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

## TL;DR

This paper shows how certain viruses evade bacterial immune systems by reducing specific DNA sequences.

## Contribution

The study reveals that Pseudomonas aeruginosa-targeting phages avoid CRISPR-Cas I-F by depleting PAM sequences.

## Key findings

- Targeted phages have lower PAM sequence frequency than non-targeted ones.
- Targeted phages have lower G+C content in their genomes.
- This suggests evolutionary pressure to escape CRISPR-Cas I-F immunity.

## Abstract

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 Pseudomonas aeruginosa 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.