A SEQUENTIAL REPAIR MODEL OF PHOTOREACTIVATION IN BACTERIA*

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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.

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