Helical Periodicity of DNA, Poly(dA)·Poly(dT) and Poly(dA‐dT)·Poly(dA‐dT) in Solution

Helical periodicity of DNA, poly(dA)·poly(dT) and poly(dA‐dT)·poly(dA‐dT) has been measured in solution by using the band shift method of Wang [Wang, J. (1979) Proc. Natl Acad. Sci. USA, 76, 200–203]. The method makes use of the effect, on the superhelicity of closed circular DNA molecules, of the insertion of specific nucleotide sequences of known length. The method was applied to a variety of recombinant plasmid DNAs which were constructed by inserting DNA, poly(dA)·poly(dT) or poly(dA‐dT)·poly(dA‐dT) into pBR322 DNA. When compared to DNA, poly(dA)·poly(dT) was found to have a smaller pitch (by about 0.5 base pair/turn), whereas poly(dA‐dT)·poly(dA‐dT) has a slightly larger pitch (by 0.1 base pair/turn). These features correlate well with the known ability of the alternating copolymer to reconstitute nucleosomes upon incubation with histones, in contrast to the non‐alternating one which fails to do so. Finally, a detailed analysis of the principles underlying the methods developed by Wang [reference quoted above and Wang, J. (1978) Cold Spring Harb. Symp. Quant. Biol. 42, 29–33] leads to an increase in the estimate of the helical periodicity of DNA of 0.15 base pair/turn, over the reported value of 10.4 base pairs/turn (references quoted above). This essentially accounts for the discrepancy observed with the value of 10.6 base pairs/turn obtained by nuciease digestion of DNA immobilized on a surface [Rhodes, D. & Klug, A. (1980) Nature (Lond.) 286, 573–578].