Two pathways of development: lysis or coexistence
Unlike virulent strains, temperate bacteriophages have two possible life cycle scenarios upon entering a cell.
The first pathway is the productive (lytic) pathway. It strictly follows the classic cycle: the virus enters the cell, its genome replicates sequentially, new virions are assembled, and the process concludes with the death (lysis) of the host bacterium.
The second pathway is the integrative (lysogenic) pathway. In this case, host destruction does not occur. The viral DNA enters the cytoplasm, forms a ring, and integrates into the bacterial chromosome via crossing-over. The integrated viral DNA is called a prophage, and the bacterium carrying this genome is referred to as a lysogenic culture. With each cell division, the prophage replicates synchronously and is passed on to daughter cells. This state of peaceful coexistence itself is called lysogeny.
It is important to note that integration does not occur randomly, but rather at specific homologous sites. For example, in the bacterium Escherichia coli, integration of lambda phage occurs at a strictly defined locus located between the galactose and biotin metabolism genes.
How lysogeny is maintained and immunity is established
The essence of lysogeny is that the production of new phage progeny is completely blocked, preventing cell destruction. This status quo is genetically controlled. The prophage genes themselves encode the synthesis of a specific low-molecular-weight protein known as the specific repressor. This protein acts as the main "restraining" factor. Its mechanism of action is simple: the repressor suppresses the transcription (reading) of those regions of the viral genome that are responsible for vegetative reproduction.
The presence of the pre-synthesized repressor in the cytoplasm gives rise to an interesting phenomenon: the immunity of lysogenic bacteria. If the DNA of a similar (homologous) phage attempts to enter the cell, superinfection occurs. However, the new virus will not be able to replicate: the repressor protein immediately blocks the transition of the fresh viral DNA into a vegetative state, preventing cell lysis.
Prophage induction: return to lysis
The term "lysogeny" itself implies only a potential capacity of the cell for lysis. The transition from the latent state to the active state is reversible and is called induction (or derepression). During induction, the prophage is excised from the bacterial chromosome, the synthesis of viral components is triggered, new virions are assembled, and the cell dies. The basis of this mechanism is the inactivation of the repressor protein by external factors.
Two types of induction are distinguished:
- Spontaneous: Occurs autonomously with an extremely low frequency (from $10^{-2}$ to $10^{-6}$). Due to this rarity, mass culture die-off is not observed.
- Directed (induced): Triggered artificially by exposure to mutagens. Physical agents include ultraviolet and ionizing radiation. Chemical agents include peroxides, mitomycin C, and others.
In biotechnology, spontaneous induction carries serious risks: sudden lysis of industrial producer bacteria can lead to the loss of a valuable product, such as a batch of antibiotics or vitamins. Conversely, in genetic engineering, directed induction is actively used for research purposes.
Phage conversion and plasmid localization
The integrated viral genome can confer new biological properties upon the host—morphological, biochemical, cultural, or antigenic. This phenomenon is called phage conversion.
The most famous clinical example is associated with the causative agent of diphtheria (Corynebacterium diphtheriae). This bacterium is capable of producing its main pathogenicity factor—diphtheria exotoxin—exclusively when a specific prophage is present in its genome. Without phage integration, the bacterium remains nontoxigenic.
There are also exceptions to the general rules of temperate phage behavior:
- Defective phages. They have permanently lost the ability to form functional viral particles and cannot complete their developmental cycle either in a natural environment or upon artificial induction.
- Plasmid localization. The genome of certain phages (e.g., phage P1) does not integrate into the bacterial chromosome at all. It exists autonomously in the cytoplasm, similar to a plasmid. Such autonomous viruses are widely used as vectors in genetic engineering for the targeted transfer of genetic information.
Briefly: answers to topical questions
What sequential stages are included in the productive (lytic) cycle of bacteriophage reproduction?
The productive (lytic) cycle of bacteriophage reproduction includes five main consecutive stages.
- Adsorption — attachment of the phage to specific receptors on the bacterial cell wall.
- Nucleic acid penetration (injection) — cleavage of the cell wall by lysozyme and injection of viral DNA into the cytoplasm.
- Biosynthesis of phage components — suppression of host cell metabolism, transcription of phage DNA, and translation of viral proteins.
- Assembly of structural components — formation of capsids and tail structures, filling empty heads with nucleic acid.
- Morphogenesis of mature particles and release — joining of the capsid with the tail assembly, destruction of the bacterial envelope, and cell lysis with the release of progeny into the environment.
Which enzymes catalyze the integration of viral DNA into the chromosome and its excision during induction?
The integration of viral DNA into the host cell chromosome is catalyzed by several enzymes, although specific excision enzymes during induction are not described in the sources. The insertion of DNA-containing viruses is ensured by:
- Restriction enzymes (restrictases) — participate in the preparation of DNA sites.
- Endonucleases — cleave nucleic acids.
- Ligases — join DNA fragments.
For RNA-containing viruses, reverse transcriptase (reversase) operates prior to integration, synthesizing a complementary DNA strand. Prophage excision during induction occurs due to the inactivation of the repressor protein under the influence of external factors, but specific enzymes for this step are not specified.
What known bacterial exotoxins, besides diphtheria toxin, are encoded by temperate bacteriophage genes?
In addition to diphtheria exotoxin, streptococcal pyrogenic exotoxins are encoded by temperate bacteriophage genes.
- Streptococcal pyrogenic exotoxins (Dick erythrogenic toxin, rash toxin, scarlet fever toxin, erythrogenin) are produced by strains of Streptococcus pyogenes.
Their synthesis is directly linked to the phenomenon of lysogeny: bacterial strains lacking the temperate phage are unable to produce these toxins. These exotoxins cause a systemic toxic effect and a characteristic bright red rash in scarlet fever due to necrotic action on the vascular endothelium and activation of T-lymphocytes.
To easily remember the essence of induction, imagine that the repressor protein acts as a "brake." Ultraviolet radiation or chemical agents "break the brake," and the prophage rapidly transitions into the vegetative phase, culminating in cell lysis.
Class and exam questions
What is the main difference between a temperate and a virulent phage?
Virulent phages always follow the lytic pathway and kill the bacterium. Temperate phages are capable of integrating their genome into the host chromosome, keeping it alive and replicating alongside it (lysogeny).
Why is a lysogenic bacterium not destroyed upon superinfection?
The cytoplasm of a lysogenic cell already contains the synthesized repressor protein. It blocks the transcription of genes from any entering homologous phage, preventing it from proceeding to replication.
What will happen to a biotechnological production facility during spontaneous induction?
Spontaneous induction in industrial strains leads to the transition of the prophage into the lytic cycle, mass culture die-off, and, consequently, the loss of the manufactured product (vitamins, antibiotics).
What else to study on this topic
- Molecular mechanism of crossing-over integration
- Phage localization: galactose and biotin loci
- Use of autonomous phages as genetic vectors
- Structure and operational principles of the repressor protein
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