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Genetic Engineering Publications - GEG Tech top picks
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BigField GEG Tech's insight:
The ability to form new memories of invasion depends on the ability of a CRISPR to incorporate new spacers. The mechanistic basis for this acquisition from DNA is increasingly understood and has often been assumed to be the sole mode of CRISPR adaptation. Silas et al. describe an intriguing twist on bacterial adaptive immunity, identifying a subset of CRISPR systems with the remarkable ability to incorporate new spacers directly from RNA. 
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In this report, the scientists present the high-resolution crystal structures of the three SpCas9 variants in complexes with a single-guide RNA and its altered PAM-containing, partially double-stranded DNA targets. A structural comparison of the three SpCas9 variants with wild-type SpCas9 revealed that the multiple mutations synergistically induce an unexpected displacement in the phosphodiester backbone of the PAM duplex, thereby allowing the SpCas9 variants to directly recognize the altered PAM nucleotides. These findings explain the altered PAM specificities of the SpCas9 variants and establish a framework for further rational engineering of CRISPR-Cas9.