Strain-specific outcomes of cytosine-base editing in Streptomyces.

UNLABELLED: The clustered regularly interspaced short palindromic repeats (CRISPR)-Cas system has facilitated gene editing of different organisms. Specifically, programmable base editing enables stable conversion of a single nucleotide to another nucleotide without causing DNA double-strand breaks. Cas9-derived base editors, including adenine and cytidine base editors, catalyze the formation of transition mutations with high efficiency. The third-generation cytidine base editors comprise a nickase-Cas9 fused to a cytidine deaminase and uracil DNA glycosylase inhibitor to enable the transition from C:G to T:A. We observed that in certain Streptomyces spp., this cytidine base editor produced C:G to G:C transversions at a surprisingly high rate of 44, while in other strains, it yielded the expected C:G to T:A transition mutations. There was also a notable timing difference for base editing between distinct strains. Bioinformatics analysis revealed differences in DNA mismatch repair and nucleotide excision repair pathways, including the presence of uvrD helicase gene only in the C:G to G:C transversion strain. Expression of uvrD in the C:G to T:A transition strain led to higher rates of C:G to G:C transversions. These discoveries will aid in the development of efficient C:G to G:C base editors. IMPORTANCE: Base editors are useful tools that allow genetic engineers to precisely change single bases in an organism's genome. Most base editors catalyze transition mutations (e.g., A-to-G or C to-T). Transversion mutations, which convert a purine base to a pyrimidine base, have been described but operate at much lower efficiency. Here, we describe a surprising discovery that a transition base editor produced high-efficiency transversion mutations in a species of Streptomyces. This information was used to engineer a high-efficiency transversion base editor.