Fidelity of Prespacer Capture and Processing is Governed by the PAM Mediated Interaction of Cas1-2 Adaptation complex in Escherichia coli

Abstract

During CRISPR adaptation, short sections of invader derived DNA of defined length are specifically integrated at the leader-repeat junction as spacers by Cas1-2 integrase complex. While several variants of CRISPR systems utilise Cas4 as an indispensible nuclease for processing the PAM containing prespacers to a defined length for integration- surprisingly- a few CRISPR systems such as type I-E are bereft of Cas4. Therefore, how the prespacers show impeccable conservation for length and PAM selection in type I-E remains intriguing. In Escherichia coli, we show that Cas1-2/I-E- via the type I-E specific extended C-terminal tail of Cas1 -displays intrinsic affinity for PAM containing prespacers of variable length and its binding protects the prespacer boundaries of defined length from the exonuclease action that ensues the pruning of aptly sized substrates for integration. This suggests that cooperation between Cas1-2 and cellular exonucleases drives the Cas4 independent prespacer capture and processing in type I-E.