THE VARIATION OF RADIATION SENSITIVITY OF BACTERIA FED TETRAHYMENA PYRIFORMIS DURING THE GROWTH CYCLE AND FACTORS RELATED TO ITS ORIGIN

Abstract— Tetrahymena pyriformis shows a cyclic variation of sensitivity (for lethality) to u.v. light while the response to X-rays is almost constant throughout the growth (division to division) cycle. It is demonstrated in this paper, and elsewhere, that survival of caffeine treated animals is higher after higher doses than after the lower doses of radiation; such a response indicates that the increased radiation dose induces a resistant state not existing at lower doses. Response of synchronized animals to postirradiation treatment with caffeine is interpreted as indicating that recovery from the lethal effect of radiation has two components as previously postulated: (1) a caffeine sensitive recovery system (termed the ' N ' system) and, (2) an activated recovery capacity (termed the ' T' system) which is less sensitive to caffeine.
The maximum activated ( T ) system repair capacity only equals the repair capacity of the ( N ) non-activated system for X-ray induced lesions and, since the N system does not seem to show a cyclic variation of effectiveness for X-ray induced lesions, the X-ray survival remains constant throughout the growth cycle.
The repair capacity of the activated ( T ) repair system is, throughout most of the cycle and especially during the S period, much greater than that of the N system for u.v. induced lesions. Thus the u.v. response vanes in a manner largely dependent upon the capacity of the activated system and the fraction of the population showing the activated response; in G 2, a time when the activated repair system is rather ineffective, the non-activated ( N ) system appears to become unusually efficient in the repair of u.v. induced lesions.