Haloarcula hispanica CRISPR authenticates PAM of a target sequence to prime discriminative adaptation.
Bottom Line: The prokaryotic immune system CRISPR/Cas (Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated genes) adapts to foreign invaders by acquiring their short deoxyribonucleic acid (DNA) fragments as spacers, which guide subsequent interference to foreign nucleic acids based on sequence matching.Interestingly, interference utilizes only four PAM sequences, whereas adaptation-priming tolerates as many as 23 PAM sequences.This relaxed PAM selectivity explains how adaptation-priming maximizes its tolerance of PAM mutations (that escape interference) while avoiding mis-targeting the spacer DNA within CRISPR locus.
Affiliation: State Key Laboratory of Microbial Resources, Institute of Microbiology, Chinese Academy of Sciences, Beijing 100101, China University of Chinese Academy of Sciences, Beijing 100049, China.Show MeSH
Mentions: The majority of PAM mutations should block target interference because of the strict PAM selectivity. However, priming adaptation can counter these escape mutations by acquiring new spacers from the target-bearing DNA (17). Whether all types of PAM mutations can be tolerated to prime adaptation remains unknown. Therefore, for the 60 plasmids that escaped CRISPR interference, a spacer acquisition assay was subsequently performed against their transformants after a 5-day cultivation. Specific primers were used to amplify the CRISPR leader end, and arrays with new spacers incorporated were expected to produce larger-sized PCR products (20). Strikingly, CRISPR adaptation was observed for nearly one third of these escape plasmids (Figure 2A), revealing 19 different PAM sequences that are recognized to prime adaptation, which we described as PAP (priming adaptation permissive). Correspondingly, the remaining 41 tri-nucleotides were referred as PAIN (priming adaptation and interference non-permissive) sequences. Notably, PAP sequences are not all equally favoured, because faint expanded bands were observed for pTAA1, pCAG1, pCCG1 and pCGC1, whereas for most of the other plasmids, evident CRISPR expansion was observed (Figure 2A).
Affiliation: State Key Laboratory of Microbial Resources, Institute of Microbiology, Chinese Academy of Sciences, Beijing 100101, China University of Chinese Academy of Sciences, Beijing 100049, China.