Bacteriophages in Pseudomonas aeruginosa evade the CRISPR-Cas I-F system by depletion of PAM sequences

Clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated (Cas) proteins systems provide bacteria with an adaptive immune system against exogenous sequences, such as plasmids and bacteriophages (viruses of prokaryotes). To avoid autoimmunity, the recognition of a very short sequence called the protospacer adjacent motif (PAM) is essential for efficient immune response triggering. This raises the question of whether viruses targeted by certain CRISPR-Cas systems have a higher or lower frequency of their PAM sequences than non-targeted viruses. This was tested here in the opportunistic human pathogen <i>Pseudomonas aeruginosa</i> that presents five different types of CRISPR-Cas systems. The most frequent of them is the subtype I-F (present in 36% of the strains), which has the PAM 5'-CC sequence. When the viral genomes targeted by this system were analysed, their PAM frequency was found to be lower than that of non-targeted viruses. Remarkably, targeted viruses have comparatively lower G+C content. All this could be the result of selection pressure on the genome of these viruses to escape from the CRISPR-Cas I-F system.