A Sequential Repair Model of Photoreactivation in Bacteria*

Abstract

Abstract— Kinetics of photoreactivation were studied in E. coli WP2 hcr‐, a strain deficient in dark repair. Cells in aqueous suspension were subjected to u.v.‐irradiation, then exposed to photoreactivating light for different periods. Survival curves, with samples at a minimum of six u.v. doses, were obtained at several periods of photoreactivation ranging from zero to maximum. The surviving fractions do not conform to a dose‐reduction model, but instead, they fit a ‘sequential repair’ model that assumes as a limiting condition that the number of active enzyme molecules is small. The model used assumed: (1) a single enzyme molecule is active at any one time; and (2) inactivating events are nullified consecutively around the DNA molecule. The mathematics of the model are derived and presented. Photoreactivation is attributed to the action of two processes. (1) A photochemical process, that is rate limiting below 1000 ergs mnr‐2 sec‐1, was measured at a photoreactivating irradiance of 60 ergs mm‐2 sec‐1. This has a rate constant of 5 × 10‐5‘events’ erg‐1 mm2. (2) A dark process, measured at photoreactivating irradiances of 4000 and 6000 ergs mm‐2 sec‐1, has a rate constant of 2.2 ‘events’ min‐1.

Near-UV mutagenesis: photoreactivation of 365-nm-induced mutational lesions in Escherichia coli WP2s

“…This result indicates that PR of mutational lesions induced at 365 nm is similar to the results obtained at 254 nm with this strain by Witkin (32). The rate of PR of 365-nm-produced mutational lesions (T1/2 = 7 min) at a 10 _ 00-256C and 0C (9) was considerably less than the rate of PR for survival previously reported (TI/2 = 2.2 min) under similar conditions at 254 nm. Partial destruction of the PR enzyme by 365-nm radiation (23,26) may account for the reduced PR rate for mutagenesis at 365 nm.…”

Absence of Repair of Sublethal U.V. Light Damage in Chlamydomonas

“…These ratios are similar to those given in the table, although the period of holding cells in the dark was 72 hours, compared with a period of 24 hours used by Davies . The lack of dose-modification obtained for Chlamydomonas is in contrast to those results obtained with other systems where dose modification was observed (Kelner 1949, Novick and Szilard 1949, Davies, Tyler and Webb 1970 .…”

Life under Conditions of Ionizing Radiation

“…DNA repair by photolyase in the presence of light (photoreactivation) is the best studied repair reaction of this type to remove UV-induced DNA lesions from the genome. Genes coding for photolyase activities (PHR genes) were found in many prokaryotic and eukaryotic organisms, but not in all species [40]. Another example of direct damage reversal is repair of O 6 -methyl guanine by transfer of the alkyl group from the DNA to a cysteine in a protein, an O 6 -methylguanine-DNA methyltransferase, which appears to be present in all living organisms [41].…”

“…Other examples of socalled radio-fossils are 40 K and 87 Rb. Essentially, exposure from external irradiation comes from 40 K and the γ-emitting intermediate products of the persisting decay series. There are considerable geographic variations of the natural radiation due to varying concentrations of radionuclides in soil and minerals.…”