Evidence for a salt-induced conformational transition in UV-irradiated superhelical PM2 DNA.
Upon treatment with UV irradiation, native (supercoiled) PM2 DNA undergoes an increase in electrophoretic mobility relative to the nicked circular form in the presence of 1 M NaCl or 5 mM CaCl2 or MgCl2. This effect is dependent upon supercoiling in that the relative electrophoretic mobility decreases with decreasing superhelical density of the molecule. These findings indicate that supercoil-dependent aspects of the secondary and tertiary structure of nonirradiated PM2 DNA can be altered by a combination of UV irradiation and any of the ionic environments above. We show that the alteration is not the result of a conversion of Z-DNA segments to a right-handed helix or to a renaturation of denatured regions in PM2 DNA. Circular dichroism studies do not support a simple model in which A-form DNA induced by superhelical stress is converted to B-form DNA by UV-induced photodamage and salt. We, therefore, present three alternative explanations for these observations two of which invoke conformational transitions in secondary structure and a third which requires a change in tertiary structure due to an increase in flexibility.
Peterson
C A
CA
Department of Molecular Genetics, University of Texas M.D. Anderson Cancer Center, Houston 77030.
Gray
D M
DM
Gray
H B
HB
Legerski
R J
RJ
D001435:Bacteriophages / Q000235:genetics*; D002122:Calcium Chloride; D002942:Circular Dichroism; D004277:DNA, Single-Stranded / Q000737:chemistry; D004278:DNA, Superhelical / Q000737:chemistry*; D004279:DNA, Viral / Q000737:chemistry; D004587:Electrophoresis, Agar Gel; D015636:Magnesium Chloride; D008968:Molecular Conformation; D012965:Sodium Chloride; D014466:Ultraviolet Rays
D004277:DNA, Single-Stranded; D004278:DNA, Superhelical; D004279:DNA, Viral; D015636:Magnesium Chloride; D012965:Sodium Chloride; D002122:Calcium Chloride