Synthetic Dual-InputHybrid RiboswitchesOptimizedGenetic Regulators in Yeast

Synthetic riboswitches, genetic regulatory elements composed entirely of RNA, have been engineered to control a variety of mechanisms at the level of both transcription and translation in all domains of life. The efficiency of riboswitch regulation can be increased by inserting two of them into an mRNA sequence in close proximity, resulting in a tandem riboswitch. The tandem state results in improved regulation beyond that of a single riboswitch by allowing both binding pockets to contribute to a higher dynamic range. The focus of this study was to create a novel tandem riboswitch design by integrating the binding pockets of two different riboswitches into one continuous structure, thereby creating a dual-input hybrid riboswitch. These hybrids remain compact in size with a shorter sequence length compared to a tandem riboswitch, while taking advantage of the binding pockets and scaffold sequences provided by both parental riboswitches. Through rational design, hybrid constructs derived from the combination of tetracycline-, tobramycin-, neomycin-, and paromomycin-binding riboswitches were engineered that significantly increase the dynamic range (e.g., from 14- to 36-fold for tobramycin) while increasing their expression levels in the absence of ligand (e.g., 28% to 68% expression for tetracycline). This study expands the toolbox of synthetic riboswitches and establishes general design guidelines applicable to similar riboswitches. Additionally, the dual-input state makes hybrid riboswitches an interesting target for the design of genetic regulators following Boolean logic.