Pseudomonas aeruginosa CRISPR-Cas primes a minimal proline-codon toxin to abort anti-CRISPR phages

Pseudomonas aeruginosa typically carries type I-F CRISPR-Cas systems, which are targeted by diverse (pro)phage-encoded anti-CRISPR (Acr) proteins. Here, we show that ∼26% of P. aeruginosa Cas effectors are reprogrammed by a regulatory RNA guide (CreA) to transcriptionally silence a conserved small RNA toxin (CreT) that arrests cell growth upon Cas inactivation. This toxin unprecedentedly employs two consecutive proline codons, thus termed “proline-codon toxin”. Combined phylogenetic and genetic analyses unraveled that CreTA has forced P. aeruginosa to reject prophages whose Acrs disrupt Cas-DNA binding, which is essential for CreT repression. We further show that Acr-armed lytic phages designed to overcome CRISPR adaptive immunity can be instead aborted by CreTA-mediated defense in vitro and in a mouse model. Our findings unravel intra-genomic arms races between CreTAs and anti-CRISPR phages, and underscore the necessity of rational engineering of strain-specific therapeutic phages to penetrate the layered CRISPR and TA barriers of multidrug-resistant P. aeruginosa .