Phage-encoded single-guide RNAs subvert CRISPR-Cas9 immunity

Central to CRISPR technologies is the single-guide RNA (sgRNA), an engineered fusion of a processed CRISPR RNA and tracrRNA that directs Cas9 and many Cas12 nucleases to bind and cleave target DNA ^1–4 . Here, we report the discovery of similarly compact viral sgRNAs (vsgRNAs) encoded by bacteriophages that counteract bacterial CRISPR-Cas9 immunity. vsgRNAs inhibit Cas9 function via two complementary routes: by sequestering the Cas9 apoenzyme, and by re-directing the Cas9 nuclease to transcriptionally silence its own promoter. vsgRNAs also can cooperate with co-encoded anti-CRISPR proteins (Acrs), including AcrIIA25.1 that blocks DNA binding by Cas9 complexed with a standard sgRNA but not with the vsgRNA. We predict that phages evolved vsgRNAs by co-opting and repurposing host-encoded long-form tracrRNAs (tracr-L) responsible for Cas9 auto-repression and a countermeasure to Acrs ^5,6 . Our search also uncovered Cas9-regulating small CRISPR-associated RNAs (scaRNAs), which we predict were inserted upstream of tracrRNAs to form tracr-L but were also co-opted by phages as viral scaRNAs to suppress Cas9 immunity. Finally, we found that vsgRNAs can enable genome editing in mammalian cells, offering a natural guide RNA template for CRISPR technologies. Overall, these findings reveal that bacteriophages devised their own compact sgRNAs tailored to subvert Cas9 immunity, long preceding their rational design to program RNA-guided nucleases ² .