Interaction of drugs with Z-DNA: cooperative binding of actinomycin D or actinomine to the left-handed forms of poly(dG-dC).cntdot.poly(dG-dC) and poly(dG-m5dC).cntdot.poly(dG-m5dC) reverses the conformation of the helix

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Abstract
The interaction of actinomycin D and actinomine with poly(dG-dC) .cntdot. poly(dG-dC) and poly(dG-m5dC) .cntdot. poly(dG-m5dC) under B- and Z-form conditions has been investigated by optical and phase partition techniques. Circular dichroism data show that the conformation at the binding site is right-handed, even though adjacent regions of the polymer have a left-handed conformation. Actinomycin D binds in a cooperative manner to poly(dG-dC) .cntdot. poly(dG-dC) under both B-form and Z-form conditions. Analysis of the circular dichroism data shows that 5 .+-. 1 base pairs of left-handed poly(dG-dC) .cntdot. poly(dG-dC) in 4.4 M NaCl switch to a right-handed conformation for each bound actinomycin D. When the left-handed form of poly(dG-dC) .cntdot. poly(dG-dC) is stabilized by the presence of 40 .mu.M [Co(NH3)6]Cl3, 25 .+-. 5 base pairs switch from a left-handed to a right-handed conformation for each bound actinomycin D. Actinomine binds cooperatively to left-handed poly(dG-dC) .cntdot. poly(dG-dC) in 40 .mu.M [Co(NH3)6]Cl3 and to left-handed poly(dG-m5dC) .cntdot. poly(dG-m5dC) in 2 mM MgCl2. Actinomine does not bind to left-handed poly(dG-dC) .cntdot. poly(dG-dC) in 4.4 M NaCl at concentrations as high as 100 .mu.M. Each bound actinomine converts 11 .+-. 3 base pairs of left-handed poly(dG-dC) .cntdot. poly(dG-dC) in 40 .mu.M [Co(NH3)6]Cl3 and 7 .+-. 2 base pairs of left-handed poly(dG-m5dC) .cntdot. poly(dG-m5dC) in 2 mM MgCl2. The binding isotherm data also indicate that the binding site has a right-handed conformation. The actinomycin D-poly(dG-dC) .cntdot. poly(dG-dC) binding isotherm in 4.4 M NaCl may be fit by an allosteric binding model, which also indicates that the conformation of poly(dG-dC) .cntdot. poly(dG-dC) is altered only in the vicinity of the bound ligands and that intercalation into a right-handed binding site is much more favorable than intercalation into a left-handed helix. The conversion of the polymer in 4.4 M NaCl from a left-handed to a right-handed conformation occurs in a sequential manner, forming regions of right-handed poly(dG-dC) .cntdot. poly(dG-dC) that are nearly saturated with actinomycin D. The important factors controlling the binding are the charge of the drug and the forces stabilizing the left-handed conformation. The biological implications of the simultaneous existence of B and Z forms by these polymers are discussed.