Nonrandom correlation between selective distribution of virogenic 5-bromodeoxyuridine and DNA-binding nonhistone proteins in rat DNA.

In order to more accurately determine a mechanism for the bromodeoxyuridine (BrdU)-mediated activation of endogenous type C virus from normal rat embryo cells, nonhistone nuclear protein-DNA interactions were analyzed in vitro. Native, as well as kinetically fractioned DNA samples previously labeled with either [3H]-thymidine or [3H]-BrdU were combined with DNA-binding nonhistones and characterized according to distribution of the isotopes, and extent and localization of protein-binding sites. As before, [3H]-BrdU was relatively more concentrated in repetitive DNA as compared to [3H]-thymidine. Using a membrane filter retention assay, nearly 60% of complete, 37% of repetitive, and 12% of nonrepeated DNA-protein reconstituted complexes were retained on the filters regardless of isotopic precursor. However, a proportionately greater amount of [3H]-BrdU than [3H]-thymidine was recovered following extensive digestion of renatured complexes with DNase I, even though comparable amounts of DNA were acid-insoluble. The disproportionate binding of nonhistones to repetitive DNA, especially BrdU-substituted regions, may be related to the highly specific, well-characterized modifications in eukaryotic transcription attributed to the analog.