EARLY INTERMEDIATES IN SPERMIDINE-INDUCED DNA CONDENSATION ON THE SURFACE OF MICA

The folding pathway of spermidine-induced DNA condensation on the surf ace of mica was examined by varying the concentration of spermidine in a dilute DNA solution and visualizing intermediates by atomic force m icroscopy (AFM). Images reveal that spermidine-induced DNA condensatio n on mica involves multiple well-defined structural intermediates; At 1.5-3 mu M spermidine there are no interesting morphologies of the DNA , although there is a reduction of the apparent persistence length. At 7.5-15 mu M spermidine, intramolecular loops (mean diameter 40 +/- 15 nm) form. Loops initially appear to form independently of each other, but individual molecules with multiple loops tend to crossover at the same point producing ''flower''-shaped structures. At 30 mu M spermid ine, the tendency to form single crossover points increases and multim olecular flowers form. After initial flower formation disklike condens ates appear, sometimes as apparent outgrowths from flowers. The resolv able strands in:the disks are thicker than double-stranded DNA, sugges ting a close association of two or more DNA strands and, thus, a stabi lization of strand-strand interactions along the length of the DNA, Th is strand-strand stabilization is further indicated by the formation o f very large (>500 nm) multimolecular aggregates, at 150 mu M spermidi ne, composed predominantly of flowers and disks. These aggregates are initially planar with a monomolecular thickness and few crossover poin ts. At the highest spermidine concentrations examined growth in the th ird dimension is seen as additional layers; of condensates formed. The se results suggest that there are several intermediates early in Sperm idine-induced DNA condensation on mica, with well-defined characterist ics. Some of these intermediates have novel intra- and intermolecular contacts.