Evidence for a Further Dark Repair Process in Bacteria

Abstract

Resistant (repair proficient) strains of Escherichia coli stop DNA synthesis almost immediately after low doses of ultraviolet (UV). Synthesis ceases for a period that is a function of the UV dose and is resumed at the same rate as unirradiated bacteria^1–4. The time at which synthesis restarts correlates well with the time at which excision-repair is completed. Billen and his collaborators showed that when DNA synthesis resumes, a new replication point is initiated preferentially at the chromosomal origin, the existing replication point remaining inactive^5–7. In contrast, excision-deficient (Uvr⁻) or recombination-deficient (RecA⁻) bacteria show decreased rates of synthesis as the dose is increased until (when the surviving fraction is very low) synthesis ceases to be measurable^1,3,8. Moreover in Uvr⁻ bacteria, DNA synthesis after UV continues largely at the existing replication point⁹. Thus in repair-proficient bacteria there exists after irradiation a “stalled” replication-complex, which must be repaired if the new replication point is to traverse the same region. At the least this must involve the elimination of one replicated arm by some breakage and rejoining process and may also involve the repair of radiation-induced lesions in that region.