GROWTH, RESPIRATION, AND NUCLEIC ACID SYNTHESIS IN ULTRAVIOLET-IRRADIATED AND IN PHOTOREACTIVATED ESCHERICHIA COLI

Abstract

U-v. (2537 A) radiation causes some basic cellular change in the cell, which in turn causes the familiar u.-v. effects, such as inhibition of growth or death. Proposed criteria for this basic u.-v.-induced change are that it occur immediately after irradiation, at low doses of u.-v., and be immediately and completely reversible by reactivation light (3600-4900 A). Such a reaction was sought in the behavior of u.-v.-irradiated E. coli, strain B/r, (+ or - reactivation light) immediately after radiation. U.-v. has no immediate effect on aerobic cellular respiration as measured by O2 absorption in succinate or in nutrient broth. Doses of u.-v. light inactivating 90% of the cells stops cellular division but has little immediate effect on growth (if defined as cellular enlargement). Greater doses of u.-v. light immediately inhibit cellular enlargement, and this inhibition is reversed partially by reactivating light. Since doses of u.-v. killing the majority of the cells has little immediate effect on growth, growth inhibition is not considered to be the important basic and immediate effect of u.-v. light. Following irradiation of a culture of E. coli in the logarithmic growth phase with a minimal lethal dose of u.-v. light, a period of abnormal logarithmic growth occurs, during this period cells continue to grow at a logarithmic rate little less than that of the control; following this period, growth suddenly ceases or its rate is reduced sharply. This is the period of growth inhibition. With higher doses of u.-v. a 3d period of partial lysis is observed. Reactivating light has little effect on the period of abnormal logarithmic growth but abolishes the period of growth inhibition. Low doses of u.-v. light have little immediate effect on the synthesis of ribose nucleic acid by the cells, but stop immediately and completely the synthesis of desoxyriDOse nucleic acid. After reactivating light treatment, synthesis of desoxyribose nucleic acid is resumed at an accelerated rate. It is suggested that the immediate and specific inhibition of desoxyribose nucleic acid synthesis may be the basic immediate effect of u.-v. radiation.