Cationic metals promote sequence-directed DNA bending

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Abstract
A DNA segment of .apprx. 200 base pairs (bp) from Crithidia fasciculata kinetoplast minicircles was previously shown by electron microcospy (EM) to bend into a small circle due to its unique nucleotide sequence containing repeated blocks of 4-6 A''s. When this segment was flanked by 207 bp of plasmid DNA on one side and 460 bp on the other, the resulting 890-bp DNA was found to appear either relatively straight or extremely bent as visualized by EM. The bend was located one-third the distance from one end. The fraction of molecules with the extreme bend increased from .apprx. 2% to 50-60% following incubation of the DNA with increasing concentrations of Zn2+, Co2+, Ba2+, and Mn2+. These observations suggest that sequence-directed bending in DNA is an inducible and not a static phenomenon. Possible roles of transitions between the bent and straight conformations in the control of gene expression are discussed.