Nisanur Çakmakçı - Retroviruses

# Nisanur Çakmakçı - Retroviruses Retroviruses have properties which are characteristic of both RNA and DNA viruses. Virions contains RNA, but their intracellular from is DNA which is integrated into cellular DNA. After the virus nucleoprotein has been introduced into the cytoplasm of cells, the virion-associated polymerase converts virion-RNA into DNA which then can be integrated among cellular genes. As a consequence, virus-DNA can be transferred from one generation of cells to the other in the form of a stable, integrated molecule of DNA, a process which has been defined as reverse transcription. Retroviruses represent the only known example of information flow in the opposite direction to that which is seen in normal cells. --- All retrovirus genomes contain between 5000 and 9000 nucleotides and each virion contains two identical RNA molecules which are coupled at their 5' ends. The RNA is combined with four capsid proteins that are deriverd by cleavafe from the polyprotein gag(group a ntigen). The capsid also contains a reverse transcriptase, pol(polymerase) and the envelope of the virus contains one glycoprotein, env(envelope). --- One retrovirus which has been studied extensively is avian sarcomavirus(ASV). The ASV genome contains four genes with the following sequence from the 5' end of RNA: gag, pol, env an src(sarcoma). The src gene is responsible for transformation of the host cell. --- ![](https://i.imgur.com/H1UMgv6.png) Figure 1. Schematic description of the gene structure of some different retroviruses. (a) Avian sarcome virus(ASV). (b) Avian and murine leukaemia viruses(ALV and MLV). (c) Avian myelocytomatosis virus(AMCV). --- Avian and murine leukaemia viruses(ALV and MLV, respectively) have a genome organization which is similar to that of ASV but they lack src. Between env and poly(A) in the genome of ALV there is a sequence, named 'c' of about 300 nucleotides which is shared between ASV, ALV and certain other avian retroviruses. This sequence most likely does not code for any protein, since there are stop codons in all reading frames. The c region probably also occurs in murine retroviruses. --- In addition, cell of both virds and mice contain endogenous retroviruses which are integrated into their genomes. One difference between endogenous avian retroviruses and ALV is that the c region may be absent in the former. --- Defetive leukaemia viruses, which lack pieces of one or more of the genes needed for virus replication, are common in birds and mice. These defective viruses all require an intact helper viruse, such as ALV or MLV, for their replication. One example is avian myelocytomatosis virus(AMCV). The RNA of these viruses lacks pol and parts of gag and env, which are substituted for by sequences of cellular origin. --- **Replication and Gene Expression of Retroviruses** --- During replication the reverse transcriptase starts near the 5' end of the genome with assistance from cellular tRNA which partly is hybridized to virus-RNA. This figure describes the different steps in the replicaiton which eventually lead to a circular retrovirus DNA being integrated into host_cell DNA. The mechanism for integration is unknown and it is not clear if linear or circular DNA are integrated. Synyhesis of viral RNA occurs in the cell nucleus with the aid of RNA polymerase II from the host cell and integrated DNA most likely represents the template during this reaction. --- ![](https://i.imgur.com/CiVlMI8.png) Figure 2. Reversed transcription of retrovirus-RNA. A cellular tryptophane tRNA initiator hybridizes to 35S-virion-RNA in avian viruses about 100 nucleotides from thw 5' end(a). The first product which is formed is a DNA fragment, which initially is covalently bounf to the tRNA initiator and reaches to the 5' end of the RNA template(b). In the subsequent step the transcription is transferred from the 5' to the 3' end of virus-RNA because of the occurrence of a short sequence which is repeared in both parts of the genome(c). Virus-DNA is then copied and a new transfer from the 5' to the 3' end occurs(d). The complete double-stranded DNA copy which has repetitive 3' and 5' sequences in each end (e) can be circularized at the 5' and the 3' sequences is two forms retaining the repetition(f) and excluding one set of repeated sequences(g). Finally, integration of virus-DNA into the cellular genome occurs, probablt via the repeated srquences in the end(h). --- Three different kinds of cytoplasmic RNA have been described as products of viral TNA sythesis of ASV. The largest form of RNA contains 9500 nucleotides and codes for all gene products and therefore most likely is identical with virion-RNA. This mRNA directs the sythesis of a polyprotein which after cleavage yields the different gag products. It is also possible that the polymerase is formed from the same mRNA. The second mRNA is 5400 nucleotides long and it codes for env and, in ASV, also for the src and c sequences. However, in ASV only yhe env part of mRNA can be translated, which leads to the appearance of a glycoprotein with a molecular weight of 79 000- 90 000. The smallest form of mRNA contains 3500 nucleotides and codes for src. The leukaemia viruses which lack the src gene obviously cannot generate this mRNA. The product of the src-mRNA is a polypeptide with a molecular weight of 60 000 and it carries protein kinase activity. All three forms of mRNA have a non-translated leader sequence of about 100 nucleotides. This sequence has been added by use of a splicing process. Similar to the situation with the eukaryotic mRNA, only one protein is directed by each mRNA molecule. ---