Mre11-Rad50 enhances spacer acquisition in a haloarchaeal Type I-B CRISPR-Cas system

Clustered regularly interspaced short palindromic repeat (CRISPR) loci and their associated (cas) genes provide adaptive immunity to bacteria and archaea. CRISPR-Cas systems acquire short DNA fragments from the genomes of infecting plasmids and viruses, which are inserted into the CRISPR locus as a "spacer" sequence in between repeats. Spacers constitute a memory of infection that is used to recognize and attack invading genetic elements in future infections. Despite the evolutionarily divergent genetic backgrounds of bacteria and archaea, the same CRISPR-Cas systems are functional in both of these prokaryotic domains. In bacteria, efficient spacer acquisition requires the DNA repair nucleases RecBCD/AddAB. These nucleases, however, are not present in archaea. Here we investigated the importance of the DNA repair systems in the Haloferax volcanii Type I-B CRISPR-Cas response. We found that elimination of the DNA repair nuclease Mre11-Rad50, but not Fen1, substantially reduces spacer acquisition. CRISPR immunity against H. volcanii pleomorphic virus 1 (HFPV-1), on the other hand, was not affected by these deletions. Our results describe how CRISPR-Cas systems have adapted to provide anti-viral defense to hosts from different domains of life.