Chimeric Guides Probe and Enhance Cas9 Biochemical Activity

DNA substitutions in RNA can probe the importance of A-form structure, 2'-hydroxyl contacts, and conformational constraints within RNA-guided enzymes. Using this approach, we found that Cas9 biochemical activity tolerated significant substitution with DNA nucleotides in the clustered regularly interspaced short palindromic repeat RNA (crRNA). Only minimal RNA content was needed in or near the seed region. Simultaneous substitution at all positions with predicted crRNA Cas9 2'-hydroxyl contacts had no effect on enzyme activity. The trans-activating crRNA (tracrRNA) also tolerated >50% substitution with DNA. DNA substitutions in the tracrRNA-pairing region of crRNA consistently enhanced cleavage activity while maintaining or improving target specificity. Together, results point to a prominent role for guide:target A-form-like helical structure and a possible regulatory role for the crRNA tracrRNA pairing motif. A model chimeric crRNA with high activity did not significantly alter RNP assembly or target binding but did reduce Cas9 ribonucleoprotein stability, suggesting effects through conformation or dynamics. Cas9 directed by chimeric RNA-DNA guides may represent a cost-effective synthetic or molecular biology tool for robust and specific DNA cleavage.