Warner, K.D. et al. Nat. Struct. Mol. Biol. 21, 658–663 (2014).

Analogously to the use of GFP and its rainbow of derivatives to label proteins with a fluorescent tag, Spinach is a useful tool for fluorescently labeling RNA via genetic encoding. Spinach is made up of an RNA aptamer and a small-molecule analog of the GFP chromophore, which only fluoresces upon binding to the aptamer. Now Warner et al. provide a detailed investigation into the structural basis of Spinach fluorescence by solving the cocrystal structure of the aptamer bound to its chromophore. They found that a G-quadruplex architecture was crucial for generating fluorescence. The structural results allowed them to design a miniaturized 'Baby Spinach' that is half the size of Spinach but retains 95% of its fluorescence intensity; such a miniature version may reduce imaging artifacts.