Adenoviruses are common viruses that can cause a range of illnesses, from mild respiratory infections to gastroenteritis. Their genome contains all the instructions the virus needs to enter a host cell, take over its machinery, and replicate. The study of the adenovirus genome has also provided a model for understanding fundamental cellular processes.
Composition and Structure of the Genome
The adenovirus genome is a single, linear molecule of double-stranded DNA (dsDNA) about 36,000 base pairs long. Unlike the circular DNA in many bacteria or the RNA genomes of other viruses, this DNA is housed compactly within the virus’s protein shell, or capsid. The genetic material is organized with genes on both DNA strands to maximize its coding capacity.
At each end of the linear DNA are Inverted Terminal Repeats (ITRs), which are identical sequences of about 100 base pairs. These ITRs act like bookends, protecting the ends of the DNA from degradation. They also contain the origin of replication, the specific site where DNA copying begins.
Covalently attached to the 5′ end of each DNA strand is a 55-kilodalton Terminal Protein (TP). This protein is a remnant of the replication process and acts as a primer, providing a starting point for the viral DNA polymerase to begin synthesis. This mechanism solves the challenge of fully replicating the ends of linear DNA, a problem host cells solve differently using telomeres.
Key Genetic Regions and Functions
The adenovirus genome is organized into regions expressed in a timed sequence after infection, broadly categorized as “early” and “late” genes. The early genes are transcribed before DNA replication begins and are responsible for preparing the host cell for viral propagation. They are grouped into four main regions:
- E1: Acts as the master regulator of the infection cycle by pushing the host cell into the S phase, where DNA is synthesized.
- E2: Encodes the machinery needed to copy the viral DNA, including the viral DNA polymerase and the Terminal Protein precursor.
- E3: Contains proteins dedicated to immune evasion that interfere with the host’s ability to recognize and destroy infected cells.
- E4: Assists in viral DNA replication, controls the expression of late genes, and helps shut down host cell protein synthesis.
After the viral DNA has been copied, the late genes (L1 through L5) are expressed. These genes code for the structural proteins that form new virus particles, including the hexons, pentons, and fiber proteins of the capsid. These components self-assemble in the nucleus to form new virions.
Replication Inside a Host Cell
The replication cycle begins when the virus attaches to a host cell and injects its DNA into the cytoplasm. The genome and its attached Terminal Protein then travel to the nucleus. Inside the nucleus, the virus hijacks the cell’s resources, starting with the transcription of the early genes.
Expression of the E1 genes initiates a cascade that activates other early regions and prepares the cell for viral DNA replication. The replication process then begins at the ITRs, using the Terminal Protein as a primer. This process can produce up to a million new copies of the viral genome in under two days.
With a sufficient pool of new genomes, the process shifts to the late phase, and structural proteins are produced. These proteins assemble with the new DNA in the nucleus to form infectious virions. The final step involves the cleavage of a precursor Terminal Protein (pTP) to its mature form (TP) by a viral protease. The accumulation of new virions eventually leads to the rupture of the host cell, releasing the progeny to infect neighboring cells.
Role in Gene Therapy and Vaccines
Scientists harness the adenovirus’s cell-entry mechanism for medical purposes like gene therapy and vaccine development. By modifying the viral genome, researchers create a replication-incompetent “vector” that can deliver genetic material into human cells without causing disease. This engineered virus can infect cells but cannot multiply within them.
The most common modification involves deleting the E1 and E3 genetic regions. Removing the E1 region eliminates the virus’s ability to replicate. Deleting the E3 region creates space in the genome for a therapeutic gene and helps reduce the host’s immune response to the vector. The engineered virus is then grown in special lab cell lines that provide the missing E1 proteins, allowing for mass production.
In gene therapy, this modified virus carries a healthy copy of a gene to replace a faulty one. For vaccines, the vector carries a gene from another pathogen, like the spike protein from SARS-CoV-2. When the adenovirus vector enters human cells, it instructs them to produce the foreign protein. This process primes the immune system to recognize and fight the actual pathogen, a technology used in several COVID-19 vaccines.