CRISPR/Cas9 gene editing technology has been widely used in many fields since its inception. Gene therapy developed using this technology has huge prospects in the medical and health fields. CRISPR/Cas9 technology enables gene-editing of the genome at specified sites, but a common worry about the technology is that gene-editing occurs where it shouldn't. In recent years, researchers at the University of Bath and Cardiff University in the UK have developed a new Cas9 nuclease using synthetic biology techniques. It allows the CRISPR/Cas9 gene editing system to be regulated by a low-cost, abundant, non-toxic amino acid, thereby making CRISPR/Cas9 technology safer and more controllable.

An important part of CRISPR/Cas9 technology is a nuclease called Cas9, which cuts double-stranded DNA to mediate gene editing. Ideally, Cas9 should be expressed when the gene editing task needs to be completed and inactivated after the gene editing task is completed. Scientists have made significant progress in the field of regulating Cas9 nuclease activity. Currently, the activity of Cas9 nuclease can be regulated by light, temperature, and antibiotics, but these regulation methods have different defects. For example, the use of antibiotic-regulated Cas9 nucleases often fails to completely inhibit Cas9 activity, and the use of antibiotics may affect the animal's microbiome, causing other side effects, and may lead to antibiotic resistance in bacteria.

Researchers at the Universities of Bath and Cardiff have used a technique known as codon expansion in synthetic biology to generate a new Cas9 nuclease. Proteins in nature are composed of 20 kinds of natural amino acids, and the protein synthesis mechanism of cells uses 64 kinds of gene codons to complete the protein synthesis process. The self-expanding technology of genetic code pairs specific codons with unnatural amino acids, allowing cells to add unnatural amino acids at specific sites when synthesizing proteins, thereby giving proteins new functions.

In this study, the researchers paired the "UAG" codon that originally encoded the stop signal with an unnatural amino acid called BOC, and they simultaneously introduced the UAG codon in the gene sequence encoding the Cas9 protein. This leads to the fact that the synthesis of this new Cas9 protein requires the presence of BOC amino acids in the environment. If the BOC amino acids are not present, then the Cas9 protein cannot be synthesized, and the CRISPR/Cas9 gene editing system is inactive. The addition of BOC amino acids to the environment leads to the synthesis of the Cas9 protein, which stimulates the activity of the gene editing system.

Using in vitro cell culture models and transgenic mouse models, the researchers demonstrated that this novel CRISPR/Cas9 gene editing system exhibits gene editing activity only in the presence of BOC unnatural amino acids. The advantage of this regulatory system is that it stops the production of the Cas9 protein, thereby completely inhibiting the activity of the Cas9 nuclease without the need for gene editing. Moreover, BOC amino acid is a derivative of lysine, which is non-toxic, abundant, low-cost, and does not have adverse effects on the environment.

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