Distribution of bromouracil in the pyrimidine oligonucleotides of Drosophila DNA.

DNA was isolated from Drosophila melanogaster larvae fed radioactive thymidine (TdR), bromodeoxyuridine (BUdR), or one of these nucleosides together with 5-fluorouracil (FU) under identical conditions. The four DNA samples designated T-DNA, B-DNA, T(F)-DNA, and B(F)-DNA respectively were hydrolyzed with formic acid-diphenylamine, and the pyrimidine oligonucleotides of various chain lengths (isotichs) were ractionated by DEAE-celluclose chromatography. The distributions of the labeled thymine and bromouracil (BU) residues among the pyrimidine isostichs were determined, and compared for the four DNAs. These comparisons indicated the following: (1) Similarity of T-DNA and T(F)-DNA with respect to labeled thymine distribution among the pyrimidine isostichs (using the mononucleotide to pentanucleotide tracts for statistical comparisons) showed that TdR entering DNA synthesis via the salvage pathway has the same distribution whether the synthetic pathway is operating normally or is inhibited by FU treatment. (2) Quantitative comparison of the distribution of BU among the isostichs of B-DNA and B(F)-DNA with the distribution of labeled thymine in T-DNA showed significant differences, indicating that the organism does not accept BUdR indiscriminately at all thymine sites for DNA synthesis. (3) Comparison of the distribution of BU in B-DNA and B(F)-DNA showed that isostichs 1 and 3 differ significantly. The elution profiles of the isostichs also differed qualitatively between these DNAs. This analysis confirms differential incorporation of BUdR into Drosophila DNA in the presence of a thymidylate inhibitor. The observations on BUdR incorporation in Drosophila DNA have been discussed with relation to the high frequency of somatic cell mutations induced in imaginal disc cells by treatment with the analog in the presence of inhibitors of thymidylate synthetase.