Not so selfish after all: Viruses weaponize selfish genetic elements known as “mobile introns” to disrupt the ability of competing phages to reproduce (Jul 2024) An intron endonuclease facilitates interference competition between coinfecting viruses Phages

[Michael Harrop]

https://www.asbmb.org/asbmb-today/science/092824/viruses-use-freeloading-genes-as-weapons
https://www.science.org/doi/10.1126/science.adl1356

Publishing in the journal Science, researchers in the School of Biological Sciences studied selfish genetic elements in bacteriophages (phages), viruses that are considered the most abundant organisms on Earth. To their surprise, researchers found that selfish genetic elements known as “mobile introns” provide their virus hosts with a clear advantage when competing with other viruses: phages have weaponized mobile introns to disrupt the ability of competing phage viruses to reproduce.

“This is the first time a selfish genetic element has been demonstrated to confer a competitive advantage to the host organism it has invaded,” said study co-first author Erica Birkholz, a postdoctoral scholar in the Department of Molecular Biology. “Understanding that selfish genetic elements are not always purely ‘selfish’ has wide implications for better understanding the evolution of genomes in all kingdoms of life.”

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Editor’s summary​

Viruses frequently harbor mobile introns containing homing endonucleases, but their function or selective advantage is in doubt. Birkholz et al. have discovered that a mobile intron containing a homing endonuclease found within a viral RNA polymerase gene in one bacteriophage blocks productive infection by a related and competing coinfecting bacteriophage. The homing endonuclease of one phage aims for the homologous sequence in the polymerase of the competitor phage to block assembly of the rival’s progeny. Competition experiments confirmed that even small selective advantages such as this are quickly amplified because of the rapid rates of viral replication. Similar mobile introns are ubiquitous across phage, fungi, and archaea, and may provide selective advantages in a range of interactions. —Caroline Ash

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Abstract​

Introns containing homing endonucleases are widespread in nature and have long been assumed to be selfish elements that provide no benefit to the host organism. These genetic elements are common in viruses, but whether they confer a selective advantage is unclear.

In this work, we studied intron-encoded homing endonuclease gp210 in bacteriophage ΦPA3 and found that it contributes to viral competition by interfering with the replication of a coinfecting phage, ΦKZ. We show that gp210 targets a specific sequence in ΦKZ, which prevents the assembly of progeny viruses.

This work demonstrates how a homing endonuclease can be deployed in interference competition among viruses and provide a relative fitness advantage. Given the ubiquity of homing endonucleases, this selective advantage likely has widespread evolutionary implications in diverse plasmid and viral competition as well as virus-host interactions.

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