Identification of spacer and protospacer sequence requirements in the Vibrio cholerae Type I-E CRISPR/Cas System

Abstract

The prokaryotic adaptive immune system CRISPR/Cas serves as defense against bacteriophage and invasive nucleic acid. A Type I-E CRISPR/Cas system has been detected in classical biotype isolates of Vibrio cholerae, the causative agent of the disease cholera. Experimental characterization of this system revealed a functional immune system that operates using a 5-TT-3 protospacer-adjacent motif (PAM) for interference. However, several designed spacers against the 5-TT-3 PAM do not interfere as expected, indicating further investigation of this system is necessary. In this study, we identified additional sequence requirements of a pyrimidine in the 5 position of the spacer and purine in the complementary position of the protospacer using 873 unique spacers and 2267 protospacers mined from CRISPR arrays in deposited sequences of V. cholerae. We present bioinformatic evidence that during acquisition the protospacer purine is captured in the prespacer and that a 5-RTT-3 PAM is necessary for spacer acquisition. Finally, we demonstrate experimentally that a 5-RTT-3 PAM is necessary for CRISPR interference by designing and manipulating spacer and cognate PAMs in a plasmid conjugation assay and discover functional consequences of base pairing with the 5 spacer pyrimidine in spacer efficacy. ImportanceBacterial CRISPR/Cas systems provide immunity by defending against phage and other invading elements. A thorough comprehension of the molecular mechanisms employed by these diverse systems will improve our understanding of bacteriophage-bacterial interactions and bacterial adaptation to foreign DNA. The Vibrio cholerae Type I-E system was previously identified in an extinct classical biotype and was partially characterized for its function. Here, using both bioinformatic and functional assays, we extend that initial study. We have found that the Type I-E system still exists in modern strains of V. cholerae. Furthermore, we defined additional sequence elements in both the CRISPR array and in target DNA that are required for immunity. CRISPR/Cas systems are now commonly used as precise and powerful genetic engineering tools. Knowledge of the sequences required for CRISPR/Cas immunity is a prerequisite for the effective design and experimental use of these systems. Our results greatly facilitate the effective use of one such system. Furthermore, we provide a publicly available script that assists in the detection and validation of CRISPR/Cas immunity requirements when such a system exists in any bacterial species.