De novo Design of Translational RNA Repressors

A central goal of synthetic biology is the development of methods for the predictable control of gene expression. RNA is an attractive substrate by which to achieve this goal because the relationship between its sequence, structure, and function is being uncovered with increasing depth. In addition, design approaches that use this relationship are becoming increasingly effective, as evidenced by significant progress in the de novo design of RNA-based gene regulatory mechanisms that activate transcription and translation in bacterial cells. However, the design of synthetic RNA mechanisms that are efficient and versatile repressors of gene expression has lagged, despite their importance for gene regulation and genetic circuit construction. We address this gap by developing two new classes of RNA regulators, toehold repressors and looped antisense oligonucleotides (LASOs), that repress translation of a downstream gene in response to an arbitrary input RNA sequence. Characterization studies show that these designed RNAs robustly repress translation, are highly orthogonal, and can be multiplexed with translational activators. We show that our LASO design can repress endogenous mRNA targets and distinguish between closely-related genes with a high degree of specificity and predictability. These results demonstrate significant yet easy-to-implement improvements in the design of synthetic RNA repressors for synthetic biology, and point more broadly to design principles for repressive RNA interactions relevant to modern drug design.