An anti-CRISPR uses guide RNA to block Cas9 and helps design better gene editors

The structure of the anti-Cas9 protein AcrIIA27 bound to SpyCas9-sgRNA reveals critical insights into its inhibitory mechanism.

• AcrIIA27 binds to the phosphate backbone of sgRNA, inhibiting various Cas9 orthologs.
• The positioning of AcrIIA27 near the DNA-binding pocket of SpyCas9 prevents substrate DNA recognition.
• Solvent-exposed regions of sgRNAs may lead to nonspecific binding, potentially reducing genome-editing efficiency.
• Truncating the solvent-exposed regions of sgRNAs significantly improves genome-editing efficiency in human cells.
• The findings describe a novel inhibition mechanism of an anti-Cas protein that could inform the design of better genome-editing tools.

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