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Bioengineers have been attempting to utilize variety of organisms and their enzymatic pathways to execute beneficial applications such as production of chemicals or treatment of pollutants. In recent years, design and implementation of synthetic pathways have been studied to enlarge the range and variety of biological pathways. Design of synthetic pathways needs a gene expression regulator module which is designable, tunable, extendable and modular.(1) RNA is one of promising candidates that meets aforementioned requirements for regulator module. There are numerous examples of RNA-mediated regulation of gene expression in nature. Moreover, engineers have produced various RNA-based regulators by mimicking their natural counterparts.(2) Due to its structural and functional versatility engineers expect to select or screen any RNA regulator with desired function from a pool of RNAs whose sequences are randomized. We have tried to select an RNA switch which controls expression level of transcriptionally fused downstream gene in response to amino acid L-tryptophan. L-tryptophan is an aromatic amino acid used as a diet supplement for animal feed or a treatment for depression. At first, previously selected L-tryptophan aptamer(3) was fused to tetA-sGFP fusion gene via randomized linker. Escherichia coli W3110 cells transformed with the vectors which harbor above gene construct were cultured under two different conditions for selection of L-tryptophan responsive RNA switch. After two rounds of selection(4) - positive and negative - we got several RNA switch candidates. We investigated performance of the switch candidates using fluorescence strength of sGFP.