Structure and functional reselection of the Mango-III fluorogenic RNA aptamer

No prior work had resolved the precise atomic arrangement responsible for the high fluorescence of the Mango-III aptamer. It was already known that this RNA binds thiazole orange derivatives with significant affinity. Prior research has shown that this molecule displays a complex thermal melting profile. That uncertainty drove the need to visualize its internal architecture. This gap motivated the current structural investigation. Previous studies identified the aptamer as a promising candidate for cellular imaging. However, the exact folding pattern remained elusive until now. That lack of structural data hindered further optimization of its light-emitting properties.

The study aims to determine the structural basis of the Mango-III aptamer's high fluorescence. Researchers sought to explain why this specific RNA exhibits unique thermal melting characteristics. They intended to map the interactions between the aptamer and its thiazole orange ligand. The team wanted to understand how the pseudoknot-like connectivity influences the dye's conformation. They aimed to identify the role of the A10U mutation in enhancing light emission. The project sought to develop improved variants for better performance in cellular imaging. They intended to compare the brightness of these molecules against standard fluorescent proteins. The investigation was motivated by the need for more efficient tools to visualize RNA in living cells.

The team employed X-ray crystallography to resolve the atomic coordinates of the RNA complexes. They performed structure-guided mutagenesis to test the influence of specific nucleotides on fluorescence. The review approach involved comparing the wild-type sequence against the A10U mutant. Researchers synthesized the iMango-III variant through functional reselection techniques. They assessed the thermal stability of the molecules using melting experiments. The group calculated the quantum yield for each variant to quantify brightness. They utilized thiazole orange derivatives as the primary ligands for binding assays. The investigation integrated structural modeling with biochemical characterization to validate the findings.

The crystal structures reveal a globular architecture formed by a pseudoknot-like connection between a G-quadruplex and a non-canonical duplex. The A10U mutation increases the quantum yield from 0.55 to 0.66. The improved variants fluoresce approximately fifty percent brighter than enhanced green fluorescent protein. The dye is held in a planar state by the G-quadruplex and a long-range trans Watson-Crick pair. A pyrimidine oriented perpendicular to the nucleobase planes further stabilizes the fluorophore. The biphasic thermal melting profile corresponds to the presence of this complex tertiary structure. The reselected iMango-III mutant exhibits enhanced performance compared to the original sequence. These results confirm the structural basis for the high affinity binding of TO1-Biotin.

The authors suggest that the globular fold is responsible for the high quantum yield observed. They propose that the pseudoknot-like arrangement stabilizes the dye in a planar state. The researchers claim that the A10U mutation enhances brightness by optimizing the long-range base pairing. They conclude that the reselected iMango-III variant offers superior performance for biological applications. The study indicates that these aptamers surpass enhanced green fluorescent protein in signal intensity. They imply that the structural insights facilitate the design of future RNA-based probes. The findings demonstrate that non-canonical duplexes are vital for maintaining the specific architecture. The team maintains that these tools are suitable for real-time tracking of cellular transcripts.