Which viruses are capable of virogeny
The integrative type of interaction is not characteristic of all pathogens. A key condition for virogeny is the virus's ability to introduce its genetic material into the chromosome of the infected cell.
The main groups capable of integration include:
- Temperate DNA-containing bacteriophages (bacterial viruses capable of existing inside a bacterial cell without immediately destroying it).
- Oncogenic viruses, which cause tumor transformation of tissues.
- Certain infectious viruses of humans and animals, including both DNA-containing (e.g., hepatitis B virus) and RNA-containing (e.g., human immunodeficiency virus — HIV).
Molecular mechanisms of DNA virus integration
For a DNA-containing virus to integrate into the host genome, a number of strict molecular conditions and specific enzymes are required.
- Preparation of the genome form: A mandatory condition is the presence of a circular form of double-stranded viral DNA. Without circularization, the integration process is impossible.
- Target recognition (attachment): The viral genome finds specific regions with homologous (similar) nucleotide sequences on the cell DNA and attaches to them.
- Insertion (integration): The viral DNA is inserted strictly into a specific locus of the cell chromosome.
- Enzymatic support: The entire process of cutting the cell DNA and splicing it with the viral DNA is carried out by a set of enzymes, which includes restriction enzymes, endonucleases, and ligases.
Features of RNA virus integration
The mechanism of integration of RNA viruses (retroviruses, a prominent representative of which is HIV) is much more complex and represents a multi-step cascade of reactions, as the cellular apparatus is unable to directly insert RNA into a DNA chromosome.
- Reverse transcription: At the first stage, the synthesis of a complementary DNA strand occurs, using the viral RNA as a template. This process is carried out by a key viral enzyme — reverse transcriptase (also known as revertase).
- Formation of the DNA transcript: Then the second strand is synthesized, resulting in a fully-fledged double-stranded DNA molecule carrying the genetic information of the virus.
- Integration: The resulting double-stranded viral DNA is integrated into the host cell chromosome, completing the virogeny process.
The "Provirus" state and outcomes for the cell
Following successful integration, the viral DNA, having become part of the cellular chromosome, is called a provirus (or proviral DNA). In this state, the virus loses its independence: the provirus replicates completely synchronously with the cell genome and is guaranteed to be transmitted to daughter cells upon division. Thus, the state of virogeny is inherited across cell generations.
The presence of a provirus can lead to two main outcomes:
- Transition to an autonomous state (induction). Under the influence of various chemical or physical factors, stability is disrupted. The provirus is able to "excise" (exclude itself) from the chromosome. Following this, a productive type of infection develops — the virus switches to active autonomous reproduction, which usually ends in cell death.
- Genetic transformation. The provirus introduces fundamentally new genetic information into the cell genome. As a result, the cell acquires new properties not previously characteristic of it. The most dangerous consequence of such transformation is the cell's ability to undergo uncontrolled division, which leads to oncogenic tissue transformation.
Briefly: answers to topical questions
What factors induce provirus induction with transition to productive infection?
Provirus induction with transition to productive infection is caused by various physical or chemical factors. Under their influence, the viral DNA (provirus) can "excise" (exclude itself) from the host cell chromosome, transitioning into an autonomous state. As a result of this process, active reproduction of the virus begins, which characterizes the development of the productive type of interaction.
How does the productive type of infection differ from the integrative type?
The productive type of infection concludes with a full reproduction cycle and the formation of infectious progeny. In the integrative type, the viral DNA is integrated into the cell chromosome with the formation of a provirus, replicates jointly with the cellular genome, and coexists with the cell. Cell transformation is also possible in the integrative type.
Which human body cells are most frequently infected by HIV during virogeny?
The primary target of HIV is CD4-bearing T-helper lymphocytes. During viral DNA integration, the reservoirs of infection are memory T cells and "dormant" macrophages. HIV also infects monocytes, macrophages, Langerhans cells, dendritic cells, and neuroglial cells; additional target cells include intestinal and cervical epithelial cells, vascular endothelium, eosinophils, and megakaryocytes.
To memorize the DNA virus integration enzymes, use the acronym REL: Restriction enzymes and Endonucleases — "cut", Ligases — "glue" (splice) the viral DNA with the cellular DNA.
Class and exam questions
What is a provirus?
A provirus is viral DNA that is physically integrated into the host cell chromosome, functions as part of it, and replicates synchronously with the cellular genome.
Why do RNA viruses need reverse transcriptase?
Reverse transcriptase (revertase) is necessary for the synthesis of a complementary DNA strand using viral RNA as a template. Without preliminary translation of genetic information from RNA to DNA, integration into the cell chromosome is impossible.
Does virogeny always proceed asymptomatically for the cell?
No, not always. The integrated virus introduces new genetic information, which can cause genetic transformation of the cell, for example, leading to its uncontrolled division (oncogenesis). Furthermore, induction with a transition to the active reproduction stage of the virus is possible.
What else to study on this topic
- Structure and properties of the reverse transcriptase (revertase) enzyme
- Physical and chemical factors of provirus induction
- Mechanisms of oncogenic cell transformation by proviruses
- Features of temperate bacteriophage integration
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