Systematic comparison and rational design of theophylline riboswitches for efficient gene repression

Riboswitches are promising regulatory tools in synthetic biology. To date, 25 theophylline riboswitches have been developed for gene expression regulation in bacteria. However, no one has systematically evaluated their regulatory effects. To facilitate rational selection of theophylline riboswitches, we examined 25 theophylline riboswitches in Escherichia coli and surprised to find that none of the five repressive riboswitches were more than 2-fold effective. To solve this problem, we rationally designed a transcriptional repressive riboswitch and demonstrated its effect not only in various bacterial strains but also in different growth media or different temperatures. By introducing two copies of theophylline riboswitches and a RepA protein degradation tag coding sequence at the 5'-end of a reporter gene, we successfully constructed a dual gene expression regulatory system with up to 150-fold potency, namely the R2-RepA system. R2-RepA system is only 218 bp in length, expression of any protein could be repressed efficiently by simply inserting this system upstream of the target protein-coding sequence. This study represented a crucial step toward harnessing theophylline riboswitches and expanding the synthetic biology toolbox.