Incessant encounters of all cellular life forms with mobile genetic elements (MGEs) have driven the evolution of diverse defence mechanisms, including CRISPR–Cas and restriction–modification systems. In this Perspective, Koonin, Makarova, Wolf and Krupovic describe the surprisingly intricate interplay between MGEs and host defence systems. Not only do defence systems commonly show high horizontal mobility but many molecular components are ‘guns for hire’ that have been co-opted by defence systems from MGEs and vice versa.
- Eugene V. Koonin
- Kira S. Makarova
- Mart Krupovic