Partial recovery of ultraviolet-damaged denatured or native transforming DNA from Hemophilus influenzae, has been obtained by exposing the irradiated DNA in the denatured form to nitrous acid. Some factors that affect this recovery are described. An erythromycin marker (E20) was not reactivated. The UV damage reactivable by nitrous acid is different from that repaired by the photoreactivating enzyme from bakers' yeast. The pretreatment with nitrous acid affords a slight protection for denatured C25 DNA and Sm250 DNA against ultraviolet irradiation, but this pretreatment sensitized the E20 DNA to this irradiation.

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