Figure 3.
crRNA processing pathways in Type II CRISPR-Cas systems. In Type II systems, the precursor transcript of the CRISPR repeat-spacer array forms duplexes with the trans-activating tracrRNA through pre-crRNA repeat:tracrRNA anti-repeat interactions. The duplex RNAs stabilized by the protein Cas9 are recognized and cleaved by the bacterial endoribonuclease III (RNase III). A second processing by unknown nucleases (trimming by an exonuclease and/or cleavage by an endoribonuclease) generates the mature crRNAs. An alternative pathway for the production of mature crRNAs was described in a Type II-C of N. meningitidis. Here, the transcription of short crRNAs occurs directly from promoters contained within the repeats of the array, and thus independently of cleavage by RNase III. The mature dual tracrRNA:crRNAs complexed with the protein Cas9 form the interference complex that target and cleave site specifically double-stranded DNA.