RELATIVE INDUCTION OF CYCLOBUTANE DIMERS and CYTOSINE PHOTOHYDRATES IN DNA IRRADIATED in vitro and in vivo WITH ULTRAVIOLET‐C and ULTRAVIOLET‐B LIGHT

SV40 DNA was irradiated in vitro and in vivo with UV‐C (240–280nm) and UV‐B (280–320nm) light, and damaged sites sensitive to digestion with Escherichia coli endonuclease III (endo III) and bacteriophage T4 endonuclease V (endo V) were quantified. The frequency of endo Ill‐sensitive sites (primarily cytosine photohydrates) induced was1–2% of the frequency of endo V‐sensitive sites (cyclobutane dimers) in both purified SV40 DNA and intracellular episomal SV40 DNA. Endo III‐ and endo V‐sensitive sites in DNA were induced in the same relative proportion at both UV‐C and UV‐B wavelengths. We found no evidence to support earlier inferences that intracellular conditions enhance the formation of cytosine photohydrates or other monobasic forms of DNA damage.