Abstract
Bromination of DNA affects only guanine and cytosine which are converted into 8bromoguanine (8-BrC) and S-bromocytosine (5.BrC) respectively [I] . If the Br:nucleotide ratio is below 1, the percentage of brominated bases is proportional to the amount of bromine added. The brominated DNA molecules are more flexible than native DNA [2] and electronmicroscope photographs of material brominated to a low degree reveal that they are composed of two species: clusters resembling the denatured DNA and extended chains simulating native molecules [3]. The presence of two distinct species is also revealed by the melting curve profile of partially brominated DNA and these two species can be separated by fractionation on a hydroxylapatite column. DNA isolated from antibiotic-resistant strains of Haemophilus influenzae looses some of its transforming activity after bromination. A similar inactivation is observed in brominated Bacillus subtilis DNA; however, in both cases, a significantly different degree of inactivation is found for various markers [4] . In the course of this study, the template activity of calf thymus DNA, brominated to different extents, was tested with two DNA polymerases: E. coli DNA polymerase, insensitive to the secondary structure of the primer; and regenerating rat liver replicative DNA polymerase requiring preferentially denatured DNA [51. The action of both polymerases is inhibited by the presence of 8BrC and S-BrC in the template DNA and Ear the mammalian enzyme, modifications of the secondary structure of DNA are not responsible for the inhibition. North-Holland Publishing Company Amsterdam 2. Methods
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