crRNA biogenesis

Mature crRNAs are key elements in CRISPR-Cas defense against genome invaders. These short RNAs are composed of unique repeat/spacer sequences that guide the Cas protein(s) to the cognate invading nucleic acids for their destruction. The biogenesis of mature crRNAs involves highly precise processing events. Interestingly, different types of CRISPR-Cas systems have evolved distinct crRNA maturation mechanisms. The CRISPR repeat-spacer array is transcribed as a precursor CRISPR RNA molecule (pre-crRNA) that undergoes one or two maturation steps. In type I CRISPR-Cas systems, pre-crRNA is cleaved within the repeat regions by a specific Cas6-like endoribonuclease that at least in some cases is a subunit of a Cascade complex to yield the mature crRNAs. In type III systems, the standalone endoribonuclease Cas6 processes pre-crRNA by cleavage within the repeats, producing an intermediate molecule that is further trimmed to generate the mature crRNAs. Type II systems have evolved a unique crRNA biogenesis pathway, in which a trans-acting small RNA (encoded by the CRISPR-Cas locus) base pairs with each repeat sequence of the pre-crRNA to form a double-stranded RNA template that is cleaved by the housekeeping endoribonuclease III in the presence of protein Cas9 (Csn1). The generated intermediates are then subjected to further maturation by a yet to be revealed mechanism. In this chapter, we present a detailed comparative analysis of pre-crRNA recognition and cleavage mechanisms involved in crRNA biogenesis in the three types of CRISPR-Cas systems