The objective of the proposed research is to expand and develop a novel and simple method for imaging RNA and small molecules in living cells. Unlike green fluorescent protein (GFP), which can be used to monitor proteins in cells, there are currently no analogous simple and straightforward approaches to track RNA movement in living cells. Currently available approaches have critical limitations that have precluded their widespread use. We have engineered both an RNA receptor and a small molecule ligand that interact to form a fluorescent complex. The RNA switches on the fluorescence of an otherwise nonfluorescent small molecule. We have characterized this RNA-fluorophore complex, optimized its sequence to improve its fluorescence properties, and shown that it can be used to monitor RNAs in living cells. Additionally, we have shown that the RNA aptamer can be fused to other aptamers to generate allosterically regulated fluorescent analyte sensors. In order to develop our system into a widely-used, simple and sensitive technique that would permit the study of the cell biology of RNA, the specific aims of this application are: (1) To develop a palette of RNA-fluorophore complexes; (2) To optimize the fluorescence properties of RNA-fluorophore complexes. In this aim, we will use affinity maturation and other techniques to increase the fluorescence of the RNA-fluorophore complexes; (3) To use RNA- fluorophore complexes to visualize mRNAs in cells and mRNA trafficking in axons. In this aim, we describe experiments to optimize the fluorescence of RNA-fluorophore complexes in cells and to monitor mRNA trafficking and mRNA degradation during axon turning; (4) To develop fluorescent sensors from RNA-fluorophore complexes. We present a simple and generalizable approach for generating fluorescent analyte sensors and using them in cells. Together, the experiments in these four aims will result in a versatile RNA imaging and fluorescent sensor technology that is simpler and more specific than any other methodology that is currently available.