Puzzle of Bacteria’s CRISPR RNA-Guided Cas9-Based Destruction of Foreign DNA Solved

A central question has been answered regarding a protein that plays an essential role in the bacterial immune system and is fast becoming a valuable tool for genetic engineering. A team of researchers with the Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley has determined how the bacterial enzyme known as Cas9, guided by RNA, is able to identify and degrade foreign DNA during viral infections, as well as induce site-specific genetic changes in animal and plant cells. Through a combination of single-molecule imaging and bulk biochemical experiments, the research team has shown that the genome-editing ability of Cas9 is made possible by the presence of short DNA sequences known as “PAM,” for protospacer adjacent motif. “Our results reveal two major functions of the PAM that explain why it is so critical to the ability of Cas9 to target and cleave DNA sequences matching the guide RNA,” says Dr. Jennifer Doudna, the biochemist who led this study. “The presence of the PAM adjacent to target sites in foreign DNA and its absence from those targets in the host genome enables Cas9 to precisely discriminate between non-self DNA that must be degraded and self DNA that may be almost identical. The presence of the PAM is also required to activate the Cas9 enzyme.” With genetically engineered microorganisms, such as bacteria and fungi, playing an increasing role in the green chemistry production of valuable chemical products including therapeutic drugs, advanced biofuels, and biodegradable plastics from renewables, Cas9 is emerging as an important genome-editing tool for practitioners of synthetic biology.
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