FIGURE 2.

CRISPR/Cas systems as genome editing tools and regulators of gene expression. (A) General outline of a type II CRISPR/Cas system; the scaRNA component is present in certain lineages only. Pre-crRNA is processed by the combined action of Cas9 and RNase III to form mature crRNAs, each containing a repeat (R) element and a spacer (S1–S3) region. (B) Cas9 normally functions to cleave viral or plasmid DNA in the bacterial cell upon association of the mature crRNA with a complementary foreign DNA molecule. The tracrRNA and crRNA components can be replaced by a guide RNA, and the Cas9 enzyme may be mutated to achieve custom DNA target cleavage. (C) The novel scaRNA of certain type II CRISPR systems mediates Cas9 cleavage of a target mRNA transcript by associating with the mRNA at its RBS.