CRISPR-Cas orchestrates hidden anti-phage defenses into a layered bacterial immunity

A Nature study reveals that type I-C CRISPR–Cas loci embed diverse anti-phage defences and use small crRNA-like RNAs to guide the Cascade complex to repress promoters of these immune cassettes (CRISIS), maintaining their basal expression for antiviral activity while limiting fitness costs. When CRISPR–Cas is mutated or countered by anti-CRISPR proteins, these embedded systems become derepressed, boosting innate immunity at the expense of host fitness, thereby showing that CRISPR–Cas coordinates a layered, CRISPR-supervised bacterial immune network.