Using Cryoem to Understand How Phages Evade Bacterial CRISPR Defense System


Saikat Chowdhury, MaryClare F. Rollins, Joshua Carter, Ryan Jackson, Lyn'Al Nosaka, Blake Wiedenheft, Gabriel C. Lander


Biophysical Journal


Bacteria and archaea have evolved different CRISPR based adaptive immune system for defense against phage infections. Pseudomonas aeruginosa has type I-F CRISPR-cas defense system, where the cas-crRNA surveillance complex (csy-complex) binds to foreign DNA and recruits the cas-2/3 nuclease for degradation of the foreign DNA. Bacteriophages have evolved mechanisms of evading this CRISPR defense system by encoding different anti-CRISPR (ACR) proteins. ACR1F and ACR2F are two such ACRs that bind to specific regions of the csy-complex and suppress the immune system. We present here a 3.4Å cryo-EM reconstruction of the ACR-bound csy-complex. This structure not only shows the overall organization of the type I-F csy-complex, but also reveals how ACRs interact with this complex and prevent the binding of phage DNA.



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