Abstract
Clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated (Cas) proteins constitute a microbial immune system against invading genetic elements, such as plasmids and phages. Csn2 is an Nmeni subtype-specific Cas protein, and was suggested to function in the adaptation process, during which parts of foreign nucleic acids are integrated into the host microbial genome to enable immunity against future invasion. Here, we report a 2.2 Å crystal structure of Streptococcus pyogenes Csn2. The structure revealed previously unseen calcium-dependent conformational changes in its tertiary and quaternary structure. This supports the proposed double-stranded DNA-binding function of S. pyogenes Csn2.
Highlights
Clustered regularly interspaced short palindromic repeats (CRISPR) are a class of repetitive genetic elements found within many bacterial and archaeal genomes [1]
Mounting evidence indicates that CRISPR and Cas proteins represent a microbial immune system that protects against invading foreign genetic elements, such as plasmids and phages [1]
The results of the mobility shift assay indicate that S. pyogenes Csn2 has a non-specific doublestranded DNA (dsDNA)-binding function (Figure 1B)
Summary
Clustered regularly interspaced short palindromic repeats (CRISPR) are a class of repetitive genetic elements found within many bacterial and archaeal genomes [1]. The asymmetric unit of the structure contains two S. pyogenes Csn2 monomers, three calcium ions, 204 water molecules and two ethylene glycol molecules. The E. faecalis Csn2 structure revealed calcium ions at the equivalent sites [8], and the coordinating side-chains are conserved (Figure 2A).
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