Synthetic riboswitches (RSs) are promising regulatory devices due to their small size, lack of immunogenicity, and ability to fine-tune gene expression in the absence of exogenous trans-acting factors. Based on a gene inhibitory system developed at our lab, termed U1 snRNP interference (U1i), we have developed inducible RSs that modulate mRNA polyadenylation through selective U1 snRNP recruitment. First, we engineered different U1iRSs, which repress gene expression unless an exogenous drug is added, leading to inductions of 3-to-4-fold. Second, we developed a novel technique called  Target Capture Systematic enrichment of RSs (TACS), that involves several rounds of selection, amplification and high-throughput sequencing. TACS applied to our U1iRSs allowed us to isolate new drug-regulated RSs with enhanced U1 snRNP binding capacity and activity, achieving inducibilities of up to 8-fold. Interestingly, applying our knowledge on U1i to U1iRSs allowed us to obtained both synergistic inhibitions of more than 100-fold and inductions up to 37-fold. We demonstrate that U1iRSs are dose-dependent, reversible, and can regulate the expression of reporter and cellular genes in culture cells and mouse models, resulting in attractive systems for gene therapy applications. In addition, we have modified TACs so that it can be applied to isolate functional U1iRSs recognizing alternative ligands, including those with fast clinical translation: generic drugs with mild secondary effects after administration of high doses for long terms. Our work probes TACs as a much-needed technology for the in vitro identification of RSs capable of regulating expression in vivo and with great potential for gene therapy applications.