What is the Difference Between Prophage and Provirus

The main difference between prophage and provirus is that prophage is the viral genome integrated into a bacterial genome, whereas provirus is the viral genome integrated into a eukaryotic genome. Furthermore, prophages are bacteriophages which undergo the lysogenic cycle by integrating its genome into the bacterial genome while proviruses are retroviruses that convert their single-stranded RNA genome into double-stranded DNA, integrating into the eukaryotic genome. In addition, prophage can undergo excision from the host genome in order to enter into the lytic cycle while provirus may not undergo excision after splicing. 

Prophage and provirus are two stages of the virus, integrating into the genome of different hosts. Generally, the viral genome of both types passes over generations of the host due to genome integration. 

Key Areas Covered 

1. What is a Prophage
     – Definition, Genome Integration, Significance
2. What is a Provirus
     – Definition, Genome Integration, Significance
3. What are the Similarities Between Prophage and Provirus
     – Outline of Common Features
4. What is the Difference Between Prophage and Provirus
     – Comparison of Key Differences

Key Terms 

Bacterial Genome, Eukaryotic Genome, Genome Integration, Prophage, Provirus

What is a Prophage 

Prophage is a type of viral genome that integrates into the genome of a bacterial cell. Generally, this type of virus is known as a bacteriophage. However, bacteriophages can have two types of life cycles. They are the lysogenic cycle and the lytic cycle. Basically, in the lysogenic cycle, the viral genome integrates into the host genome. In comparison, in the lytic cycle, the virus takes over the cell and undergoes reproduction to produce new viruses.  Although viruses in the lysogenic cycle do not destroy the host cell, viruses in the lytic cycle destroy the host cell, releasing newly made phages.

Figure 1: Lytic and Lysogenic Cycles of a Bacteriophage

Furthermore, prophages go through a lysogenic cycle as they directly integrate their genome into the genome of the host. A bacterial host with an integrated prophage is called a lysogen.  Also, prophage undergoes replication along with the host genome by using cellular machinery of the bacterium. Therefore, prophage can transmit into the genomes of the daughter bacterial cells over generations. Due to the presence of extra genes that came from the prophage, the phenotype of the infected bacteria differs from that of the uninfected bacteria. This alteration is known as lysogenic conversion. As an example, it enhances the virulence of bacteria such as Vibrio cholerae and Clostridium botulinum, due to the production of toxins. Significantly, the induction of the prophage in the lysogenic cycle allows it to proceed through the lytic cycle by the excision of the prophage from the host genome.  

What is a Provirus 

The provirus is the type of viral genome that integrates into the genome of a eukaryotic cell. In comparison, retroviruses are the type of viruses that become proviruses. Usually, retroviruses contain a single-stranded RNA in their genome. Therefore, their genome also encodes for reverse transcriptase, which is the enzyme reverse transcribing RNA into DNA. Significantly, this permits the integration of the viral DNA genome into the genome of the eukaryote. Moreover, as provirus integrates into the host genome as the prophage, it undergoes the lysogenic cycle as well. Therefore, provirus passively undergoes replication along with the host genome. Generally, this process is known as lysogenic viral reproduction.

Figure 2: HIV Provirus

Moreover, the integration of the provirus may result in a latent infection or a productive infection. Typically, in the latent infection, the provirus is transcriptionally silent. However, in the productive infection, provirus undergoes transcription, directly allowing the synthesis of new viruses, which in turn infect other cells through the lytic cycle. Usually, the stage of the latent infection can become a productive infection in response to environmental conditions or health status. However, proviruses do not undergo excision after their integration into the host genome. Normally, proviruses account for 8% of the human genome, and they are inherited in the form of endogenous retroviruses. Apart from the retroviruses,  adeno-associated viruses are another example of proviruses. 

Similarities Between Prophaage and Provirus 

• Prophage and provirus are two stages of viruses infecting different types of host cells. 
• Significantly, both stages of viruses integrate their genomes into the host genome. 
• Therefore, the viral genome can replicate as a part of the host. 
• They can be retained in the lysogenic cycle. 

Difference Between Prophage and Provirus 

Definition 

Prophage refers to the genome of the bacterial viruses, integrated into the bacterial genome while provirus refers to the genome of the virus, integrated into the genome of the eukaryotic host cell. 

Integration 

While prophage integrates into the bacterial genome, provirus integrates into the eukaryotic genome. 

Examples 

Bacteriophages are an example of prophages, while retrovirus is an example of proviruses. 

Type of Viral Genome 

Prophages have a genome with double-stranded DNA, while proviruses have a genome with single-stranded RNA. 

Significance of the Integration Process 

Moreover, prophages undergo a lysogenic cycle by integrating its genome into the bacterial genome while proviruses convert their single-stranded RNA genome into double-stranded DNA, integrating into the eukaryotic genome. 

Reverse Transcriptase 

Whereas prophages do not use reverse transcriptase, proviruses use reverse transcriptase. 

Importance 

Prophage can enter into the lytic cycle through the excision of the genome from the bacterial genome, while provirus can establish a chronic infection by remaining in the host for a long time. 

Excision of the Viral Genome 

Furthermore, prophage undergoes excision from the host genome to enter into the lytic cycle while provirus does not undergo excision once integrated. 

Proceeding 

New viral particles synthesized in the lytic cycle of the prophage undergo lysogeny in the newly infected cells while the new viral particles synthesized in the lysogenic cycle through the productive infection of proviruses undergo the lytic cycle in the newly infected cells. 

Conclusion 

Prophage is the type of viral genome that integrates into the genome of a bacterial cell. Generally, it remains in the lysogenic cycle and passes over generations through the replication of the genome. However, it can undergo excision from the host genome to enter into the lytic cycle, which causes the destruction of the bacterial cell. Besides, viruses that infect bacteria are bacteriophages. Provirus, on the other hand, is a type of viral genome which integrates into the genome of a eukaryotic cell. Although it undergoes replication along with the eukaryotic genome, provirus may not undergo excision from the host genome. Therefore, it can cause chronic infections. Also, retroviruses are the type of viruses that become provirus. Hence, the main difference between prophage and provirus is the type of host and their entrance into the lytic cycle. 

References:

1. OpenStax. “The Viral Life Cycle.” Lumen|Microbiology, Available Here.

Image Courtesy:

1. “Figure 6.8” By The Viral Life Cycle – Microbiology – OpenStax (CC BY 4.0) via OpenStax CNX
2. “Figure 6.12” By The Viral Life Cycle – Microbiology – OpenStax (CC BY 4.0) via OpenStax CNX