The unique retrovirus

Certainly all of you know the retroviruses, since it is the family to which the human acquired immunodeficiency syndrome virus or AIDS virus belongs. Retroviruses form an extensive and diverse family

with unique features: they are the only viruses that have diploid genome, i.e., two identical molecules of RNA. The viral particle contains single-stranded RNA, but when it infects the cell, the RNA is transformed into double-stranded DNA. To convert RNA into DNA, they have a special enzyme, called reverse transcriptase, which gives the name to the family. They produce a large variety of diseases, many of them slow and progressive. Also retroviruses are a major cause of tumours in animals. Once we know why they are chronic diseases, we will study some of the diseases that they produce. Retroviruses are enveloped viruses, and therefore, sensitive to many physical, chemical and environmental agents. In the envelope, peplomers or molecules protrude, which are recognized by cellular receptors.

More to the inside, there is an icosahedral capsid, shaped like a truncated cone. Further inside, two nucleocapsids cover the viral RNA, accompanied by the necessary enzymes for replication, mainly the reverse transcriptase. Between the capsid and the envelope, there are filling proteins that constitute the matrix.

The genome of the retrovirus has three main genes: gag, which encodes the internal protein; pol, that encodes the enzymes such as reverse transcriptase, and env, which encodes the envelope proteins. The immense majority of virus need the cell to be active and multiplying, so that the enzymes and components that form new nucleic acids and proteins are available. But some retroviruses, the lentivirus, possess regulatory genes, represented here in red, that allow for replication, even when the cell is resting, which undoubtedly has many advantages. In addition, other retrovirus possess a gene that turns the infected cell into a tumour, and that is called an oncogene. Therefore, retroviruses have several genomic characteristics which make them unique.

To understand how retrovirus produce slow and progressive diseases it is necessary to understand, basically , their replication cycle. Cell receptors recognize certain sequences of the envelope proteins. After this, the plasma membrane fuses with the viral envelope, the capsid releases the RNA into the cytoplasm, which is always accompanied by the reverse transcriptase, represented by a star. Reverse transcriptase is the real protagonist of the cycle of retroviruses. Using one of the molecules of RNA as a template, it synthesises a complementary molecule, but not of RNA but of DNA. The reverse transcriptase possesses RNase activity, so that it degrades the RNA molecule as it forms that of DNA. Later, it synthesizes a complementary strand of DNA using the previous DNA as a template.

After this process, the result is a double-stranded DNA molecule, identical to that which forms the chromosomes of the cell that they infect. The molecule of viral double-stranded DNA, which now is called a provirus, crosses the nuclear membrane, and is inserted within the DNA of a chromosome in a process called integration. This way it can spend a lot of time, behaving as a regular gene, passing to the progeny cells when it divides. At a certain time, due to little known circumstances, it uses the cell mechanisms to form molecules of complementary RNA that travel to the cytoplasm to be translated into the proteins of the envelope, the capsid, and the others that we have spoken about.

The nucleocapsids form with more newly synthesized RNA molecules, assembling like a puzzle. When everything is assembled, the particles exit the cell by budding, dragging along part of the plasma membrane with them.

So we have seen what are the features that make retroviruses unique. They are related to their replication cycle, which we will be focusing on in the next step.