Repair of ultraviolet light-induced damage to the deoxyribonucleic acid of Neurospora crassa.

A method is described for labeling a specific pyrimidine in the deoxyribonucleic acid (DNA) of Neurospora crassa. In cells grown in the presence of [5-(3)H]-uridine, more than 97% of the radioactivity associated with the DNA had been incorporated into cytosine. The specific activity of the labeled DNA was approximately 3 x 10(3) counts per min per mug. The DNA was isolated by elution from hydroxyapatite columns with sodium phosphate buffer (0.40 m, pH 6.8). This procedure was used to demonstrate that in vegetative cells of N. crassa both photoreactivation and excision repair are operative, as measured by the removal of ultraviolet light-induced cytosine-containing dimers.