Virus particles gene expression

In a normally growing culture of lysogenic bacteria, the majority of bacteria manage to keep their prophages in a dormant state. In a very small minority of cells, however, the prophage genes express themselves. This results in the multiplieation of the virus, lysis of the cells and liberation of infectious particles into the medium. 

The temperate virus does not exist in its mature, infectious state inside the cell, but rather in a latent form, called the provirus or prophage state. In considering virulent viruses we learned that the DNA of the virulent virus contains information for the synthesis of a number of enzymes and other proteins essential to virus reproduction. The prophage of the temperate virus carries similar information, but in the lysogenic cell this information remains dormant because the expression of the virus genes is blocked through the action of a specific repressor coded for by the virus. As a result of a genetic switch, the repressor is inactivated, virus reproduction occurs, the cell lyses, and virus particles are released. 

Aston-Jones and Card, 2000 Husak et al., 2000 Kelly and Strick, 2000) as virus particles. The foreign genes they express can, therefore, be used as markers for neuronal connections. This property is not shared by rAAV particles, which are completely defective for viral replication. 

Knowledge of viral replication strategies has helped to optimize the approaches for the genetic modification of the viruses for better expression of the heterologous genes. It has also led to the introduction of bioprocesses for the production of infectious viral particles or viral-like particles, as will be discussed next. 

The infection cycle is shown in Figure 19.2. When the polyhedra are dispersed in the environment, the viral particles inside them (ODVs) are deposited on the plant leaves. When the caterpillars feed on the virus-contaminated foliage, they ingest the polyhedra. The alkaline environment in the caterpillar medium intestine causes breakdown of the polyhedra and the viral particles are released from the polyhedra. The infection of the cells occurs via a receptor-mediated fusion process. Once in the cytoplasm, the nucleocapsids without membrane are transported to the nucleus of the cell, where gene expression and genome replication begins. 

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