Dangerous weapons: a cautionary tale of CRISPR defence

The Cas/CRISPR system of prokaryotes is a recently recognized defence system that protects cells from invasion by foreign nucleic acids, such as plasmids or bacteriophages. Using the archaeon Sulfolobus, Gud-bergsdottir et al. have demonstrated that when foreign DNA is both a target of the CRISPR system and also carries a gene that is essential for cell growth, surviving cells display mutations in the CRISPR system that eliminate the target directed spacer. This simulates natural scenarios where the invading DNA cannot be readily destroyed, such as a temperate bacteriophage that manages to evade CRISPR attack and integrate into the genome, so becoming 'self'. CRISPR deletions ranged from precise removal of a single spacer up to elimination of an entire CRISPR array but the mechanism involved is not understood. Another surprising feature was that from one to two base mutations in the same target protospacer did not affect recognition by the Sulfolobus CRISPR system, something unexpected from studies in bacteria where spacer and target protospacer must match exactly. This study shows that even though the CRISPR system is a powerful weapon it can also be a dangerous one for the cell that carries it.