Ultraviolet inactivation and photoproducts of transforming DNA irradiated at low temperatures.

Solutions of Haemophilus influenzae transforming DNA were irradiated at temperatures ranging from 25 degrees C to - 196 degrees C. Temperature dependence of the formation of thymine-containing dimers was closely correlated with inactivation of transforming activity; in general, both dimerization and inactivation decreased with decreasing temperature. The fraction of nonphotoreactivable damage increased with increasing dose at low temperatures. The nonphotoreactivable spore-type photoproduct was formed at low temperatures with a maximum at - 100 degrees C, a temperature at which the nonphotoreactivable biological inactivation was also a maximum. Intrastrand cross-linking, like dimer formation, decreased with decreasing irradiation temperature.