Casposons: mobile genetic elements that gave rise to the CRISPR-Cas adaptation machinery

- Casposons are self-replicating mobile genetic elements found in bacteria and archaea.
- Casposase is an active casposon integrase homologous to CRISPR-Cas1.
- Casposons gave rise to the adaptation machinery of CRISPR-Cas.
- CRISPR repeats and the Leader sequence have evolved from the casposon target site.
- Recruitment of Cas2 was crucial for transforming the casposase into Cas1.

A casposon, a member of a distinct superfamily of archaeal and bacterial self-synthesizing transposons that employ a recombinase (casposase) homologous to the Cas1 endonuclease, appears to have given rise to the adaptation module of CRISPR-Cas systems as well as the CRISPR repeats themselves. Comparison of the mechanistic features of the reactions catalyzed by the casposase and the Cas1-Cas2 heterohexamer, the CRISPR integrase, reveals close similarity but also important differences that explain the requirement of Cas2 for integration of short DNA fragments, the CRISPR spacers.

144