[Detection of left-helical segments in eukaryotic DNA].

The method of DNA binding to nitrocellulose filters was applied to DNA isolated from mouse liver and Ehrlich ascite carcinoma (EAC), calf thymus, and lymphocytes from patients with chronic lymphoid leukemia. In those and phage PM2 DNA the increase in the DNA binding to the filters with a rise in NaCl concentration from 0.5 up to 4.5 M was sigmoidal being suggestive of a conformational transition. No such activity was found in the case of phage lambda or single-stranded DNA. The binding decreased dramatically after mild cleavage of DNA with DNAase I or treatment with phospholipase C or Eco RI and Hin PI restrictases. Incubation of DNA with ethidium bromide led to decrease in the amount of bound DNA. This effect was enhanced with a rise in the dye concentration. The isotherms of ethidium bromide binding to eukaryotic DNA obtained in Scatchard plots by optic titration had a component with a positive slope at low values of r. Bivalent ions (Mg2+, Zn2+) shifting the equilibrium towards the Z-form increased the proportion of macromolecules retained on the filters at NaCl concentrations of 1-3 M. Local changes in the helix conformation were studied with the help of chemical probes: diethylpyrocarbonate (guanine Z-DNA) and osmium-pyridine reagent (pyrimidines of boundary B-Z sites). These probes incorporation into samples of liver DNA, EAC, and lymphocytes resulted in chemical modification of all these samples. Modification of DNA by osmium-pyridine reagent led to inhibition of subsequent restriction by Eco RI restrictase. The data obtained are suggestive of the presence of Z-regions in the B-helix of eukaryotic DNA. A topological model of Z-site stabilization in small superhelical loops of DNA fixed by protein or lipoprotein molecules is proposed.