precrRNA

PrecrRNA, or precursor CRISPR RNA, is the initial transcript produced from a CRISPR locus in bacteria and archaea. A CRISPR array consists of short repeats interspersed with spacers derived from prior invaders. The precrRNA is transcribed as a single long RNA and is subsequently processed into mature crRNA molecules that each contain a single spacer derived from invader DNA, flanked by repeat-derived sequences.

Processing and maturation of precrRNA vary by CRISPR-Cas system. In many archaeal and bacterial systems (types I and III), endoribonucleases such as Cas6 cleave precrRNA within the repeats to release individual crRNA units with a characteristic repeat-derived handle. In type II systems, maturation involves a different pathway in which the pre-crRNA forms a duplex with tracrRNA and is processed by RNase III in the presence of the Cas9 nuclease, yielding a mature crRNA that associates with tracrRNA to form an active guide complex. Other organisms employ alternative nucleases or processing steps. Across systems, mature crRNA generally comprises a spacer sequence that matches a fragment of invading DNA or RNA and a portion of the repeat that assists stable association with the effector complex.

Function and role: The mature crRNA serves as the guide for the CRISPR-Cas immune machinery. It base-pairs with complementary sequences in invading nucleic acids, directing the associated Cas effector protein to cleave the target. Target recognition often depends on adjacent motifs such as PAMs in DNA-targeting systems, shaping the specificity of interference.

Relevance: The precrRNA to crRNA maturation step is essential for CRISPR-Cas–mediated immunity and underpins many genome-editing technologies, where the crRNA guide is combined with tracrRNA or engineered into a single-guide RNA for programmable targeting.