Studies on ultraviolet irradiation of Escherichia coli containing 5-bromouracil in its DNA.

Thymine-requiring Escherichia coZi 15, tand E. coli B, twhich 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 defined 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 E. coli 15, twas 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 E. coli 15, t-. Bacteria containing 5-bromouracil were similarly hypersensitive to heat but showed no increased sensitivity to H,O, and several nitrogen mustards.