Provirus

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A provirus is a virus genome that has become part of a host cell’s DNA. In bacteria this is often called a prophage, but proviruses and prophages are different and should not be used interchangeably. Unlike prophages, proviruses do not cut themselves out of the host DNA when the cell is stressed.

A provirus can be in two states: latent or productive. In a latent infection, the provirus is quiet and does not make new viruses; it is copied every time the host cell copies its DNA and passed on to future cell generations. In a productive infection, the provirus is active and uses the cell’s machinery to produce viral RNA and new viruses, which can then go on to infect other cells.

Endogenous retroviruses are proviruses that are always in the provirus state and inherited in the genome. Nonendogenous retroviruses insert their DNA into the host genome by reverse transcription and integration; the provirus itself does not copy DNA by itself and is instead replicated along with the host genome.

A provirus can become active if conditions in the host change, potentially destroying the host cell as it makes more viruses. Proviruses make up a significant part of our genome—for example, about 8% of the human genome consists of inherited endogenous retroviruses.

The term provirus is not limited to retroviruses; it also describes other viruses that integrate into host chromosomes, such as adeno-associated viruses. Not only eukaryotic viruses do this; many bacterial and archaeal viruses also use this strategy. Among viruses with circular DNA genomes or those that replicate through a circular intermediate, many have temperate members that involve proviral integration.