Figure 4.

CrRNA biogenesis in Type II CRISPR-Cas9 systems. (A) A typical Type II-A CRISPR loci express a long pre-crRNA, and a tracrRNA cofactor that hybridizes to all the pre-crRNA repeats. The host factor RNase III cleaves the pre-crRNA/tracrRNA duplexes, yielding pairs of matured crRNA and tracrRNA species. Black rectangles, repeats; colored diamonds, spacers; purple, tracrRNA locus; black arrows, transcriptions. (B) A unique crRNA biogenesis pathway in meningoccus. Transcriptions from repeat-embedded internal promoters result in nested set of pre-crRNA transcripts of varying lengths. RNase III- and tracrRNA-mediated RNA processing (in red box) occurs in cells but is dispensable for the interference function.