Viruses Capable of Virogeny
The integrative type of interaction is not characteristic of all pathogens. A key requirement for virogeny is the virus's ability to introduce its genetic material into the chromosome of the infected cell.
Major groups capable of integration include:
- Temperate DNA bacteriophages (bacterial viruses capable of persisting inside a bacterial cell without immediately destroying it).
- Oncogenic viruses, which cause tumor transformation of tissues.
- Certain human and animal infectious viruses, including both DNA-containing (e.g., hepatitis B virus) and RNA-containing viruses (e.g., human immunodeficiency virus — HIV).
Molecular Mechanisms of DNA Virus Integration
For a DNA-containing virus to integrate into the host genome, strict molecular conditions and specific enzymes are required:
- Genome Conformation: A circular double-stranded viral DNA molecule is mandatory. Without circularization, integration cannot occur.
- Target Recognition (Attachment): The viral genome locates specific sites with homologous (similar) nucleotide sequences on the cellular DNA and attaches to them.
- Insertion (Integration): The viral DNA integrates strictly into a specific locus of the cellular chromosome.
- Enzymatic Machinery: The entire process of cleaving cellular DNA and ligating it with viral DNA is mediated by a set of enzymes, including restriction endonucleases, endonucleases, and ligases.
Features of RNA Virus Integration
The integration mechanism of RNA-containing viruses (retroviruses, exemplified by HIV) is significantly more complex and represents a multi-step cascade of reactions, as the cellular machinery cannot directly integrate RNA into a DNA chromosome.
- Reverse Transcription: In the first step, a complementary DNA strand is synthesized using viral RNA as a template. This process is mediated by a key viral enzyme — reverse transcriptase (also known as revertase).
- DNA Transcript Formation: The second strand is then synthesized, yielding a complete double-stranded DNA molecule carrying the viral genetic information.
- Integration: The resulting double-stranded viral DNA integrates into the host cell chromosome, completing the virogeny process.
The Provirus State and Cellular Outcomes
Following successful integration, the viral DNA—now part of the cellular chromosome—is termed a provirus (or proviral DNA). In this state, the virus loses its independence: the provirus replicates in absolute synchrony with the cell genome and is reliably transmitted to daughter cells during division. Thus, the virogeny state 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 can excise (excision) from the chromosome. Subsequently, a productive infection develops—the virus shifts to active autonomous replication, typically resulting in cell death.
- Genetic Transformation. The provirus introduces fundamentally new genetic information into the cell genome. As a result, the cell acquires novel properties it did not previously possess. The most dangerous consequence of such transformation is the capacity for uncontrolled cell proliferation, leading to oncogenic tissue transformation.