CASPOSONS - SILENT HEROES OF THE CRISPR-CAS SYSTEMS EVOLUTIONARY HISTORY

Many archaeal and bacterial organisms possess an adaptive immunity system known as CRISPR-Cas. Its role is to recognize and degrade foreign DNA showing high similarity to repeats within the CRISPR array. In recent years computational techniques have been used to identify cas1 genes that are not associated with CRISPR systems, named cas1-solo. Often, cas1-solo genes are present in a conserved neighborhood of PolB-like polymerase genes, which is a characteristic feature of self-synthesizing, eukaryotic transposons of the Polinton class. Nearly all cas1-polB genomic islands are flanked by terminal inverted repeats and direct repeats which correspond to target site duplications. Considering the patchy taxonomic distribution of the identified islands in archaeal and bacterial genomes, they were characterized as a new superfamily of mobile genetic elements and called casposons. Here, we review recent experiments on casposons' mobility and discuss their discovery, classification, and evolutionary relationship with the CRISPR- Cas systems.

Automated summary

Many archaeal and bacterial organisms have a type of adaptive immune system called CRISPR-Cas, which recognizes and degrades foreign DNA similar to repeats in the CRISPR array. Cas1 genes, known as cas1-solo, have been identified using computational techniques. These genes are often found near PolB-like polymerase genes and are considered a new superfamily of mobile genetic elements called casposons.