We report here the direct evidence for the formation of alternative DNA structures in a plasmid DNA, termed pTIR10, containing a pyrimidine/purine-biased (Pyr/Pur) stretch isolated from the rat genome. Long Pyr/Pur sequences are abundant in eukaryotic genomes, and they may modulate the biological activity of genes and genomes via formation of various types of triplex-related structures. The plasmid DNA in sodium acetate buffer (pH ) was deposited on APS-modified mica, and after drying it was imaged with an atomic force microscope in air. .
