CODEX Entry 6061: Retroviruses
Many different amino acids and other biomolecules may have arrived on Earth from space as evidenced by the Murchison Meteorite. The last universal common ancestor (LUCA) is thought to be the ancient organism that preceded all cellular life on Earth. However, LUCA was not the beginning of life, since it was already complex and capable of protein synthesis and DNA replication. Simpler replicating elements that preceded LUCA may have originated from RNA, or self replicating ribozymes, viroids or viruses. RNA has been shown to be able to evolve from an enzymatically inactive to active form. Large pools of RNA molecules give rise to species with various three-dimensional conformations. Ribozymes can cleave, join and replicate other ribozymes. Protein synthesis is achieved by ribosomes.
A retrovirus is a type of virus that inserts a copy of its RNA genome into the DNA of a host cell that it invades, changing the genome of that cell. Once inside the host cell’s cytoplasm, the virus uses its own reverse transcriptase enzyme to produce DNA, the reverse of the usual pattern. The new DNA is then incorporated into the host cell genome by an integrase enzyme. The host cell then treats the viral DNA as part of its own genome, transcribing the viral genes along with the cell’s own genes, producing the proteins required to assemble new copies of the virus. This is the technique that Moderna and Pfizer are using to create antibodies in their covid-19 vaccine. Remnants of the RNA world still exert key functions in the present DNA-protein world. Viroids have been shown to be able to increase in complexity by acquiring a coding capacity. Positively charged proteins help to disentangle RNA structures, which allows for faster replication rates, and protects the RNA from degradation.
Ancient giant viruses, such as mimiviruses, possess the genes required for autonomous replication, and are as complex as some living species. The giant viruses confirm that viruses can acquire genetic information via horizontal gene transfer, to become complex cell-like entities. Unlike present viruses which are parasites that require cells to replicate, the ancestors of present-day viruses were free-living retroviruses. Even some of today’s viruses, such as poliovirus, can replicate outside of cells, in a rich soupy environment.
50% of the human genome consists of retrovirus-like elements. Animals that survive infections incorporate retroviruses, which then protects against superinfection. Viruses supply new genes, and are key drivers of evolution. Bats carry SARS coronavirus without signs of the disease due to this effect.
As with all other cellular life forms, archaea also harbor viruses. Archaea can use various energy sources, including sugars, ammonia, hydrogen gas, and sunlight. They populate almost all habitats on Earth, and are part of the microbiota of humans and other species. They are particularly abundant in the oceans, and as part of plankton, may be the most abundant group of organisms on our planet. Archaea may be evolutionarily older than bacteria. The archaeal viruses are unique, in that most of the encoded proteins do not share significant sequence similarities with known proteins, and more than a quarter of the proteins whose structure has been determined display unique folds and other original structural material. They are relatively small compared to viruses and cells yet remain versatile, mutabile and capable of autonomous replication, in the absence of the genetic code and proteins.