PAM and repeat-end correlation.

Abstract

<p>(A): PAMs of observed spacers and the co-occurring trinucleotide repeat-ends associated with these spacers. Notice that the spacer-proximal nucleotide of the repeat end is identical to the protospacer-proximal nucleotide of the PAM. (B): Schematic of the proposed mechanism for spacer acquisition during CRISPR adaptation. A protospacer with specific PAM is selected after which it is processed into the pre-spacer (at least 33–34 bp), which contains the last nucleotide of the PAM (the pre-spacer could be single-stranded or double-stranded). The pre-spacer is than integrated at the leader proximal end of the CRISPR locus. The nucleotide derived from the PAM forms the last nucleotide of the repeat. (C): R-loop formation by mature crRNA (61 nucleotides) during CRISPR interference. Notice that the last nucleotide of the repeat (the nucleotide derived from the PAM) is complementary to the target DNA sequence. It remains unknown whether base-pairing between these nucleotides is important for interference.</p

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