Mechanism of foreign DNA recognition by a CRISPR RNA-guided surveillance complex from Pseudomonas aeruginosa.

The Type I-F CRISPR-mediated (clustered regularly interspaced short palindromic repeats) adaptive immune system in Pseudomonas aeruginosa consists of two CRISPR loci and six CRISPR-associated (cas) genes. Foreign DNA surveillance is performed by a complex of Cas proteins (Csy1­4) that assemble with a CRISPR RNA (crRNA) into a 350-kDa ribonucleoprotein called the Csy complex. Here, we show that foreign nucleic acid recognition by the Csy complex proceeds through sequential steps, initiated by detection of two consecutive guanine­cytosine base pairs (G­C/G­C) located adjacent to the complementary DNA target. We show that this motif, called the PAM (protospacer adjacent motif), must be double-stranded and that single-stranded PAMs do not provide significant discriminating power. Binding assays performed with G­C/G­C-rich competitor sequences indicate that the Csy complex interacts directly with this dinucleotide motif, and kinetic analyses reveal that recognition of a G­C/G­C motif is a prerequisite for crRNA-guided binding to a target sequence. Together, these data indicate that the Csy complex first interacts with G­C/G­C base pairs and then samples adjacent target sequences for complementarity to the crRNA guide.