Biochemical study demonstrates that Cas1-2 protects prespacer boundaries during exonuclease trimming in CRISPR type I-E, indicating how host enzymes offset the loss of Cas4.
Although Cas1-2/I-E does not prune the prespacers, its binding protects the prespacer boundaries from exonuclease action. This ensures the pruning of exposed ends by exonucleases to aptly sized substrates for integration into the CRISPR locus. In summary, our work reveals that in a few CRISPR-Cas variants, such as type I-E, the specificity of PAM selection resides with Cas1-2, whereas the prespacer processing is co-opted by cellular non-Cas exonucleases, thereby offsetting the need for Cas4.
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Yoganand et al. (2019) studied this question.
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