E. coli DNA photolyase is a monomeric light-harvesting enzyme that utilizes a methenyltetrahydrofolate (MTHF) antenna cofactor to harvest light energy for the repair of thymine dimers in DNA. For this purpose, the enzyme evolved to bind the cofactor and red-shift its absorption maximum by 25 nm. Using the crystal structure as a guide, we mutated each protein residue that contacts the cofactor in an effort to identify the interactions responsible for this selective stabilization of the cofactor's excited state. Hydrogen bonding, packing, and electrostatic interactions were examined. Remarkably, a single residue, Glu109, appears to play an important, if not exclusive, role in inducing the observed red-shift. Thus, this protein, the simplest light-harvesting system known, appears to have evolved a remarkably simple mechanism to tune the photophysical properties of the antenna cofactor appropriately for biological function.