Repair of single-strand deoxyribonucleic acid breaks in ultraviolet light-irradiated Haemophilus influenzae.

The wild-type strain and mutants of Haemophilus influenzae, sensitive or resistant to ultraviolet light (UV) as defined by colony-forming ability, were examined for their ability to perform the incision and rejoining steps of the deoxyribonucleic acid (DNA) dark repair process. Although UV-induced pyrimidine dimers are excised by the wild-type Rd and a resistant mutant BC200, the expected single-strand DNA breaks could not be detected on alkaline sucrose gradients. Repair of the gap resulting from excision must be rapid when experimental conditions described by us are employed. Single-strand DNA breaks were not detected in a UV-irradiated sensitive mutant (BC100) incapable of excising pyrimidine dimers, indicating that this mutant may be defective in a dimer-recognizing endonuclease. No single-strand DNA breaks were detected in a lysogen BC100(HP1c1) irradiated with a UV dose large enough to induce phage development in 80% of the cells.