The lysogenic cycle represents a reproductive strategy used by certain viruses, primarily those that infect bacteria, known as bacteriophages. This cycle is characterized by the viral genetic material entering the host cell and incorporating itself into the host’s own genome, rather than immediately taking over the cell’s machinery to produce new virus particles. Unlike the lytic cycle, which rapidly destroys the host cell, the lysogenic cycle allows the virus to lie dormant, replicating passively as the host cell divides and survives. Phages capable of this non-destructive path are called temperate phages.
The Phases of Lysogeny
The establishment of the lysogenic state is a multi-step process that begins with the physical encounter between the virus and the host bacterium. The virus first attaches to the outer surface of the host cell using specific proteins, a step called adsorption, followed by penetration where its genetic material—often DNA—is injected into the bacterial cytoplasm. This viral DNA then circulates within the host cell, where it must decide between the lytic and lysogenic pathways, a choice often influenced by the cell’s physiological health.
If the lysogenic path is chosen, the viral DNA physically splices itself into the host cell’s chromosome, a process known as integration or recombination. This integration is typically mediated by a phage-encoded enzyme called an integrase, which facilitates the precise insertion of the viral DNA into a specific site on the bacterial chromosome. Once integrated, the viral genome is referred to as a prophage, and the host cell carrying it is called a lysogen. For example, the well-studied Lambda phage inserts itself into the E. coli chromosome at a specific attachment site.
The prophage remains in a state of latency, meaning its genes responsible for viral particle production are repressed and inactive. During this time, the prophage’s DNA is replicated along with the bacterial chromosome every time the host cell prepares to divide. This passive replication ensures that every daughter cell inherits a copy of the viral genome, effectively spreading the virus through the bacterial population without expending resources on producing new viral particles.
The Mechanism of Induction
The dormant state of the prophage is not permanent, and the virus can switch from the lysogenic state back into the destructive lytic cycle through a process called induction. Induction is the mechanism by which the prophage excises itself from the host chromosome, a switch generally triggered by environmental stress signals. These stress signals often indicate that the host cell’s survival is threatened, making the long-term, passive strategy of lysogeny no longer viable.
Common environmental triggers include exposure to ultraviolet (UV) radiation, certain chemicals, or conditions that cause damage to the host cell’s DNA. This damage often activates a bacterial distress response known as the SOS response, which involves the RecA protein. The activated RecA protein then initiates the cleavage and inactivation of the repressor protein that had been keeping the prophage dormant.
With the repressor inactivated, the genes necessary for the lytic cycle are expressed, beginning with the precise excision of the prophage DNA from the host chromosome. This newly separated viral DNA then takes over the host’s cellular machinery to begin the production of new viral components and particles. The induction process is the point of no return for the lysogen, as it leads directly to the biosynthesis, assembly, and eventual release of progeny phages, culminating in the lysis of the host cell.
Consequences for the Host Cell
The presence of a prophage confers several significant biological and evolutionary advantages to the host bacterium, even as it harbors a potential time bomb. One immediate effect is known as superinfection immunity, where the lysogen becomes resistant to infection by other, similar phages. The repressor protein that maintains the lysogenic state also binds to the corresponding sites on the invading viral DNA, preventing the invader from initiating its own lytic cycle and thus protecting the host cell.
A second consequence is lysogenic conversion, which occurs when the prophage carries genes that provide a new trait to the bacterium. These traits are often virulence factors that enhance the bacterium’s ability to cause disease in a larger host, such as a human. For instance, the bacteria that cause diphtheria and botulism only produce their harmful toxins when they are lysogenized by a specific phage that carries the necessary toxin gene.
From the virus’s perspective, having both the lytic and lysogenic cycles is an effective survival strategy, allowing the virus to adapt its reproductive method to the surrounding conditions. When host cells are abundant and healthy, the lytic cycle provides rapid, explosive reproduction, while the lysogenic cycle ensures the virus’s survival and spread when host cells are scarce or stressed.