Biological studies suggest that a significant proportion of the cytotoxicity observed in mammalian cells after uv irradiation may be due to damage other than cyclobutane dimers in DNA. Although pyrimidine-pyrimidone (6-4) photoproducts have been implicated as major contributors to cell lethality, their induction has been measured at considerably less than cyclobutane pyrimidine dimers when measured by chromatographic techniques. Because the yield of (6-4) photoproducts may be reduced by their lability to extreme heat and pH, we have devised an alternative, immunological quantification which does not require DNA hydrolysis. Affinity-purified rabbit antisera were used to precipitate low molecular weight32 P-labeled PM2 DNA irradiated with increasing fluences of uv light. DNA of known molecular weight was used to determine rates of induction for antibody-binding sites associated with (6-4) photoproducts and cyclobutane dimers. These rates were calculated to be 0.6 (6-4) photoproducts and <tex-math>$1.2\ \text{cyclobutane dimers}/10^{8}/{\rm Da}/{\rm J}/{\rm m}^{2}$</tex-math>. At low uv fluences (6-4) photoproducts were induced at one-half the rate of cyclobutane dimers, whereas at higher fluences (6-4) photoproducts predominated.

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