Effect of photoreactivation on the filling of gaps in deoxyribonucleic acid synthesized after exposure of Escherichia coli to ultraviolet light.

Daughter-strand gaps in deoxyribonucleic acid (DNA) synthesized after exposure of excision-deficient Escherichia coli to ultraviolet light are filled during subsequent incubation in buffer, and the rate of filling is increased when the incubation in buffer is carried out in the presence of 360-nm light. It is concluded that daughter-strand discontinuities are prevented from being rapidly sealed in the dark not because of some structural feature of the daughter-strand but because of the presence of a pyrimidine dimer on the opposite (parental) strand. "Photoreactivation-stimulated gap filling" is dependent on the polA(+) and recA(+) but not the exrA(+) genes. It is suggested that the removal of the dimer allows gap-filling by DNA polymerase I and polynucleotide ligase. The recA(+) gene may be needed at a very early stage, possibly for gap stabilization.