A Programmable Dual-RNA-Guided DNA Endonuclease in Adaptive Bacterial Immunity

How do Type II CRISPR/Cas systems, which rely on the Cas9 protein, achieve sequence-specific silencing of invading DNA? The researchers sought to determine the mechanism by which Cas9 recognizes and cleaves target DNA and whether this system could be simplified for use as a programmable genome-editing tool.

Using the Cas9 protein from Streptococcus pyogenes, the authors performed in vitro biochemical assays to test its endonuclease activity against plasmid and linear DNA substrates. They investigated the roles of the CRISPR RNA (crRNA) and the trans-activating crRNA (tracrRNA) in target recognition and cleavage. By creating point mutations in the Cas9 protein's predicted nuclease domains (HNH and RuvC-like) and testing various truncated RNA constructs, they mapped the functional requirements for DNA cleavage. Finally, they engineered a single chimeric RNA that fused the crRNA and tracrRNA to test if the system could be simplified into a single-guide format.

• Dual-RNA Requirement: Cas9 is not an autonomous nuclease; it requires a complex of mature crRNA and tracrRNA to bind and cleave target DNA. The tracrRNA is essential for both target recognition and activating the Cas9 enzyme.
• Mechanism of Cleavage: Cas9 utilizes two distinct nuclease domains to perform double-stranded breaks. The HNH domain cleaves the DNA strand complementary to the crRNA, while the RuvC-like domain cleaves the non-complementary strand.
• PAM Dependency: Efficient cleavage requires a Protospacer Adjacent Motif (PAM)—specifically a GG dinucleotide—located immediately downstream of the target sequence. This motif is critical for licensing the Cas9 complex to unwind the DNA and form an R-loop.
• Programmability: The researchers successfully demonstrated that the dual-RNA system can be replaced by a single chimeric RNA molecule. This engineered "single-guide" RNA retains the ability to direct Cas9 to specific DNA sequences, providing a robust and versatile platform for targeted genome editing.