Studies of Chloroplast Development in Euglena XIV. Sequential Interactions of Ultraviolet Light and Photoreactivating Light in Green Colony Formation

Photoreactivation (PR) of green colony-forming ability in Euglena is pH-insensitive from pH 6.0 to 8.0 and temperature-sensitive with a maximum rate at 35°C. There is no PR at 0°C. The rate of PR varies with the growth stage of the cells; PR of exponential phase cells is slower than that of stationary phase cells. The reciprocity rule holds for PR over a 6-fold range of intensity. The shape of PR curves is a function of the UV dose; there appears to be a progressive increase in multiplicity until a limiting multiplicity is reached as indicated by the fact that curves for high doses are superposable. Dark-grown and light-grown cells give the same PR response for comparable UV doses. UV inactivation of cells which have been treated with UV and then with PR light shows that, if the PR dose is sufficiently large, the same UV-inactivation curve is obtained as for nonpretreated control cells. Doses of PR lower than the saturating dose produce UV-inactivation curves, the ultimate slopes of which are parallel to the slope of the control curve, but which show reduced multiplicity. The multiplicity of these curves increases with increasing PR dose. The UV inactivation of green colony-forming ability in Euglena is completely photoreactivable at the doses studied, in contrast with the UV inactivation of colony-forming ability, which occurs at considerably higher UV doses and behaves like most other photoreactivable systems, showing a photoreactivable sector of 0.32