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 separated 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 more compact prime editor architectures that also broaden the types of RTs used for prime editing.