General Information/The casposases

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The casposases

More recently, TE related to CRISPRs, Casposons have been identified[1] and a reassement of their ends has led to the identification of an 14-15 bp target duplication [2]. Moreover, the purified Cas1 enzyme encoded by these ancestral transposons has been demonstrated to catalyse strand transfer of a pre-cleaved transposon in vitro but does not appear to promote transposon strand cleavage in this assay[3]. Cas1 "casposases" use similar chemistry to that used by the CRISPR Cas1-Cas2 complex but with opposite substrate specificities since CRISPR Cas1-Cas2 integrates "random" sequences into a specific site in the CRISPR locus whereas casposases integrate specific site (the casposon ends) into random target sequences (Fig.24.1).

Fig.24.1. Organization of the Aciduliprofundum boonei casposon. The casposon is shown as an unfilled box. Black filled arrow-heads represent the terminal inverted repeats (TIR); green arrows, the target site duplication (TSD); the genes are as follows: type B DNA polymerase (red box); HNH endonuclease (green); transcriptional regulator with HTH domain (pink); hypothetical protein with HTH domain (orange); cas1 (dark blue) with a c-terminal HTH (light blue); N6-methyltransferase (yellow). From Hickman and Dyda (2015). Non-CRISPR-associated cas1 form two distinct clades. One contains three distinct casposon families (1, 2, and 3) which share features with eukaryotic Polinton/Maverick eukaryotic transposons labeled ‘self-synthesizing’ since they include B family DNA polymerase genes. Members of the three families differ in gene content and evolutionary provenance of the DNA polymerases (protein-primed or RNA-primed).

Bibliography

  1. Krupovic M, Makarova KS, Forterre P, Prangishvili D, Koonin EV . Casposons: a new superfamily of self-synthesizing DNA transposons at the origin of prokaryotic CRISPR-Cas immunity. - BMC Biol: 2014 May 19, 12;36 [PubMed:24884953] [DOI]
  2. Hickman AB, Dyda F . CRISPR-Cas immunity and mobile DNA: a new superfamily of DNA transposons encoding a Cas1 endonuclease. - Mob DNA: 2014, 5;23 [PubMed:25180049] [DOI]
  3. Hickman AB, Dyda F . The casposon-encoded Cas1 protein from Aciduliprofundum boonei is a DNA integrase that generates target site duplications. - Nucleic Acids Res: 2015 Dec 15, 43(22);10576-87 [PubMed:26573596] [DOI]