Microbiology: slicer for DNA

Abstract

Many bacteria and archaea protect themselves from viruses and other invasive genomes through a genetic interference pathway. The small RNAs that guide this defence specify the direct cleavage of foreign DNA.

Figure 1. RNA-guided DNA cleavage by the core CRISPR machinery

a, CRISPR loci include variable numbers of repeat and spacer sequences, with the latter derived from invasive DNAs previously encountered by the cell or its ancestors. The repeat/spacer region is flanked by a ‘leader’ sequence and by cas genes. b, CRISPR transcription and processing of precursor crRNA yields mature crRNAs. c, These presumably associate with an effector complex that includes protein products of the cas genes. Garneau et al. report that the CRISPR pathway in Streptococcus thermophilus induces double-strand breaks at cognate protospacers embedded within invasive DNAs, implying that the core effector complex achieves genetic interference, and so infection blockage, by crRNA-guided DNA cleavage.