Direct Demonstration of the Monomerization of Thymine‐containing Dimers in U.V.‐IRRADIATED Dna by Yeast Photoreactivating Enzyme and Light*

Ultraviolet‐irradiated E. coli DNA (3H‐thymine‐labelled) was mixed with un‐irradiated E. coli DNA (14C‐thymine‐labelled) and exposed to light in the presence of purified yeast photoreactivating enzyme. As the 3H‐thymine‐containing cyclobutane dimers disappeared during the photoreactivation, there was a stoichiometric increase of monomeric 3H‐thymine as determined from the 3H/14C ratio in thymine. This is the first direct demonstration that thymine‐containing dimers in u.v.‐irradiated DNA are monomerized by yeast photoreactivating enzyme in the presence of light.