‐hand on a single virion.

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.

In vivo uncoating of tobacco mosaic virus after infection of tobacco leaves

In vivo uncoating of tobacco mosaic virus (TMV) was studied. As Shaw had reported, initiation of uncoating reaction takes place very efficiently. Coat protein is removed from the virus as a peptide which is precipitable with trichloroacetic acid. Short rod particles with partly exposed RNA are thus formed. Further uncoating to coat protein‐free TMV‐RNA (28S) seems to take place with very low efficiency which is comparable to that of formation of local lesions on the inoculated leaf. From the data on the intracellular distribution of these products of uncoating reaction, mechanisms and significance of these reactions are discussed.