Lysogeny, or the lysogenic cycle, is one of two cycles of viral reproduction (the lytic cycle being the other). Lysogeny is characterized by integration of the bacteriophage nucleic acid into the host bacterium's genome or formations of a circular replicon in the bacterial cytoplasm. In this condition the bacterium continues to live and reproduce normally. The genetic material of the bacteriophage, called a prophage, can be transmitted to daughter cells at each subsequent cell division, and at later events (such as UV radiation or the presence of certain chemicals) can release it, causing proliferation of new phages via the lytic cycle. Lysogenic cycles can also occur in eukaryotes, although the method of DNA incorporation is not fully understood. The difference between lysogenic and lytic cycles is that, in lysogenic cycles, the spread of the viral DNA occurs through the usual prokaryotic reproduction, whereas a lytic cycle is more immediate in that it results in many copies of the virus being created very quickly and the cell is destroyed. One key difference between the lytic cycle and the lysogenic cycle is that the lysogenic cycle does not lyse the host cell straight away. Phages that replicate only via the lytic cycle are known as virulent phages while phages that replicate using both lytic and lysogenic cycles are known as temperate phages. In the lysogenic cycle, the phage DNA first integrates into the bacterial chromosome to produce the prophage. When the bacterium reproduces, the prophage is also copied and is present in each of the daughter cells. The daughter cells can continue to replicate with the prophage present or the prophage can exit the bacterial chromosome to initiate the lytic cycle.
1.1 Fitness tradeoffs for bacteria 1.2 Lysogenic conversion
1.2.1 Bacterial survival 1.2.2 Bacterial virulence 1.2.3 Preventing lysogenic induction
Bacteriophages are viruses that infect and replicate within a
bacterium. Temperate phages (such as lambda phage) can reproduce using
both the lytic and the lysogenic cycle. Via the lysogenic cycle, the
bacteriophage's genome is not expressed and is instead integrated into
the bacteria's genome to form the prophage. Since the
bacteriophage's genetic information is incorporated into the
bacteria's genetic information as a prophage, the bacteriophage
replicates passively as the bacterium divides to form daughter
bacteria cells. In this scenario, the daughter bacteria cells
contain prophage and are known as lysogens.
Lysogens can remain in the
lysogenic cycle for many generations but can switch to the lytic cycle
at any time via a process known as induction. During induction,
prophage DNA is excised from the bacterial genome and is transcribed
and translated to make coat proteins for the virus and regulate lytic
The model organism for studying lysogeny is lambda phage. Prophage
integration, maintenance of lysogeny, induction, and control of phage
genome excision in induction is described in detail in the lambda
Fitness tradeoffs for bacteria
Bacteriophages are parasitic because they infect their hosts, use
bacterial machinery to replicate, and ultimately lyse the bacteria.
Temperate phages can lead to both advantages and disadvantages for
their hosts via the lysogenic cycle. During the lysogenic cycle, the
virus genome is incorporated as prophage and a repressor prevents
viral replication. Nonetheless, a temperate phage can escape
repression to replicate, produce viral particles, and lyse the
bacteria. The temperate phage escaping repression would be a
disadvantage for the bacteria. On the other hand, the prophage may
transfer genes that enhance host virulence and resistance to the
immune system. Also, the repressor produced by the prophage that
prevents prophage genes from being expressed confers an immunity for
the host bacteria from lytic infection by related viruses.
In some interactions between lysogenic phages and bacteria, lysogenic
conversion may occur, which can also be called phage conversion. It is
when a temperate phage induces a change in the phenotype of the
infected bacteria that is not part of a usual phage cycle. Changes can
often involve the external membrane of the cell by making it
impervious to other phages or even by increasing the pathogenic
capability of the bacteria for a host. In this way, temperate
bacteriophages also play a role in the spread of virulence factors,
such as exotoxins and exoenzymes, amongst bacteria. This change then
stays in the genome of the infected bacteria and is copied and passed
down to daughter cells.
Lysogenic conversion has shown to enable biofilm formation in Bacillus
anthracis Strains of B. anthracis cured of all phage were unable to
form biofilms, which are surface-adhered bacterial communities that
enable bacteria to better access nutrients and survive environmental
stresses. In addition to biofilm formation in B. anthracis,
lysogenic conversion of Bacillus subtilis, Bacillus thuringiensis, and
Preventing lysogenic induction Strategies to combat certain bacterial infections by blocking prophage induction (the transition from the lysogenic to the lytic cycle) by eliminating in vivo induction agents have been proposed. Reactive oxygen species (ROS), such as hydrogen peroxide, are strong oxidizing agents that can decompose into free radicals and cause DNA damage to bacteria, which leads to prophage induction. One potential strategy to combat prophage induction is through the use of glutathione, a strong antioxidant that can remove free radical intermediates. Another approach could be to cause an overexpression of CI repressor since prophage induction only occurs when the concentration of CI repressor is too low. References
^ a b c Campbell and Reece (2005). Biology. San Francisco: Pearson.
^ Lodish; et al. (2008). Molecular Cell Biology. New York: W.H.
Freeman. pp. 158–159.
^ a b c d Watson; et al. (2008). Molecular Biology of the Gene. Cold
Spring Harbor, New York: Cold Spring Harbor Laboratory Press.
^ a b Chen; et al. (21 June 2005). "Population Fitness and the
Regulation of Escherichia coli