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Seven mutants of Haemophilus influenzae were isolated by the criterion of sensitivity to ultraviolet (UV) inactivation of colony formation. These mutants and the wild type were characterized with regard to X-ray inactivation of colony formation, UV induction of division inhibition, the ability of the eight strains to act as recipients to UV-irradiated H. influenzae phage and transforming deoxyribonucleic acid (DNA), and the influence of acriflavine on the survival of UV-irradiated transforming DNA with these strains as recipients. The photoreactivable sector of transforming DNA with yeast photoreactivating enzyme was measured for the most UV-sensitive mutant and was found to be greater than that of wild type. Judged by the above criteria, the order of the strains' sensitivities shows some, but by no means complete, correlation from one type of sensitivity characterization to another, indicating that a minimum of two variables is needed to explain the differences in the strains. Acriflavine increases the UV sensitivity of transforming DNA except in the most sensitive mutant. This effect is usually, but not always, more pronounced in the case of the more UV-resistant marker. The acriflavine effect is postulated to be the result of at least two factors: (i) interference with repair of transforming DNA in the host cell, and (ii) interference with the probability of recombination between transforming DNA and host DNA.

Ultraviolet radiation inactivates cells, viruses, and biologically active DNA's, destroys the priming activity of DNA in the calf thymus polymerase system,' and causes mutations. It is reasonable to suppose that much of the deleterious action of ultraviolet light arises from photochemical changes in DNA, such as chain breakage, cross-linking of strands, hydration of the pyrimidines, and formation of dimers between adjacent thymine residues in the polynucleotide chains (see refs. 2 and 3 for reviews). It has not been possible to show that any of these changes leads to biological inactivation, although many such interpretations have been made. A fundamental difficulty in identifying photochemical products with ultraviolet-induced biological damage is that increasing ultraviolet doses result in increasing numbers of all types of products, and those made in even small amounts may have devastating biological effects. If some of the photochemical events could be increased or decreased relative to others during biological inactivation, one could correlate biological alteration with relative amounts of photochemical products per DNA molecule.The ratio of thymine dimers to other products in irradiated DNA can apparently be altered under appropriate conditions, and the present work gives evidence that ultraviolet-produced thymine dimers in Hemophilus influenzae transforming DNA lead to a loss in transforming activity. Thymine dimers are not, however, sufficient to explain all the observed inactivation by ultraviolet radiation.Materials and Methods.-In the biological experiments, transforming DNAt from streptomycin-and cathomycin-resistant H. influenzae was suspended in 0.1 M NaCl at a concentration of 1 ,qg/ml and irradiated at room temperature. Meth-

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