CRISPR goes retro

Genetic invasions threaten nearly all cells and genomes. Bacteria and archaea “remember” encounters with invaders through an adaptive immunity strategy involving clustered, regularly interspaced, short palindromic repeats (CRISPRs). These repeats can store snippets of an invader's genome as “spacers” ( 1 ), which then constitute a heritable memory that can instruct an immune response against future encounters with foreign DNAs carrying those same sequences. The ability to form new memories of invasion depends on the ability of a CRISPR to incorporate new spacers. The mechanistic basis for this acquisition from DNA is increasingly understood ( 2 ) and has often been assumed to be the sole mode of CRISPR adaptation. On page 932 of this issue, Silas et al. ( 3 ) describe an intriguing twist on bacterial adaptive immunity, identifying a subset of CRISPR systems with the remarkable ability to incorporate new spacers directly from RNA.