Replication of plant and animal viruses

REPLICATION OF ANIMAL AND PLANT VIRUSES

Basic differences between replication of bacteriophages and replication of animal and plant viruses were pointed out earlier in this chapter. The following sections discuss the replication of animal viruses in more detail and point out how this process differs from plant virus replication.

Attachment (Adsorption)

A prerequisite step in the viral infective process of animal and bacterial cell is attachment, or adsorption, of the virion on specific receptor sites on the surface of a susceptible cell. A cell lacking a receptor for a specific virus is not infected by that virus.

For naked virions, the surface capsid proteins are probably responsible for the binding to a specific cell receptor. For enveloped virions, the surface glycoproteins of the envelope membrane (the spikes) are responsible for recognition of a cell surface receptor, which is followed by attachment.

This specific virion-cell interactive explains why certain virions infect only particular types of cells. For example, the influenza virus infects epithelial cells of the upper respiratory tract, and the human immunodeficiency virus (HIV) invades specific cells in the body, including white blood cell called CD4 lymphocytes. In contrast to specific adsorption of animal and bacterial viruses on certain cell types, plant viruses do not appear to require specific receptor sites to attach to cells.

Penetration

The second step in the viral infective process is penetration of the virion into the host cell. The virions of a few viruses such as poliovirus undergo a structural change so that their nucleic acid is released directly into the cytoplasm. But most enveloped viruses enter the host cell by fusion of the host-cell membrane and the viral envelope. This releases the naked nucleocapsid into the cell cytoplasm. Some enveloped and most non-enveloped virions are engulfed within the host cell in the membrane-bounded vacuole. Then these nucleocapsids are released into the cytoplasm by one of the mechanisms. The viral nucleic acid must then be released from the capsid so that it become available.

For transcription, translation, and replication. To release the nucleic acid, some viruses are uncoated in the phagocytic vacuoles in the host cell by the action of host-cell enzymes. These enzymes digest the capsid, freeing the viral nucleic acid. Depending on the virus, uncoating occurs within the vacuoles, in the cytoplasm, or in the nucleus. In most cases, the uncoating process is poorly understood. Penetration of plant cells is complicated by the presence of the thick wall surrounding the cell. Probably the most important way in which plants viruses penetrate this wall is by insects. Insects that have the viruses on their mouth parts or that harbor them within their tissues can inoculate these viruses directly into plant cells during feeding. Plant viruses may also penetrate plant cell walls by means of the scattered pores that extend through the wall. These pores normally allow water and nutrients to pass into the cell; they are also used by the cell to excrete substances such as waxes.

Biosynthesis of Virus Components

As is the case for bacteriophages, the biosynthesis of animal and plant viruses within the host cell can be divided into early and late functions. You will recall that early functions are those biochemical events that take over the host cell and synthesize early viral mRNA. Late functions are those that later synthesize other proteins and assemble the nucleocapsid .

Maturation and Assembly

When a critical number of the various components have been synthesized, they are assembled into mature virus particles in the nucleic acid/or cytoplasm of the infected cell. The time period from the uncoating until the assembly of new, mature virions is referred to as the eclipse period, because of the host cell is broken open during this period no infectious virus is found. The same is true in the bacteriophages life cycles. However, unlike that of bacteriophages, the process of assembly does not appear to involve special biosynthetic enzymes, but rather occurs spontaneously as a result of the highly specific molecular interaction of capsid macromolecules with the viral nucleic acid. Thus maturation may be defined as that phase of viral infection during which structural components of the newly replicated viral nucleic acid to form the nucleocapsid structure.

Release

The release of mature virions from the host cell is the final step in viral multiplication. The mechanism of release varies with the type of virus. In some animal virus infection, the host cell disintegrates, releasing the virions. Naked virions are generally released in a burst-like fashion (all at once) as the cells disintegrate, or lyse. Alternatively, they may be extruded from the cell over a period of time by exocytosis, a process that is essentially the reverse of the penetration process shown in FIGURE 15.28B. With a few naked animal and plant viruses, the host cells are not destroyed. The virions leave the cells by special channels (tubules) over an extended period of time.

Enveloped animal viruses (and presumably enveloped plant viruses) are released by budding through special areas of the host-cell membrane coded for by the virus. In this process, the virions acquire a portion of the host membrane. Budding may take place along the cytoplasmic membrane, the nuclear membrane, or even other intracytoplasmic membranes. It involves a protrusion outward, or bud, by a membrane that is associated with a nucleocapsid. This protrusion soon pinches off to form a closed sac that completely surrounds the nucleocapsid. Then the bud is excised by the action of special enzymes on the envelope surface. For example, the spikes that project from the envelope of influenza viruses are composed of a releasing enzyme called neuraminidase [SEE FIGURE 15.6]. If a virion becomes envelope by budding at the cytoplasmic membrane, it is automatically released from the cell at the same time. If the envelope comes from internal membranes, the mature virus is usually released by exocytosis or through special channels. Poxviruses appear to acquire their envelope de novo by biosynthesis within the cytoplasm.

The yield of virus particles per cell varies with the virus, the type of cell, and the growth conditions. The average yield of plant and animal virions range from several thousand to about 1 million per cell, compared with a yield of only several hundred phages from a bacterial cell.

As an example of the viral replicative process in the eukaryotic cell, FIGURE 15.29 illustrates the replication of the herpes simplex virus, which is the cause of “fever blisters” (“cold sores”) and genital herpes. As you can see in the figure, the event related to biosynthesis occurs in both the nucleus and the cytoplasm, with the assembly of the virion initiated in the nucleus. The nucleocapsids of these viruses then migrate to the cytoplasmic membrane (after envelopment by budding through the nuclear membrane). There the mature enveloped viruses apparently reach the surface of the cell through cytoplasmic channels.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.