Malaluan, R. P. P.; Dy, R. L. V.

CRISPR-Cas systems form the adaptive immunity of prokaryotes, conferring sequence-specific protection against genetic parasites. Here, we functionally characterized the CRISPR-Cas system of Pseudomonas aeruginosa ATCC 10145 (PA10145), which led us to discover the existence of an isolated CRISPR array, unique to this system. PA10145 possesses a type I-F CRISPR-Cas composed of a cas operon flanked by two divergently organized CRISPRs. The isolated CRISPR array, CRISPR3, is located ~1.3 million bp away from the cas loci. The cas and three CRISPR arrays are active. Plasmids with an engineered protospacer matching any of the three arrays were targeted and stimulated hyperactive adaptation in all CRISPR arrays of PA10145 if the plasmids possessed an intact protospacer adjacent motif (PAM), whereas minimal to no adaptation was observed when PAM was mutated. Spacer acquisition via interference-driven adaptation proceeds through strand-biased priming in PA10145. Interestingly, the isolated CRISPR3 and the cas-adjacent CRISPR2 have nearly identical leader sequences with only 3 bp mismatches. From a survey of CRISPR loci in 1,198 P. aeruginosa genomes, isolated arrays only occur as type I-F with similarly matching leaders to CRISPR2. Highly-transmissible mobile genetic elements (MGEs) associate with CRISPR2 and CRISPR3, suggesting that isolated arrays might have originated from recombination events involving CRISPR2 as facilitated by these MGEs. Tracing evolutionary trajectories of the isolated CRISPR3 relative to cas-adjacent arrays revealed that CRISPR3 is laterally transferred across P. aeruginosa genomes. Taken together, these results implicate the role of horizontally-acquired isolated arrays in CRISPR-mediated pan-immunity as gateways to mobilize genetic memories.