Oligo(A), oligo(TG), and Alu repeats of DNA in chromatin are available for sequence-specific chemical modification with oligodeoxynucleotide derivatives.

Reaction of 4-(N-2-chloroethyl-N-methylamino) benzylphosphamides of oligonucleotides, which are targeted to the poly(A), poly(TG), and Alu repeats of eukaryotic DNA in chromatin and isolated nuclei from HeLa cells, has been investigated. It was found that the reagents alkylate DNA and some proteins due to specific complex formation. The affinity character of the reaction was proved by the fact that free corresponding oligonucleotides taken in excess or preliminary treatment of chromatin with S1 nuclease both prevent the biopolymers from the modification. Deproteinated DNA from the same cells does not react with oligonucleotide derivatives. This suggests that the chromatin DNA must have some structural features allowing oligonucleotide binding. Reactivity may be attributed to the existence of strongly negative supercoiled DNA regions containing single-stranded sequences or regions where DNA can unwind in the presence of complementary oligonucleotides. Results obtained suggest that in eukaryotic chromatin there are open DNA sequences available for affinity modification with oligonucleotide derivatives not only due to formation of triple helixes.