KEYWORDS: Fluorescence resonance energy transfer, Proteins, Luminescence, Fluorescent proteins, Sensors, Molecules, Genetics, Etching, Computer programming, Signal to noise ratio
Riboswitches are a type of natural genetic control element that use untranslated sequence in the RNA to recognize and
bind to small molecules that regulate expression of that gene. Creation of synthetic riboswitches to novel ligands depends
on the ability to screen for analyte binding sensitivity and specificity. In our work, we have coupled a synthetic
riboswitch to an optical reporter assay based on fluorescence resonance energy transfer (FRET) between two genetically-coded
fluorescent proteins. Specifically, a theophylline-sensitive riboswitch was placed upstream of the Tobacco Etch
Virus (TEV) protease coding sequence, and a FRET-based construct, BFP-eGFP or eGFP-REACh, was linked by a
peptide encoding the recognition sequence for TEV protease. Cells expressing the riboswitch showed a marked optical
difference in fluorescence emission in the presence of theophylline. However, the BFP-eGFP FRET pair posses
significant optical background that reduces the sensitivity of a FRET-based assay. To improve the optical assay, we
designed a nonfluorescent yellow fluorescent protein (YFP) mutant called REACh (for Resonance Energy-Accepting
Chromoprotein) as the FRET acceptor for eGFP. The advantage of using an eGFP-REACh pair is the elimination of
acceptor fluorescence which leads to an improved detection of FRET via better signal-to-noise ratio. The EGFP-REACh
fusion protein was constructed with the TEV protease cleavage site; thus upon TEV translation, cleavage occurs
diminishing REACh quenching and increasing eGFP emission resulting in a 4.5-fold improvement in assay sensitivity.
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