Disclosed are methods, compositions and devices for screening a protein library for proteins having a desired activity, such as capable of catalyzing the formation of a bond between two reactants. In an exemplary embodiments, a plurality of proteins are expressed in vitro from a plurality of nucleic acids, the plurality of proteins are exposed with two single stranded oligonucleotides having complementary sequences, each oligonucleotide having a reactant and a fluorophore, the fluorescence of the protein-reactant-oligonucleotide-fluorophore complexes is detected and the complexes showing detectable fluorescence energy transfer are isolated, thereby isolating proteins having the desired enzymatic activity.