Fig 1.

A reaction-independent complementation assay for detecting enzyme activities of genes or artificial libraries of genes. (Upper) A specific “three-hybrid” assay for detecting lactamase activity. The two dimerized proteins consist of dihydrofolate reductase (rose) and glucocortocoid receptor ligand-binding domains (blue). These are fused to LexA DNA binding and B42 RNA polymerase activation domains and will reconstitute active RNA polymerase complex to transcribe the reporter gene β-galactosidase when the two proteins are noncovalently ligated via the dimerizer dexamethasone-cephem-methotrexate molecule. (Lower Left) Examples of ligation or cleavage reactions that could be used in the assay. (Lower Right) Application of the assay to protein engineering. Figure was provided by Debleena Sengupta and Virginia Cornish (Columbia University, New York).