ISSN:
1573-0689
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Physics
Notes:
Abstract On the assumption that the dependence of the electrophoretic mobility of superhelical DNA upon the number of tertiary turns (Wr) is a monotonously increasing function devoid of points of inflection, it is concluded that the inflection (change of sign of the first derivative) observed on the curve gives evidence for a conformational transition in DNA secondary structure that begins long before the B→Z or B→ cruciform transitions. The transition consumes ≈ 60% of the topological turns at native levels of supercoiling. It is proposed that the conformation produced belongs to the A-family. Provided that this transition indeed yields the A form (11 base pairs per turn), the energy of the B→A conformational transition is estimatd to be 5.8–10.3 cal per base pair for different nucleotide sequences at physiological ionic strength. The energies of B→Z and B→ cruciform transitions in superhelical DNA estimated from electrophoretic mobilities by the present method coincide perfectly with the values obtained by other authors using other methods. In addition, on the basis of the data of Brady et al. (1983) on the number of tertiary turns in superhelical DNA determined by X-ray scattering, it is concluded that the initial assumption is justified and the ratio of bending to twisting stiffnesses of superhelical DNA is estimated as 7∶1 (in the fully supercoiled molecule containing 50% of the supposed A-conformation).
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01857795
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