Bacterial adaptive immunity uses CRISPR (clustered regularly interspaced short
palindromic repeats)-associated (Cas) proteins together with CRISPR transcripts
for foreign DNA degradation. In type II CRISPR-Cas systems, activation of Cas9
endonuclease for DNA recognition upon guide RNA binding occurs by an unknown
mechanism. Crystal structures of Cas9 bound to single-guide RNA reveal a
conformation distinct from both the apo and DNA-bound states, in which the
10-nucleotide RNA "seed" sequence required for initial DNA
interrogation is preordered in an A-form conformation. This segment of the guide
RNA is essential for Cas9 to form a DNA recognition-competent structure that is
poised to engage double-stranded DNA target sequences. We construe this as
convergent evolution of a "seed" mechanism reminiscent of that used by
Argonaute proteins during RNA interference in eukaryotes.
