Engineered CRISPR prime editors with compact, untethered reverse transcriptases

The CRISPR prime editor PE2 consists of a Streptococcus pyogenes Cas9 nickase (nSpCas9) fused at its C-terminus to a Moloney Murine Leukemia Virus reverse transcriptase (MMLV-RT). Here, we show that separate nSpCas9 and MMLV-RT proteins function as efficiently as intact PE2 in human cells. We use this Split-PE system to rapidly identify and engineer prime editor architectures that are more compact and that expand the RT repertoire for prime editing.