Abolition of intrinsically bent DNA structure components in AT clusters by netropsin interaction; titration viscometric investigations.

It is argued that the enhancement of the apparent DNA contour length by the specifically binding non-intercalating drug netropsin (Nt) (Reinert et al., NAR 9, 2335, 1981) at very low Nt/DNA-phosphate ratios essentially is the result of an abolition of periodically arranged intrinsic helix bends in A.T rich tracts of base pairs. In the preceding paper the existence of pronounced DNA tertiary structure components has been postulated for (two species of) natural eukaryotic DNA. The resulting model suggests local apparent solenoid-related DNA tertiary structure components at high sodium ion concentration cs, partly/totally molten out at 45/60 C. With decreasing cs the tertiary structure components have been found to be gradually reduced, at least below cs = 0.010 M, as titration viscometrically revealed by a gradual rise of the apparent DNA contour length (Reinert et al., JBSD 9, 537, 1991). Hence, we performed titration viscometric analyses about Nt interaction with calf thymus DNA (ctDNA) at cs = 0.075 M, 0.010 M and 0.004 M Na+. The concomitant DNA conformational changes are quantitatively described in terms of the relative changes of both DNA persistence length and hydrodynamically operative apparent DNA contour length for the three first resolved interaction modes below a Nt/DNA-P ratio of 0.03. These experiments, together with previous respective analyses at cs = 0.20 M Na+ and different temperatures (l.c.), suggest that those DNA sites binding Nt most strongly predominantly are responsible for the formation of solenoid-related DNA tertiary structure components. Most probably these are A tract-containing sequences. As the essential factor for their apparent elongation effect at low Na+ concentrations, a gradual alteration of the number of base pairs per helix turn seems to occur below cs = 0.010 M Na+ and, concomitantly, a change in phasing between intrinsic helix bends and helix screw.