Photoreactivation of ultraviolet-induced suppression of division in Chilomonas paramecium

Abstract The two phases of division-suppression found after UV-irradiation of Chilomonas paramecium populations were examined for photoreactivability. The second, or delayed, phase was readily photoreactivated whether the tests were run on cultures growing in complete medium, on nitrogen-depleted cells in nitrogen-deficient medium, or nitrogen-depleted cells returned to complete medium. Photoreactivability declined as cells progressed toward division, and could not be demonstrated if PR exposure was performed later than a few hours before the first post-UV division. Photoreactivability was prolonged when division was prevented by nitrogen deprivation. The initial, or immediate, phase of division-suppression was also photoreactivable, but this, for unknown reasons, could only be demonstrated if lag was prolonged by nitrogen-deprivation. These results may be interpreted as indicating that both phases of the division-suppression are the result of UV-damage to DNA, and that there is retention of photoreactivating enzyme complement even under conditions of nitrogen depletion. It is noted, however, that UV may produce photoreactivable lesions in cell constituents other than DNA, and that these may also account for either or both of the two phases of division-suppression.