Six different types of CRISPR systems are utilized by microbes to form RNA-based immunity against mobile genetic elements. Type I CRISPR-Cas is the most widely distributed system. Its DNA interference mechanism is quite different from that of Cas9 and Cas12. Instead of introducing a single Double strand break (DSB) at the target site, Type I CRISPR-Cas shreds DNA into pieces in a multi-step process. This is done through the concerted actions of an RNA-guided target-searching complex called Cascade and a helicase-nuclease fusion enzyme Cas3. A total of six subtypes of Type I systems exist (I-A to I-E), each has unique mechanistic properties and may lead to distinct genome editing applications. I will present efforts from my group that lead to the high-resolution mechanistic understanding of several Type I CRISPR-Cas systems. I will further present data showing how we convert the mechanistic understanding to highly efficient deletion-editing tools (CRISPR-Cas3) in human cells.
