Role and Function of HIV-1 Reverse Transcriptase

Table of Contents
Role of Reverse Transcriptase
Function of Reverse Transcriptase
Mechanism of Function of Reverse Transcriptase
Rate of Mutation by Reverse Transcriptase
Studies on HIV-1 Mutation Accuracy

Role of Reverse Transcriptase

    The enzyme Reverse Transcriptase (RT) plays a central role in the transmission of a broad variety of genetic elements [3]. The enzyme is responsible for the transcription of viral RNA to produce a double stranded DNA (dsDNA) that can be inserted into its host genome [6].
The enzyme reverse transcriptase got is name because it reverses the normal flow of information in biological systems [4].
As such reverse transcriptase serves as the enzyme which catalysis the reverse transcription reaction; RNA to DNA [8].

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(Click to Enlarge)


RTs bring about the replication of many groups of viruses, mobilization of rectrovirus and other transposable elements, and many others that pioneer of this field are yet to understand [8].
Reverse transcriptase plays a vital role in viral replication. Bacterial and Eukaryotic Reverse transcriptases (RT) are all RNA-dependent DNA polymerases which produce complementary DNA (cDNA) [2]. Never the less, the cDNAs produced by these two rectroelement systems are fundamentally different [2].
In eukaryotes, RT replicates the entire genone or the transposable genetic units including the gene for RT [2]. This is done to ensure that RT plays its role of producing identical complementary DNAs (cDNAs) of the original units, and is able to produce infectious viruses or transposable units when integrated back into the host DNA [2].

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Function of Reverse Transcriptase

Reverse transcriptase is complex and multifunctional enzymes that have myriad functions. The general functions of reverse transcriptase in three steps are as follows:
1) It acts as an RNA dependent DNA Polymerase, which transcribes single-stranded DNA from viral RNA.
2) It demonstrates the activity of Ribonuclease H (Rnase H) which is a subunit of the Reverse Transcriptase enzyme.
3) Reverse transcriptase acts as DNA-dependent polymerase and transcribes the second DNA strand which is complementary to the Ist DNA strand [9].

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(illustration of RT function)

Emphasis will be placed on HIV-1 Reverse transcriptase mainly because of their function in the replication process and also their role in HIV infection [4]. Reverse Transcriptase plays a big role as to how HIV (Human Immunodeficiency Virus) comes to life and multiples [1].

HIV is the virus that causes AIDS (Acquired Immune deficiency Syndrome. Once it enters a human cell, it springs to life. HIV is a gene enclosed in a cocoon of proteins and lacks most of the machinery that living organisms have until it finds its way into its host [5].

HIV genome constitutes of a Pol region. It is in this region that the HIV-1 reverse transcriptase is been encoded [5]. The expression of the Pol region is brought about by an upstream frame shift during the translation process. It is necessary that this frame shift occurs to assure the production of Pol products in lower concentrations than the Gag products [5]. HIV virus carries in it the reverse transcriptase protein and two copies of its full RNA genome inside of its capsid [5]. Reverse transcriptase synthesizes a double stranded DNA (dsDNA) from the single stranded RNA genome of the virus [5]. The dsDNA is integrated into the chromosome of the host where it serves as a template for the RNA virus strand the host's replication machinery creates [5]. This activity demonstrates HIV-1 reverse transcriptase's vitality for viral replication process [5].

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Mechanism of Function of HIV Reverse Transcriptase:

Reverse transciptase begins its work in the caspid. A Transfer RNA (tRNA)primer starts the synthesis process by binding the 3' long terminal repeat of the viral RNA genome [7]. RT then forms a complementary strand of DNA (cDNA) using the tRNA primer hence forming a DNA-RNA hybrid (Misra and Knox). This synthesis is done in the normal 5'-3' direction [7]. The ribonuclease H (RNase) H activity of RT which is part of the p66 subunit digests the original RNA plus strand, hence creating room for the for the synthesis the plus DNA strand [5]. String of purines is left undigested to serve as a primer for the synthesis of the second DNA strand [7]. This lead to the creation of a DNA double Helix. Ribonucleas H again digests the t-RNA primer used in making the second DNA strand once its synthesis is completed [5].
This double stranded DNA is then spliced into the chromosomes of the host cell [1].

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Rate of Mutation by HIV-1 Reverse Transcriptase

Reverse transcriptase has no proofreading ability like DNA polymerase, as such it does the RNA to DNA conversion at an incredible high rate of error [1].
The rate of error is about one per cycle [1]. Hence each rectroviral DNA genone generated by Reverse Transcriptase contains at least one mutation [1]. The large number of errors made by Reverse Transcriptase makes it possible for the HIV virus to generate hundreds of variants which then invades the immune system and any drugs used by the host [1]

Click to see the AIDS Virus

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Studies on HIV-1 Mutation Accuracy:

The genetic variation for HIV-1 is a composite of the following:
1. Mutation rate 2. The number of replication cycles per unit time 3.The fixation rate of mutations [10].
Here the in vivo rate of mutation of HIV-1 in a single round of replication was measured using a tractable system [10]
A genetically engineered vector containg lacZ alpha peptide gene reports mutations (mansky)

vpr gene is an HIV-1 accessory gene and its gene products is incoporated into HIV-1 particles together with reverse transcriptase [10]. Researches believe that vrp gene influences the accuracy of HIV-1 reverse trancription [10]. vpr gene mutations brings about a four times increase in mutation compared to that of wild-type gene [10]. This had led researches to come to the conclusion that vpr gene of HIV-1 increases reverse transcription accuracy hence modulationg the overall in vivo mutation rate [10].
Success of this studies and other like this are going on to help design bring about an appropriate medication for combating the AIDs pandemic [10]

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1. Howard Hughes Medical Institute. "The Race against Lethal Microbes: Learning to Outwit the Shifty
            Bacterial, Viruses, and Parasites that causes Infectious Diseases." Chavy Chase, Md."
         The Institute, 1996.
2. Inouye, Sumiko and Inouye Masayori. "Strucur, Function, and Evolution of Bacterial Reverse
            Transcriptase." Virus Genes Vol, 11: 1996
3. Lenvin, Henry L. "It's Prime Time for Reverse Transcriptase." Cell 10 Jan. 1997.
4. Lodish, Harvey et. al. "Molecular Cell Biology." 3rd ed. N.Y: c1995.
5. Misra, Dipika and Willian Knox. "Structure of HIV-1 Reverse Transcriptase Complex." Online.
            Internet. Http://www.emory.edu/CHEMISTRY/CAISER/modules/rt/reverse_transcriptase.html
6. Reverse Transcriptase, Recombinant (HIV)." Online. Internet. Nov. 3 1999.
7. "Reverse Transciptase Text": http://www.bimcore.emory.edu/Kins/CHEM441/Gabe/rt/text.html
8. Skalka, Marie A. and Stephen P. Goff. "Reverse Transcriptase." Plainview, N.Y.: Cold Spring
        Harbor Lab. Press, 1993.
9. Stine, Gerald J. "Aids Update." 1997.
10. The mansky lab: HIV-Variation. Online. Internet.
11. The reverse transcription process. online. Internet.

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