CRISPR and Salty: CRISPR-Cas Systems in Haloarchaea

CRISPR-Cas (CRISPR: Clustered Regularly Interspaced Short Palindromic Repeats and Cas: CRISPR associated) systems are unique defence mechanisms since they are able to adapt to new invaders and are heritable. CRISPR-Cas systems facilitate the sequence-specific elimination of invading genetic elements in prokaryotes, they are found in 45% of bacteria and 85% of archaea. Their general features have been studied in detail, but subtype- and species-specific variations await investigation. Haloarchaea is one of few archaeal classes in which CRISPR-Cas systems have been investigated in more than one genus. Here, we summarize the available information on CRISPR-Cas defence in three Haloarchaea: Haloferax volcanii, Haloferax mediterranei and Haloarcula hispanica. Haloarchaea share type I CRISPR-Cas systems, with subtype I-B being dominant. Type I-B systems rely on Cas proteins Cas5, Cas7, and Cas8b for the interference reaction and these proteins have been shown to form a Cascade (CRISPR-associated complex for antiviral defence) -like complex in Hfx (Haloferax). volcanii. Cas6b is the endonuclease for crRNA (CRISPR RNA) maturation in type I-B systems but the protein is dispensable for interference in Hfx. volcanii. Haloarchaea share a common repeat sequence and crRNA-processing pattern. A prerequisite for successful invader recognition in Hfx. volcanii is base pairing over a ten-nucleotide-long non-contiguous seed sequence. Moreover, Hfx. volcanii and Har (Haloarcula). hispanica rely each on certain specific PAM (protospacer adjacent motif) sequences to elicit interference, but they share only one PAM sequence. Primed adaptation in Har. hispanica relies on another set of PAM sequences.