CRISPR-Cas systems provide sequence-specific adaptive immunity against foreign nucleic acids1,2. They are present in approximately half of all sequenced prokaryotes3 and are expected to constitute a major barrier to horizontal gene transfer. We previously described nine distinct families of proteins encoded in Pseudomonas phage genomes that inhibit CRISPR-Cas function4,5. We have developed a bioinformatic approach that enabled us to discover additional anti-CRISPR proteins encoded in phages and other mobile genetic elements of diverse bacterial species. We show that five previously undiscovered families of anti-CRISPRs inhibit the type I-F CRISPR-Cas systems of both Pseudomonas aeruginosa and Pectobacterium atrosepticum, and a dual specificity anti-CRISPR inactivates both type I-F and I-E CRISPR-Cas systems. Mirroring the distribution of the CRISPR-Cas systems they inactivate, these anti-CRISPRs were found in species distributed broadly across the phylum Proteobacteria. Importantly, anti-CRISPRs originating from species with divergent type I-F CRISPR-Cas systems were able to inhibit the two systems we tested, highlighting their broad specificity. These results suggest that all type I-F CRISPR-Cas systems are vulnerable to inhibition by anti-CRISPRs. Given the widespread occurrence and promiscuous activity of the anti-CRISPRs described here, we propose that anti-CRISPRs play an influential role in facilitating the movement of DNA between prokaryotes by breaching the barrier imposed by CRISPR-Cas systems.
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This work was supported by funding from the Canadian Institutes of Health Research to A.R.D. (MOP-130482) and K.L.M. (MOP-136845). A.P. was supported by an Ontario Graduate Scholarship and a CIHR Canada Graduate Scholarship Doctoral Award. R.H.J.S. was funded by a University of Otago Division of Health Sciences Career Development Post-doctoral Fellowship. P.C.F was supported by a Rutherford Discovery Fellowship from the Royal Society of New Zealand. B.N.J.W. was supported by a University of Otago Doctoral Scholarship.
The authors declare no competing financial interests.
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Pawluk, A., Staals, R., Taylor, C. et al. Inactivation of CRISPR-Cas systems by anti-CRISPR proteins in diverse bacterial species. Nat Microbiol 1, 16085 (2016). https://doi.org/10.1038/nmicrobiol.2016.85
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