Replicon initiation in normal human cells and in ataxia telangiectasia cells: its differential inhibition by cycloheximide and bleomycin.

Treatment of normal human fibroblasts (NHFs) with cycloheximide, which inhibits protein synthesis, resulted in partial inhibition of their DNA synthesis, as determined by incorporation of radioactive thymidine and resistance of the cells to subsequent treatment with bleomycin. The effects of treatments of ataxia telangiectasia fibroblasts (ATFs) with cycloheximide and then bleomycin on their DNA synthesis were very similar to those on DNA synthesis of NHFs. The fact that treatment with bleomycin only caused transient inhibition of DNA synthesis within an hour in NHFs but not ATFs was confirmed. Studies by alkali-density gradient centrifugation showed that the cycloheximide mainly inhibited formation of short fragments of DNA in both NHFs and ATFs, as bleomycin does in NHFs. These findings suggest that these two chemicals both inhibit replicon initiation, and thus provide evidence that the genetic defect in ATFs is related to replicon initiation.