Virogenic BrdU and BrdU-sensitive DNA sequences are disproportionately concentrated in the template-active chromatin of rat embryo cells.

In order to characterize the molecular mechanism responsible for the BrdU-mediated activation of endogenous retrovirus from normal rat embryo cells, the previously reported selective distribution of bromouracil in DNA was correlated with the corresponding organization of the nucleo-protein complex in regard to nucleosome structure and template - active and -inactive chromatin. Following micrococcal nuclease digestion of chromatin labeled with either [(3)H]thymidine or [(3)H]BrdU, the amount and specific activities of the respective nucleosomal DNA were indistinguishable. Comparable findings were obtained following direct examination of the nuclease-sensitive, "spacer" DNA. However, when each chromatin type was fractionated into template-active and -inactive components, it was evident that [(3)H]bromouracil was nonrandomly more concentrated in the template-active portion in comparison to the random distribution of [(3)H]thymine moieties. Furthermore, it was apparent that the template-active chromatin fraction was substantially enriched in the nucleotide sequences of rat DNA known to be sensitive to the virogenic action of BrdU.