Retrovirus

What is A Retrovirus?

Retrovirus is a virus that belongs to the family of Retroviridae. It characteristically carries the genetic blueprint in the form of ribonucleic acid (RNA). Retrovirus is named after an enzyme known as reverse transcriptase. The reverse transcriptase transcribes the RNA into DNA (deoxyribonucleic acid). It is a process that constitutes a reversal of the actual direction of cellular transcription from DNA into RNA. The reverse action makes the genetic material from a retrovirus to permanently incorporate into the DNA genome of the infected cell. The enzyme is popularly used in biological sciences to synthesize genes. The best example of a retrovirus is Human Immunodeficiency Virus (HIV).


Retrovirus Structure

Retrovirus virions consist of the outer lipid envelope of glycoprotein. The virions contain two identical single-stranded RNA molecules that are present as a dimer. Although the virions do not have the same biology or morphology, their components are very similar.

Some of the main virion components are:

  1. Envelope

It is composed of lipids and glycoprotein encoded by the env gene. The retroviral envelope conducts three distinct functions.

  • Protection from the lipid bilayer

  • Allowing the retrovirus to get in and out of the host cells through endosomal membrane trafficking.

  • Directly enabling cells by fusing their membranes.

  1. RNA

It has a dimer RNA with a cap of 5’ end and a poly tail at 3’ end. The RNA genome has terminal noncoding regions vital in replication and internal areas that encode virion protein for gene expression.

  1. Proteins

It contains gag proteins, protease (PR), pol protein, and env proteins.

  • The group-specific antigen (gag) is a significant component of the viral capsid. It possesses two nucleic acid binding domains, including matrix (MA) and nucleocapsid (NC).

  • Protease is differently expressed in different viruses. During maturation of virion, it functions in proteolytic cleavages to produce mature gag and pol proteins.

  • Env proteins are responsible for the entry of virions into the host cell. Due to the functional copy of env gene retroviruses are distinct from retroelements.

Retrovirus Life Cycle

Retroviruses have a single-stranded RNA genome that transforms into a unique form of replication. After it has entered the host cell, a reverse transcriptase enzyme synthesizes a double-stranded DNA from the RNA genome of retroviral. The copy of the DNA genome gets into the host genome inside the nucleus via an enzyme called integrase. As a result, the retroviral genome is transcribed into RNA whenever the host genome transcribes, allowing retrovirus to replicate. 

Retrovirus Replication

Step by step replication of a retrovirus

  1. Retrovirus infects normal cells

  2. Viral RNA is introduced in the host cell

  3. Reverse transcription takes place

  4. Viral DNA produces reverse transcriptase

  5. Genetic material enters the host cells nucleus

  6. Viral DNA integrates into the host genome

  7. Viral genes are transcribed and translated

  8. Virus particles gathers and come out of host cell

  9. A new virus can infect other cells.


Retroviral Vectors

A retroviral vector consists of proviral sequences that accommodate the gene of interest for incorporating both into target cells. The vector contains cellular and viral gene promoters like CMV promoters to increase the expression of a gene of interest in the target cell. The use of packaging cells has been the most crucial advancement in vector technology.

Usually, the packaging cells are fibroblast derivatives that contain sequences of coding DNA independently, known as DNA plasmids, expressing viral gene products like gag and pol. The virions containing the vector genome have produced that bud off into the culture medium when the retroviral vector, along with the gene of interest is introduced into the packaging cells by techniques of non-viral transfections. They can infect and stably integrate with the genome of dividing target cells. The retroviral vector cannot replicate further as it does not encode the viral structural proteins that are provided by the packaging cell.


Difference Between Virus and Retrovirus

  1. Genetic Material       

  • Retrovirus – they have RNA as genetic material

  • Virus – they have RNA as well as DNA as genetic material

  1. Host Cells

  • Retrovirus – retrovirus do not destroy the host cell

  • Virus – virus can destroy host cell once they start replicating

  1. Reverse Transcription

  • Retrovirus – retrovirus undergoes reverse transcription to convert the RNA into DNA.

  • Virus – viruses do not undergo reverse transcription processes.


Retrovirus Examples


HIV

Transmission of HIV is through breastfeeding, bodily fluids and needle sharing. The immune system of the person infected with HIV gets weaker and weaker as HIV destroys the CD4 T immune cells that are vital to helping body fight infection. 


Human T- cell lymphotropic virus (HTLV) type 1 and (HTLV) type 2


HTLV 1 and 2 are retroviruses that are closely related. It is transmitted through blood transfusion and needle sharing. HTLV 1 is associated with the development of acute T cell Leukaemia and neurological disorder affecting the spinal cord. HTLV 2 can be spread by sexual relations and blood transfusion from an infected person.


FAQ (Frequently Asked Questions)

1. How does one contract retrovirus?

Retroviruses are the RNA virus that contains reverse enzyme transcriptase. The reverse transcriptase copies the single-stranded RNA into double standard DNA that integrates into the host chromosome and becomes provirus. The transcript is again reversed from DNA to RNA. Similar to other viruses, retrovirus uses the cellular machinery of the organism they infect to make copies of themselves. Getting affected by a retrovirus requires an additional step. The genome needs to be reversed-transcribed into DNA before it is copied in a usual way.


2. Which viruses are retrovirus?

There are three retroviruses, Human immunodeficiency virus (HIV), which can lead to AIDS. The other two can cause human illness, which is Human T- lymphotropic Virus Type 1 (HTLV 1) and Human T-Lymphotropic Virus Type 2 (HTLV 2).  HTLV 1 is associated with the development of acute T cell Leukaemia and neurological disorder affecting the spinal cord. HTLV-2 is spread by blood transfusions, sexual contact and sharing needles. It can also be spread from mother to child during birth or breast-feeding.