2. Some viruses take over the metabolism of the host cell completely and kill it; others live in harmony with their hosts.
3. Viruses exist in two states. Outside the cell they cannot multiply. Inside infected cells, they replicate.
13.2 Virus Interactions with Host Cells (Table 13.4, Figure 13.4)
1. Bacteriophages (phages) have the same relationships to their host as do animal viruses. Phages are much easier to study and serve as excellent model systems.
2. Some phages multiply inside bacteria and lyse the cells; this is a productive infection; the phages are virulent and lytic.
3. Other phages multiply but are extruded from the cell and do not kill it.
4. Temperate phages transfer their DNA into the host cell, where it multiplies either as a plasmid or more commonly is integrated into the chromosome of the host.
5. A latent infection may show no sign that cells are infected.
Lytic Phage Replication by Double-Stranded DNA Phages (Figure 13.5)
1. This type of productive infection is one in which the host cell metabolism is taken over by a virulent phage and lyses. The infection proceeds through a number of defined steps:
a. Attachment—protein tail fibers on the tail of the phage adsorb to specific receptors on the cell wall.
338 Chapter 13 Viruses of Bacteria b. Penetration—the DNA passes through the open channel of the tail and is injected into the cell. The phage coat remains on the outside.
c. Transcription—the phage DNA is transcribed. Some is transcribed early in infection and other regions are transcribed later. The mRNA is then translated into phage proteins.
d. Replication of phage DNA and proteins—the phage DNA and proteins replicate independently of one another. A phage enzyme degrades the bacterial chromosome so that only phage proteins are synthesized.
e. Assembly (maturation)—this is a highly complex and ordered series of processes, some are catalyzed by enzymes, others not. The net result is the assembly of the phage components into a complete virus particle.
f. Release—a phage-induced lysozyme lyses the cells resulting in the release of many virus particles per infected cell. This is the burst size.
Lytic Single-Stranded RNA Phages
1. These phages attach to the sex pilus.
2. They replicate rapidly and reach burst sizes of 10,000.
3. The entering RNA codes for an unusual RNA polymerase that uses RNA as a substrate.
Phage Replication in a Latent State—Phage Lambda (Figure 13.6)
1. The temperate phage A can either go through a lytic cycle similar to T4 or integrate its DNA into a specific site in the bacterial chromosome.
2. Integration of phage DNA into the bacterial chromosome as a prophage occurs by means of site-specific recombination. (Figure 13.7)
3. The prophage is maintained in an integrated state because a repressor prevents expression of genes coding for an enzyme that excises the prophage from the chromosome.
4. Lysogens, the bacteria carrying prophage, are immune to the same phage whose DNA they carry.
5. Prophage often code for proteins that confer unique properties on the bacteria, a process called lysogenic conversion. (Table 13.3)
Extrusion Following Phage Replication—Filamentous Phages
1. The filamentous phage attach to the sex pilus of E. coli, and the single-stranded DNA enters the cell.
2. The entering positive (+) single strand of DNA is converted to a double-stranded replicative form. This DNA gives rise to the two single-stranded forms, one positive (+), the other negative ( —).
3. The negative (—) strand gives rise to its complementary strand, which serves as mRNA for the synthesis of phage proteins.
4. Filamentous phages do not take over the metabolism of the host cell completely, but multiply productively as the host multiplies.
5. Phage are released by extrusion through the cell wall, a process that does not kill the bacteria.
6. As the positive strand of DNA is extruded, the DNA becomes surrounded by a protein coat.
Lytic Infection by Single-Stranded DNA Phages
1. Single-stranded DNA phages exist that lyse cells.
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