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Prestarvation ofEscherichia coli for required amino acids results in a marked enhancement in both ultraviolet light (UV) or X-ray resistance for selective strains. Preventing protein synthesis by starvation for required amino acids results in completion of the cycle of chromosomal replication then underway. We have investigated the relationship between starvation-induced resistance enhancement (SIRE) and the excision-repair (Hcr) system in severalE. coli strains includingE. coli B/rhcr + and its isogenic mutantE. coli B/rhcr − . The following observations were made. (i) The Hcr system is the major component of SIRE in UV-irradiated strain B/r. By using the Hcr+ strain, SIRE increases the 10% survival dose from ∼400 ergs to ∼1,200 ergs/mm2 . With the Hcr cells, the increase is from ∼45 ergs to 60 ergs/mm2 . (ii) Although prestarvation leads to a moderate enhancement of resistance to X irradiation, this effect is not dependent on the Hcr system. (iii) The double mutant,E. coli Bs–1 (hcr − exr − ) is completely unable to express SIRE whether studied with UV or X irradiation. It is concluded that the Hcr system is the major system responsible for SIRE in UV-treated cells, whereas Exr (resistance to X rays) may be involved to a minor extent. The Exr character appears to be required for SIRE expression in X-ray exposed cells.

Relationship Between Radiation Response and the Deoxyribonucleic Acid Replication Cycle in Bacteria: Dependence on the Excision-Repair System