This group of families is one of several protein families that are always found associated with prokaryotic CRISPRs, themselves a family of clustered regularly interspaced short palindromic repeats, DNA repeats found in nearly half of all bacterial and archaeal genomes. These DNA repeat regions have a remarkably regular structure: unique sequences of constant size, called spacers, sit between each pair of repeats [1]. It has been shown that the CRISPRs are virus-derived sequences acquired by the host to enable them to resist viral infection. The Cas proteins from the host use the CRISPRs to mediate an antiviral response. After transcription of the CRISPR, a complex of Cas proteins termed Cascade cleaves a CRISPR RNA precursor in each repeat and retains the cleavage products containing the virus-derived sequence. Assisted by the helicase Cas3, these mature CRISPR RNAs then serve as small guide RNAs that enable Cascade to interfere with virus proliferation [2]. Cas5 contains an endonuclease motif, whose inactivation leads to loss of resistance, even in the presence of phage-derived spacers [3]. In Type I and III CRISPR/Cas systems, the Cas6 family of endoribonucleases generates functional CRISPR-derived RNAs by site-specific cleavage of repeat sequences in precursor transcripts. The structure of Cas6 [4] showed it adopts a tandem ferredoxin/RRM fold. This entry represents the C-terminal domain.

1: A guild of 45 CRISPR-associated (Cas) protein families and multiple CRISPR/Cas subtypes exist in prokaryotic genomes. Haft DH, Selengut J, Mongodin EF, Nelson KE; PLoS Comput Biol. 2005;1:e60. PMID:16292354

2: Small CRISPR RNAs guide antiviral defense in prokaryotes. Brouns SJ, Jore MM, Lundgren M, Westra ER, Slijkhuis RJ, Snijders AP, Dickman MJ, Makarova KS, Koonin EV, van der Oost J; Science. 2008;321:960-964. PMID:18703739

3: CRISPR--a widespread system that provides acquired resistance against phages in bacteria and archaea. Sorek R, Kunin V, Hugenholtz P; Nat Rev Microbiol. 2008;6:181-186. PMID:18157154

4: Evolution of CRISPR RNA recognition and processing by Cas6 endonucleases. Niewoehner O, Jinek M, Doudna JA; Nucleic Acids Res. 2014;42:1341-1353. PMID:24150936