Researchers from the Univ. of California, Berkeley, and the Lawrence Berkeley National Laboratory have discovered an intermediate checkpoint in the CRISPR-Cas9 system that may help them develop strategies for more selective control of gene editing.
Over millennia, bacteria have evolved multilayered defense systems to combat viruses. Clustered regularly interspaced short palindromic repeats (CRISPR) are segments of DNA that contain repetitive base sequences, which play a key role in CRISPR-Cas antiviral defense systems. While immunity is the natural function of CRISPR-Cas systems, researchers discovered that these complexes can be engineered to recognize, and edit, virtually any DNA sequence. Although there are many different types of CRISPR-Cas systems, research has largely focused on one specific protein, Cas9, for genome-editing applications. Cas9 is an RNA-guided endonuclease enzyme associated with the CRISPR-Cas adaptive immune system of the bacteria Streptococcus pyogenes.
To be harnessed for genome engineering, Cas9 must be complexed with programmed single-guide RNA (sgRNA). In DNA editing, Cas9 identifies a short conserved sequence — called the protospacer adjacent motif (PAM) — and checks whether the target strand and the sgRNA are complementary. If the...
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