The influence of dimer removal through excision or photoreactivation on the kinetics of DNA synthesis, sedimentation profiles of DNA molecules and survival of cells was investigated in excision-deficient and excision-proficient Escherichia coli K-12 after a flux of 20 J M-2. In excision-deficient cells photoreactivation did not influence the kinetics of DNA synthesis for a long period and the sedimentation properties of DNA synthesized immediately after photoreactivation were influenced only slightly. However, survival was increased remarkably. In excision-proficient cells where dimers were removed through excision, the kinetics of DNA synthesis increased rapidly, normal-sized DNA molecules were synthesized 60 min after irradiation and survival was substantially higher than in the above-mentioned case. This can hardly be interpreted as a more complete repair of dimers by excision because the persistence of dimers in these cells did not significantly influence either the kinetics of DNA synthesis or normalization of DNA molecules and/or survival of cells. It is concluded that persisting dimers play an important role in excision-deficient but not in excision-proficient cells, that a non-dimer damage to DNA causes inhibition of DNA synthesis after UV and that this damage is of primary importance for excision-proficient cells which can easily cope with persisting dimers.

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