Retroviruses, reverse transcriptases

Blough, H. (1990) Organic thio compormds having a sulfur-gold linkage as specific inhibitors for retrovirus reverse transcriptase, and... 

The viruses responsible for AIDS are human immunodeficiency virus 1 and 2 (HIV 1 and HIV 2) Both are retroviruses, meaning that their genetic material is RNA rather than DNA HI Vs require a host cell to reproduce and the hosts m humans are the T4 lymphocytes which are the cells primarily responsible for inducing the immune system to respond when provoked The HIV penetrates the cell wall of a T4 lymphocyte and deposits both its RNA and an enzyme called reverse transcriptase inside There the reverse transcriptase catalyzes the formation of a DNA strand that is complementary to the viral RNA The transcribed DNA then serves as the template from which the host lymphocyte produces copies of the virus which then leave the host to infect other T4 cells In the course of HIV reproduction the ability of the T4 lymphocyte to reproduce Itself IS compromised As the number of T4 cells decrease so does the body s ability to combat infections... 

Reverse transcriptase inhibitors are also used against certain viruses which although they are not retroviruses do require re verse transcriptase to repro duce The virus that causes heptatitis B is an example... 

Section 28 13 HIV which causes AIDS is a retrovirus Its genetic matenal is RNA instead of DNA HIV contains an enzyme called reverse transcriptase that allows Its RNA to serve as a template for DNA synthesis m the host cell... 

Reverse transcription is the copying of an RNA molecule back into its DNA complement. The enzymes that perform this function are called reverse transcriptases. Reverse transcription is used naturally by retroviruses to insert themselves into an organism s genome. Artificially induced reverse transcription is a useful technique for translating unstable messenger RNA (mRNA) molecules into stable cDNA. 

Herandez J, Amador L, Amantea M, Chao H, Hawley P, Paradise L (2000) Short-course monotherapy with AG1549, a novel non-nucleoside reverse transcriptase inhibitor (NNRTI), in antiretroviral naive patients. In 7th conference on retroviruses and opportunistic infections. San Francisco, CA, Abstract 669... 

Najera I, Richman DD, Olivares I, Rojas JM, Peinado MA, Perucho M, Najera R, Lopez GaHndez C (1994) Natural occurrence of drug resistance mutations in the reverse transcriptase of human immunodeficiency virus type 1 isolates. AIDS Res Hum Retroviruses 10 1479-1488 Nijhuis M, Boucher CAB, Schipper R Leitner T, Schuurman R, Albert J (1998) Stochastic processes strongly influence HIV-1 evolution during suboptimal protease inhibitor therapy. Proc Natl Acad Sci USA 95 14441-14446... 

Retroviruses Human T-cell leukaemia virus (HTLV-1) Spherical enveloped virus lOOnm in diameter, icosahedral cores contain two copies of linear RNA molecules and reverse transcriptase HTI.V is spread inside infected lymphocytes in blood, semen or breast milk. Most infections remain asymptomatic but after an incubation period of 10-40 years in about 2% of cases, adult T-cell leukaemia can result... 

Human immunodeficiency virus (HIV) is a retrovirus, i.e. its RNA is converted in human cells by the en me reverse transcriptase to DNA which is incorporated into the human genome and is responsible for producing new HIV particles. Zidovudine (azidothymidine, AZT Fig. 5.22F) is a stmctural analogue of thymidine (Fig. 5.22A) and is used to treat AIDS patients. Zidovudine is converted in both infected and uninfected cells to the mono-, di- and eventually triphosphate derivatives. Zidovudine triphosphate, the aetive form, is a potent inhibitor of HIV replication, being mistaken for thymidine by reverse transeriptase. Premature ehain termination of viral DNA ensues. However, AZT is relatively toxic because, as pointed out above, it is converted to the triphosphate by eellular enzymes and is thus also aetivated in uninfected cells. 

Enzymes in viruses We have stated that virus particles do not carry out metabolic processes. Outside of a host cell, a virus particle is metabolically inert. However, some viruses do contain enzymes which play roles in the infectious process. For instance, many viruses contain their own nucleic acid polymerases which transcribe the viral nucleic acid into messenger RNA once the infection process has begun. The retroviruses are RNA viruses which replicate inside the cell as DNA intermediates. These viruses possess an enzyme, an RNA-dependent DNA popo called reverse transcriptase, which transcribes the information in the incoming RNA into a DNA intermediate. It should be noted that reverse transcriptase is unique to the retroviruses and is not found in any other viruses or in cells. 

A primer is a very special sequence, which plays an important role in duplication, (i) In RNA, it is a short sequence that is paired with one strand of DNA and provides a free 3 -OH terminus at which a DNA polymerase starts synthesis of a deoxyribonucleotide chain, (ii) In DNA, it is another short sequence, which is complementary to a sequence of messenger RNA and allows reverse transcriptase to start copying the adjacent sequences of mRNA. (iii) In retroviruses, it is a cellular transfer RNA whose elongation initiates RNA-directed DNA synthesis by the DNA polymerase. 

The human retrovirus HIV can be controlled using chemotherapy directed at the reverse transcriptase and aspartyl protease encoded by the viral genome as with other microbial pathogens, however, resistance to drug therapy becomes a major problem. Figure 7.3 shows a crystal structure (PDB 1HXW) of the HIV protease, where mutated amino acids (shown in cyan) lead to disrupted binding of the clinically effective inhibitor ritonavir [24]. 

Reverse transcriptase. This enzyme is involved in the replication of retroviruses in vivo. It synthesizes a complementary DNA (cDNA) strand using RNA instead of DNA as its template. It is widely used to create a strand of cDNA from mRNA extracted from cells or tissue for cloning or for PCR analysis. 

Reverse transcriptase is an RNA-dependent DNA polymerase that requires an RNA template to direct the synthesis of new DNA. Retroviruses, most notably HIV, use this enzyme to repHcate their RNA genomes. DNA synthesis by reverse transcriptase in retroviruses can be inhibited by AZT. ddC, and ddl. 

The genetic material in a retrovirus is RNA not DNA. The best known retrovirus is the human immunodeficiency virus (HIV) which infects lymphocytes and hence interferes with the immune system, giving rise to the disease AIDS. Once the virus infects its host, it converts its RNA into DNA by an enzyme known as reverse transcriptase and the DNA is then inserted into the genome of the host cell (in this case the lymphocyte) ... 

The life history of a retroviras is described in chapter 17 (see Figure 17.45). A summary is presented here. The genome of a retrovirus is composed of RNA not DNA but, when a retrovirus infects a host cell its RNA is transcribed into DNA, catalysed by the enzyme, reverse transcriptase. This DNA is then incorporated into the genome of the host. On transcription of the host DNA, during cell division, viral mRNA and viral genomic RNA are produced. The... 

Many viruses and retroviruses have genomes that are single-stranded RNA instead of DNA. These include the acquired immunodeficiency syndrome (AIDS) virus and some retroviruses that cause cancer. Here, an enzyme called reverse transcriptase converts the RNA genome of the virus into the DNA of the host cell genome, thus infecting the host. 

Antibodies against the virus but also amantadine and derivatives, interfere with host cell penetration. There are nucleoside analogues such as aciclovir and ganciclovir, which interfere with DNA synthesis, especially of herpes viruses. Others like zidovudine and didanosine, inhibit reverse transcriptase of retroviruses. Recently a number of non-nucleoside reverse transcriptase inhibitors was developed for the treatment of HIV infections. Foscarnet, a pyrophosphate analogue, inhibits both reverse transcriptase and DNA synthesis. Protease inhibitors, also developed for the treatment of HIV infections, are active during the fifth step of virus replication. They prevent viral replication by inhibiting the activity of HIV-1 protease, an enzyme used by the viruses to cleave nascent proteins for final assembly of new vi-rons. 

The virus that causes AIDS, the Human Immune Deficiency Virus (HIV) is a retrovirus. Instead of double-stranded DNA it uses single-stranded RNA to store its genetic information. HIV uses the enzyme reverse transcriptase to convert its RNA into DNA in order to replicate. 

Mechanism of Action Apurine nucleoside analog that is intracellularly converted into a triphosphate, which interferes with RNA-directed DNA polymerase (reverse transcriptase). Therapeutic Effect Inhibits replication of retroviruses, including HIV. Pharmacokinetics Variably absorbed from the GI tract. Protein binding less than 5%. Rapidly metabolized intracellularly to active form. Primarily excreted in urine. Partially (20%) removed by hemodialysis. Half-life 1.5 hr metabolite 8-24 hr. 

It is a thymidine analogue. After phosphorylation in body zidovudine triphosphate selectively inhibits viral reverse transcriptase i.e. RNA dependent DNA polymerase. It is effective against retrovirus only. 

The term oncogene was coined in association with the search for the tumor-causing principle in retroviruses. Retroviruses contain RNA as the genetic material and can transcribe RNA into DNA with the help of the virus s own enzyme reverse transcriptase. The DNA form of retroviruses can integrate into the DNA of the host cell and, during cell division, is passed on to the daughter cells as a provirus. From the provirus, viral RNA and complete virus particles may be formed. 

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