The large fragment of Escherichia coli DNA polymerase I can synthesize DNAexclusively from fluorescently labeled nucleotides

A highly sensitive fluorescence-based assay for screening DNA polymerase li braries (see schematic picture) revealed a surprising activity of the wild- type Klenow fragment of Escherichia coli DNA polymerase I: This enzyme, whi ch has emerged during natural evolution as a polymerase "fit" for natural s ubstrates, retains full activity in the sole presence of artificial nucleot ides that are labeled with a fluorescent dye of the rhodamine type. This un usual finding opens up the horizon for generating DNA exclusively from rhod amine-labeled analogues-a necessary prerequisite to realizing the idea of " single-molecule sequencing".