Lysogeny refers to a process whereby a virus that specifically infects a bacterium, a bacteriophage (which means "devourer of bacteria"), achieves the manufacture of copies of its deoxyribonucleic acid (DNA ) genetic material by integrating the viral DNA into the DNA of the host bacteria . The inserted viral DNA is then replicated along with the host DNA.
The nature of lysogeny remained unresolved for many years following the discovery of the bacteriophage by Felix d'Hérelle in 1915. The sudden appearance of virus in cultures of bacteria was at first thought to be the result of viral contamination . The acceptance of lysogeny as a real phenomenon came almost 40 years later.
In lysogeny no new virus particles are made. Instead, the virus essentially remains dormant, while ensuring that its genetic material continues to be made. A stress to the bacterium, such as exposure of the bacterium to ultraviolet light, triggers the viral DNA to separate from the bacterial DNA. Then, new virus particles will form in what is known as the lytic cycle. The two processes of lysogeny and lysis are under a system of control first explained by the French biologist André Lwoff in the early 1950s.
Lysogeny is of benefit to the virus, allowing the genetic material to persist in the absence of a virus manufacture. Lysogeny can also be beneficial to the host bacterium. The primary benefit to bacteria occurs when the integrated viral DNA contains a gene that encodes a toxin. Possession of the toxin can be advantageous to those bacteria that establish an infection as part of their strategy of replication. For example, toxins encoded by bacteriophage genes are the main cause of the symptoms associated with the bacteria diseases of tetanus , diphtheria , and cholera.
The process of lysogeny has been studied most intensively in a bacteriophage that is designated as lambda. In the lambda bacteriophage, the establishment of lysogeny depends on the presence of three viral proteins. These are designated cI ("c-one"), cII, and cIII. The cI protein is manufactured first, using host molecules that interpret the information for the protein contained in the viral DNA, following the entry of the viral DNA into the host bacterium. At this point the viral DNA is not integrated into the host genome, but exists as an independent circle. CI is a so-called repressor protein that operates to occupy sequences on the viral genome that would otherwise be used to make the various viral proteins that are needed to assemble the new virus particles. By occupying these sites, cI prevents viral proteins from being produced.
At about the same time, the viral DNA becomes integrated into the host DNA and the cII and cIII proteins are manufactured. These latter proteins assist cI in the task of blocking synthesis of viral components. Accordingly, cI, cII, and cIII function to maintain the lysogenic state. The cII protein functions to make the manufacture of cI by the host's transcription machinery more efficient. The cIII protein helps protect the cII protein from being degraded by host enzymes .
Once lysogeny is established, the continued manufacture of the cI protein will maintain the integrated state of the viral DNA.
The cI protein maintains its own transcription. The binding of cI to a certain stretch of DNA promotes the recognition and use of the gene for cI to manufacture the cI protein. This is known as positive control. As well, the protein exerts a negative control of another protein (termed "cro"). In negative control, the binding of cI to a region of the DNA prevents the gene from cro from being recognized and used to manufacture the cro protein.
The "decision" to maintain lysogeny or to begin the cycle whereby new virus particles are made and the bacterium explosively releases the new particles is essentially a competition between the cI and cro proteins. This competition centers on the binding of the proteins to a stretch of DNA called the OR operator. This stretch of DNA has three sites that the proteins can occupy. Depending on which sites are occupied by which protein, the manufacture of either the cI or the cro proteins is promoted. If more cI is made, lysogeny continues. If cro is made, the process of viral assembly (i.e., the lytic cycle) begins. The lytic cycle can be triggered by events that damage the host bacterium, including exposure to environmental stressors (e.g., ultraviolet radiation exposure).
See also Bacteriophage and bacteriophage typing; Operon; Viral genetics; Virus replication