An episome, in the field of genetics, is a DNA molecule that is capable of replicating autonomously in the cytoplasm of the host cell, and that physically integrated to the host cell's chromosome, also replicates as a single molecule (at the which we call cointegrated).
The episome, therefore, can be interpreted as a form of coexistence, and not as a type of replicon. In fact, for some authors, transposons and insertion sequences can be considered episomes, since they are effectively being carried on the host cell's chromosome, although they never have an independent and autonomous existence in the cytoplasm.
In eukaryotic cells, by contrast, episome refers more to viral replicons that coexist as plasmids in infected cells than to viruses that can integrate into the genome of the host cell.
This is not the only instance in which the same word means different things in eukaryotes and prokaryotes (for example, the term transformation). Episomes have a rich history in the development of modern genetics, as they helped unravel interesting phenomena related to heredity.
Episomes that are bacteriophages
Another of the best-known examples of episomes is that of the fertility factor, or plasmid F. Sometimes, depending on the nucleotide constitution of the host bacterium (eg, E. coli), the circular plasmid recombines with homologous sites present on the chromosome. of the bacterium giving rise to a cointegrate.
That is, the plasmid can replicate in low copy number in the cytoplasm of the bacterium, or if it is integrated, replicate as a whole in a copy number corresponding to that of the bacterium without F (generally one).
In its episome state, F gives the bacteria the ability to produce a high number of recombinants after the conjugation process.
An F + bacterium (that is, that possesses an autonomous F plasmid) that undergoes the insertion of this element is said to be Hfr (for high frequency of recombination, for its acronym in English), since by a conjugation event, it is theoretically capable of "dragging" the entire bacterial chromosome into an F- bacterium (that is, lacking the fertility factor, or plasmid F).
In general, the sequences that provide the homology (and therefore, similarity and complementarity) between the F plasmid and the bacterial chromosome so that the site-specific recombination process that gives rise to the cointegrate is verified, are insertion sequences.
Episomes in eukaryotic cells
For historical reasons, the term episome (above + body) was always linked to that of plasmid, which originally derives from the world of extrachromosomal elements in prokaryotes.
When finding similar elements in eukaryotes, the use of the same was adopted to designate molecules of virus genomes capable of self-replicating in this type of infected cells with properties that were similar to those of plasmids in prokaryotes.
That is to say, in eukaryotic cells infected with viruses, we can find in some cases that, as part of its replicative cycle, the virus coexists in the cell as a circular DNA molecule similar to these other replicons described in, for example, bacteria.
The most commonly known viruses that can coexist as autonomously replicating circular DNA molecules (from the host chromosome) belong to the Herpesviridae, Adenoviridae, and Polyomaviridae families.
None of them, however, are integrated into the host genome, which is why it can be considered that they replicate as plasmids and that they do not meet the intrinsic quality that characterizes an episome: integrate into the host genome.
Although the removal of the term has been proposed, perhaps that will only add confusion to a topic that is already quite complex in its own right.
Conclusions
In summary, we can say that an episome, etymologically speaking, is a genetic element of autonomous replication that can coexist in the cell as a free DNA molecule, or physically integrated into that of the host.
From the point of view of genetics, however, an episome is a plasmid or virus that can integrate into the prokaryotic genome, or be one of the types of plasmids that a eukaryotic cell can harbor.
Interestingly, viruses that can insert into the genome of the eukaryotic host (retrovirus) are not considered episomes.
References
- Brock, TD 1990. The Emergence of Bacterial Genetics. Cold Spring Harbor Laboratory Press. Cold Spring Harbor, MA, United States of America.
- Griffiths, AJF, Wessler, SR, Carroll, SB & Doebley, J. Introduction to Genetic Analysis. WH Freeman & Co, McMillan Publishers. London, United Kingdom.
- Hayes, W. 1971. The Genetics of Bacteria and their Viruses, Second Edition. Blackwell Scientific Publications.
- Jacob, F. & Wollman, EL 1958. Les episomes, elements génétiques ajoutés. Comptes Rendus de l'Académie des Sciences de Paris, 247 (1): 154–156.
- Levy, JA, Fraenkel-Conrat, H. & Owens, OS 1994. Virology, 3rd Edition. Prentice Hall. Englerwood Cliffs, NJ, United States of America.