Reduction of DNA transforming activity in culture by 6-mercaptopurine.

The transforming activity of DNA isolated from 6-mercaptopurine-treated cultures of Bacillus subtilis strain UTH-8505 is markedly reduced when compared to that of DNA obtained from control cultures. The lower transforming activity appears to be a property of the isolated DNA; i.e., various treatments either to restore the activity or to indicate the presence of inhibitory substances that isolate with the DNA suggest a defect in the nucleic acid per se. The reduced transforming activity is not gene specific since the ability of DNA from 6-mercaptopurine-treated cultures to transform several mutants of different genetic loci is lowered. The dose-dependent effect is correlated with the extent of trichloroacetic acid-insoluble radioactivity associated with the DNA from 6-[35S]mercaptopurine-treated cultures. However, the level of apparent drug incorporation is low, being only 1 molecule equivalent to 6-mercaptopurine in 17,500 base residues of DNA having 40% of control transforming activity. The amount of 6-thioguanine incorporation possibly associated with the reduced transforming activity is even less, about one 6-thioguanine moiety per 100,000 base units. If base analog substitution accounts for the reduced transforming activity, exceedingly low levels of incorporation are sufficient to alter this biological property of B. subtilis DNA.