SUMMARY: Thymine-requiring 15, t and B, t which contained 5-bromouracil in their deoxyribonucleic acid in place of thymine showed a marked increase in sensitivity to ultraviolet (u.v.) radiation. The effect was proportional to the extent of incorporation of 5-bromouracil under denned growth conditions and for a given medium. This increase in sensitivity to u.v. was not hereditary. A maximum u.v. dose-increase effect on survival of 3.5 was obtained; there was as high as a 20,000-fold decrease in the number of survivors as compared to irradiated bacteria grown in absence of the analogue. This effect was not obtained with bacteria grown before irradiation in 2-thiothymine (an inhibitor not incorporated into DNA), nor was there an increase in u.v. sensitivity in organisms grown under conditions of thymine starvation or of 5-flurouracil inhibition. Furthermore, bacteria which did not require thymine and did not incorporate 5-bromouracil did not show this sensitization effect. The increase in u.v. sensitivity caused by 5-bromouracil was annulled by thymine. 5-Iodouracil caused a lower increase in u.v.-sensitization. The extent of photo-reactivation was not as great in irradiated bacteria which had been grown in 5-bromouracil as in bacteria containing no analogue in their DNA. A u.v.-resistant mutant of 15, t was isolated and found to be affected by 5-bromouracil in a manner similar to the slightly more u.v.-sensitive parent strain. Incorporation of 5-bromouracil did not result in a dose-increase effect on u.v.-induced mutagenesis at the thymine-dependent locus of 15, t. Bacteria containing 5-bromouracil were similarly hypersensitive to heat but showed no increased sensitivity to HO and several nitrogen mustards.


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