Finding the perfect promoter for Cas9 in homing gene drives using single cell transcriptome data.

Gene drive can modify or suppress vector populations by spreading drive alleles. In CRISPR homing drives, regulating Cas9 expression has been effective for improving drive performance, but selecting suitable promoters is often a major challenge. Here, we evaluate 35 Cas9 constructs with distinct promoters in Drosophila melanogaster and identify associations between drive performance and single-cell RNA expression patterns of the promoter-associated genes. Our results indicate that higher drive conversion is associated with elevated expression of the promoter-associated gene in reproductive cells, but embryo resistance allele formation correlates with excessive female germline expression. For males, early germline expression produces superior performance. Thus, optimal drive performance requires restricting Cas9 expression to a tight quantitative and spatiotemporal window. Additionally, we find that an in situ construct significantly reduces potentially harmful somatic expression. Based on these results, we propose criteria for selecting promoters, providing a rationale and guidance for optimization of homing gene drives.