Episome types and their characteristics

4151
David Holt
Episome types and their characteristics

A episome, In the field of genetics, it is a DNA molecule that is capable of replicating autonomously in the cytoplasm of the host cell, and that, physically integrated into the host cell's chromosome, is also replicated as a single molecule (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 as 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.

Article index

  • 1 Episomes that are bacteriophages
  • 2 Episomes that are plasmids
  • 3 Episomes in eukaryotic cells
  • 4 Conclusions
  • 5 References

Episomes that are bacteriophages

One of the classic examples of episomes is that of the bacteriophage lambda in its host bacteria, the best known of which is Escherichia coli. A bacteriophage (phage for short) is a virus that infects bacteria.

Under conditions conducive to infection of the bacterium by phage, the viral genome introduced into the cytoplasm as a linear molecule can be circularized, and by site-specific recombination events, integrate into the chromosome of the host bacterium..

Within the phage genome there is a short sequence of nucleotides (attλ) that is perfectly complementary to a binding site (attachment) on the circular chromosome of the bacterium (attB).

The recombination event between these two sites leads to the formation of a cointegrate between two circles, giving rise to a larger circle. As the bacterium's chromosome replicates, therefore, the virus genome replicates (in the episome state).

This can occur for infinite generations - unless an inductive event leads to the cleavage of the viral genome, and the subsequent entry into the autonomous replicative cycle of the virus that culminates in the lysis of the bacteria to release the newly generated virions..

Episomes that are plasmids

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 bacteria 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 has 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 to verify the site-specific recombination process that gives rise to the cointegrate, 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, its use 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, 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 fulfill 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 a virus that can integrate into the genome of prokaryotes, or be one of the types of plasmids that a eukaryotic cell can host..

Interestingly, viruses that can insert into the genome of the eukaryotic host (retrovirus) are not considered episomes..

References

  1. Brock, T. D. 1990. The Emergence of Bacterial Genetics. Cold Spring Harbor Laboratory Press. Cold Spring Harbor, MA, United States of America.
  2. Griffiths, A. J. F., Wessler, S. R., Carroll, S. B. & Doebley, J. Introduction to Genetic Analysis. W. H. Freeman & Co, McMillan Publishers. London, United Kingdom.
  3. Hayes, W. 1971. The Genetics of Bacteria and their Viruses, Second Edition. Blackwell Scientific Publications.
  4. Jacob, F. & Wollman, E. L. 1958. Les episomes, elements génétiques ajoutés. Comptes Rendus de l'Académie des Sciences de Paris, 247 (1): 154-156.
  5. Levy, J. A., Fraenkel-Conrat, H. & Owens, O. S. 1994. Virology, 3rd Edition. Prentice Hall. Englerwood Cliffs, NJ, USA.

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