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Structural biology

How CRISPR captures spacer invaders

Nature Reviews Microbiology volume 13, pages 738–739 (2015)Cite this article

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Both studies used Cas1–Cas2–protospacer complexes containing Cas1 and Cas2 from the Escherichia coli type I-E CRISPR–Cas system. As has previously been shown for DNA-free complexes, the structures revealed a hexameric architecture with a Cas2 dimer between two Cas1 dimers.

Nuñez et al. used a 33 bp protospacer that is known to be readily acquired by the E. coli CRISPR–Cas system. Rather than using a completely double-stranded DNA (dsDNA) protospacer, a 23 bp dsDNA centre was flanked by a 5 nucleotide (nt) single-stranded DNA (ssDNA) overhang at each 3′ end, which an in vitro assay had established was the preferred substrate of Cas1–Cas2. This preference was explained by the X-ray structure, which revealed the presence of a tyrosine 'wedge' at position 22 of each distal Cas1 subunit. These wedges fix the length of dsDNA at 23 bp by stacking with the base pair at either end of the duplex, which prevents downstream base pair formation and thus splays any remaining dsDNA into a fork. On either side of the dsDNA, ssDNA is threaded into a channel in Cas1, where the 3′-OH is positioned in the active site.

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References

  1. Nuñez, J. K. et al. Foreign DNA capture during CRISPR–Cas adaptive immunity. Nature http://dx.doi.org/10.1038/nature15760, (2015)

  2. Wang, J. et al. Structural and mechanistic basis of PAM-dependent spacer acquisition in CRISPR-Cas systems. Cell http://dx.doi.org/10.1016/j.cell.2015.10.008, (2015)

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  1. Naomi Attar
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Attar, N. How CRISPR captures spacer invaders. Nat Rev Microbiol 13, 738–739 (2015). https://doi.org/10.1038/nrmicro3585

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