Unidirectional trans-cleaving behavior of CRISPR-Cas12a unlocks for an ultrasensitive assay using hybrid DNA reporters containing a 3' toehold.

CRISPR-Cas12a can induce nonspecific trans-cleavage of dsDNA substrate, including long and stable λ DNA. However, the mechanism behind this is still largely undetermined. In this study, we observed that while trans-activated Cas12a didn't cleave blunt-end dsDNA within a short reaction time, it could degrade dsDNA reporters with a short overhang. More interestingly, we discovered that the location of the overhang also affected the susceptibility of dsDNA substrate to trans-activated Cas12a. Cas12a trans-cleaved 3' overhang dsDNA substrates at least 3 times faster than 5' overhang substrates. We attributed this unique preference of overhang location to the directional trans-cleavage behavior of Cas12a, which may be governed by RuvC and Nuc domains. Utilizing this new finding, we designed a new hybrid DNA reporter as nonoptical substrate for the CRISPR-Cas12a detection platform, which sensitively detected ssDNA targets at sub picomolar level. This study not only unfolded new insight into the trans-cleavage behavior of Cas12a but also demonstrated a sensitive CRISPR-Cas12a assay by using a hybrid dsDNA reporter molecule.