Bacteriophage: Bacterial Viruses
1. Distinguish between lytic and lysogenic cycles in bacteriophage.
2. Describe how viruses can contribute DNA to their hosts
Bacteriophage (both singular and plural) are viruses that infect bacteria. They are diverse, both struc-turally and functionally, and are united solely by their occurrence in bacterial hosts. Many of these
types of bacteriophage, called phage for short, are large and complex, with relatively large amounts of DNA and proteins.E. coli-infecting viruses were among the first bacteriophage to be discovered and are still some of the best studied.
Some of these viruses that infect E. coli have been named as members of a “T” series (T1, T2, and so forth); others have been given different types of names. To illustrate the diversity of these viruses, T3 and T7 phage are icosahedral and have short tails. In contrast, the so-called T-even phage (T2, T4, and T6) have an icosahedral head, a capsid that consists primarily of three proteins, a connecting neck with a collar and long “whiskers,” a long tail, and a complex base plate.
Life Cycle of Bacteriophage
The lytic cycle
When a virus lyses the infected host cell in which it is replicating, the reproductive cycle is referred to as a lytic cycle (figure , left). The basic steps of a lytic bacteriophage cycle are similar to those of a nonen-veloped animal virus. The T-series bacterio-phage are all virulent, or lytic, phage, multiplying within in-
fected cells and eventually lysing (rupturing) them.
The lysogenic cycle
In contrast to the rather simple lytic cycles, some bacteriophage do not immediately kill the cells they infect, instead they integrate their nucleic acid into the genome of the infected host cell. This integration gives them a distinct advantage; integration allows a virus to be replicated along with the host cell’s
DNA as the host divides. These viruses are called temperate, or lysogenic, phage.
The DNA segment that is integrated into a host cell’s genome is called a prophage, and the resulting cell is called a lysogen. Among the bacteriophage that do this is the binal phage lambda (λ) of E. coli. Lambda may be the best studied biological particle; the complete sequence of its 48,502 bases has been
determined. At least 23 proteins are associated with the development and maturation of phage λ, and other enzymes are in-volved in integrating this virus into the host genome.
When phage λ infects a cell, the early events constitute a genetic switch that will determine whether the virus will replicate and destroy the cell or become a lysogen and be passively replicated with the cell’s genome. This lysis/lysogeny “decision” depends on the expression of early genes.
Early on, two regulatory proteins are produced that will compete for binding to sites on the phage’s DNA. Depending on which protein “wins” either the genes necessary for replication of the genome will be expressed beginning the lytic cycle, or the enzymes necessary for integrating the viral genome into the
chromosome will be expressed and the lysogenic cycle initiated (figure , right).
A lysogenic phage has the expression of its genome re-pressed (see chapter 16) by one of the two viral regulatory proteins mentioned earlier. This is not a permanent state, however; in times of cell stress, the prophage can be derepressed, and the enzymes necessary for excision of the genome expressed. The
viral genome then is in the same state as the initial stage of infection, and the lytic cycle can commence, leading to formation of viral particles and lysis of the cell.
The switch from a lysogenic prophage to a lytic cycle is called induction because it requires turning on the gene expression necessary for the lytic cycle . It can be stimulated in the laboratory by stressors such as starvation or ultraviolet radiation. The molecular events of induction take advantage of host proteins that respond to stress to produce a protease that can destroy the repressor protein that is keeping the viral genome silent.
The normal function of this protease is to degrade a host repressor that controls DNA repair genes. The two repressor proteins are similar enough that both are degraded by the protease.