Inhibition of induced ornithine decarboxylase activity in Chinese hamster cells by gamma radiation, far ultraviolet light and psoralen plus near-ultraviolet light: a comparative study.

Inhibition by radiation of the transcriptionally controlled induction of ornithine decarboxylase (ODC) was studied in plateau-phase Chinese hamster cells. The effects of gamma radiation, far ultraviolet (254nm, UV-C) light and psoralen plus near-U.V. light (360nm, PUV-A) were compared. Increasing doses of all three treatments caused an exponential decrease in the capacity for ODC induction. This was preceded by a large shoulder region in the case of PUV-A. There was no shoulder for gamma radiation and possibly none or a small one for UV-C. When compared on the basis of cell killing UV-C is 13 times more efficient in inhibiting ODC induction than PUV-A and 69 times more than gamma radiation. The doses required to produce an inactivating hit for inhibition of ODC induction (D0) are 106 krad for gamma radiation, 2400 J m-2 for PUV-A and 14 J m-2 for UV-C. These doses produce about one gamma ray lesion, one psoralen adduct and one pyrimidine dimer in the structural gene of ODC. However, the number of lesions per transcription unit may be up to 10, depending on the contribution to the effect by lesions in transcribed and non-transcribed spacers and introns. Thus, assuming that most of the inhibition is due to effects on RNA synthesis, the various DNA lesions appear to have a similar efficiency in terminating RNA transcription, in spite of their greatly different efficiency in cell killing.