In contrast to other types of CRISPR systems, Cas13 cleaves invading RNAs, in both sequence specific and independent manners. Luciano Marraffini and co-authors use phage libraries to decipher how these activities elicit defence mechanisms in bacteria. Only transcribed targets are cleaved, and they reduce infection independent of their function. Cas13 degradation activity is shown to induce growth arrest in infected cells, which prevents infection to progress and allow uninfected cells to grow. The results show that induction of host dormancy can provide herd immunity against phage infection in bacteria. By acting on the host, Cas13 can elicit non-specific, broad immunity against escaper phages and other viruses. The findings highlight a novel, altruistic, population-level CRISPR-based defence mechanism.
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