Resource competition shapes CRISPR-mediated gene activation

CRISPR-mediated gene activation (CRISPRa) allows concurrent transcriptional control of many genes and is widely used in genome-wide screening, bioproduction, and therapeutics. Multi-gene control is possible due to the sequence specificity by which guide RNAs (gRNAs) recruit dCas9 and an activator protein to target genes. Still, the optimization of CRISPRa systems remains difficult. Here, we show that, despite sequence specificity, different gRNAs interfere with each other by competing for dCas9 and the activator protein. This competition breaks modularity and hinders CRISPRa. We also discover that gene activation is biphasic, wherein increased level of a gRNA leads to target repression instead of activation. We introduce a chemical reaction-network model that captures these effects and use it for improving the dynamic range of CRISPRa. Our results demonstrate that CRISPRa is not as modular or scalable as previously thought and establish a predictive modeling tool that enables systematic design and optimization of multi-gRNA CRISPRa systems. A record of this paper's transparent peer review process is included in the supplemental information.