Letter | Published:

Theoretical analysis of transitions between B- and Z-conformations in torsionally stressed DNA

Naturevolume 286pages637638 (1980) | Download Citation



Recently, a new structure called Z-DNA has been proposed for alternating poly(dG-dC)·poly(dG-dC) sequences based on crystallographic analysis of the hexanucleotide1. The Z-form is a left-handed double helix containing 12 base pairs per turn. In contrast, the Watson–Crick B-form helix is thought to have 10.4 pairs per turn of right-handed helix2,3,8. A cooperative, salt-induced conformational transition has been observed in poly(dG-dC)·poIy(dG-dC)4, which has been interpreted as being between the B-form (low salt) and the Z-form (high salt)1. We now analyse the possibility that such transitions could occur in susceptible sequences in physiological conditions as a consequence of the torsional stresses imposed by superhelicity. As in vivo DNA commonly occurs in a negatively supercoiled, hence underwound, state, these transitions could serve important biological functions. Both thermodynamic and statistical mechanical theories of stress-induced two-state transitions have been developed previously5,6. Here we apply these theories to transitions between the B-form and the Z-form in regions of appropriate base sequence. Assuming that the unstressed duplex is entirely B-form, we show that when the molecule is constrained to be underwound, susceptible regions may transform to the Z-form, thereby absorbing most of the torsional deformation. Interestingly, this transition is relatively independent of temperature.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.


  1. 1

    Wang, A. et al. Nature 282, 680–686 (1979).

  2. 2

    Wang, J. Proc. natn. Acad. Sci. U.S.A. 76, 200–204 (1979).

  3. 3

    Levitt, M. Proc. natn. Acad. Sci. U.S.A. 75, 640–644 (1978).

  4. 4

    Pohl, F. & Jovin, T. J. molec. Biol. 67, 375–396 (1972).

  5. 5

    Benham, C. Proc. natn. Acad. Sci. U.S.A. 76, 3870–3874 (1979).

  6. 6

    Benham, C. J. chem. Phys. 72, 3633–3639 (1980).

  7. 7

    Barkley, M. & Zimm, B. J. chem. Phys. 70, 2991–3007 (1979).

  8. 8

    Crick, F., Wang, F. & Bauer, W. J. molec. Biol. 129, 449–461 (1979).

Download references

Author information


  1. Department of Mathematics, University of Kentucky, Lexington, Kentucky, 40506

    • Craig J. Benham


  1. Search for Craig J. Benham in:

About this article

Publication history



Issue Date



Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.