A novel strategy to analyze l-tryptophan through allosteric Trp repressor based on rolling circle amplification

Rolling circle amplification (RCA) has been considered as a powerful tool for nucleic acids detection. Here, a novel repressor-RCA-based method for l-tryptophan (l-Trp) detection was developed. This method utilizes the specific interaction between the RCA circular template and the Trp repressor protein (TrpR) involved in trp operon of Escherichia coli (E. coli). In the absence of l-Trp, the TrpR protein could not bind to the RCA template, and the RCA process can be continued. When l-Trp is present, the activated TrpR will bind to the operon sequence on the RCA template and inhibit the RCA reaction. Thus, the concentration of l-Trp is correlated directly with the fluorescent RCA signals. We succeeded in detecting l-Trp in a single step in simple homogeneous reaction system. The detection limit was estimated to be 0.77 μM (S/N=3) with good linearity. The method can unambiguously distinguish l-Trp from other 19 standard amino acids and l-Trp analogs. This strategy is also promising for detecting many small molecules such as other amino acids and carbohydrates.