A Sequential Repair Model of Photoreactivation in Bacteria*

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 mm 2 . (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 .