PHOTOREACTIVATION OF ESCHERICHIA COLI Bs‐3 AFTER IN ACTIVATION BY 313 nm RADIATION IN THE PRESENCE OF ACETONE

The colony‐forming ability of E. coli B s‐3 can be inactivated by light of 313 nm wavelength in an acetone‐sensitized photochemical reaction. This ability can subsequently be restored quantitatively by illumination with photoreactivating light. A small fraction of the population cannot be inactivated; this is assumed to be due to a complete dark repair of the lesions, whatever the dose of radiation has been. Thus, such triplet energy‐transfer experiments can successfully be applied to whole cells. Since thymine dimers are formed almost exclusively, this suggests a new way of studying these lesions in relation to the biologically observable effects.