Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 102

A fundamental and important principle is that viruses are reproduced via the assembly of preformed components into particles: the parts are first made in cells and then assembled into the final product. This simple build-and-assemble strategy is unique to all viruses, but the details of how this process transpires are astonishingly diverse among members of different virus families. There are many ways to build a virus particle, and each one tells us something new about virus structure and assembly.

Modern investigations of viral reproduction strategies have their origins in the work of Max Delbrück and colleagues, who studied the T-even bacteriophages starting in 1937. Delbrück believed that these bacteriophages were perfect models for understanding the basis of heredity. He focused his attention on the fact that one bacterial cell usually makes hundreds of progeny virus particles. The yield from one cell is one viral generation; it was called the burst because the viruses that he studied literally burst from the infected cell. Under carefully controlled laboratory conditions, most cells make, on average, about the same number of bacteriophages per cell. For example, in one of Delbrück’s experiments, the average number of bacteriophage T4 particles produced from individual single-cell bursts from Escherichia coli cells was 150 particles per cell.

Another important implication of the burst is that a cell has a finite capacity to produce virus. Multiple parameters limit the number of particles produced per cell. These include metabolic resources, the number of sites for genome replication in the cell, the regulation of release of virus particles, and host defenses. In general, larger cells (e.g., eukaryotic cells) produce more virus particles per cell: yields of 1,000 to 10,000 virions per eukaryotic cell are not uncommon.

A burst occurs for viruses that kill the cell after infection, namely, cytopathic viruses. However, some viruses do not kill their host cells, and virus particles are produced as long as the cell is alive. Examples include filamentous bacteriophages, most retroviruses, and hepatitis viruses.