Article,
Early intermediates in spermidine-induced DNA condensation on the surface of mica
Journal of the American Chemical Society, 120 (35): 8903--8909 (1998)
Abstract
The folding pathway of spermidine-induced DNA condensation on the
surface of mica was examined by varying the concentration of spermidine
in a dilute DNA solution and visualizing intermediates by atomic
force microscopy (AFM). Images reveal that spermidine-induced DNA
condensation 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 spermidine, the tendency to form single crossover points
increases and multimolecular flowers form. After initial flower formation
disklike condensates appear, sometimes as apparent outgrowths from
flowers. The resolvable strands in:the disks are thicker than double-stranded
DNA, suggesting a close association of two or more DNA strands and,
thus, a stabilization of strand-strand interactions along the length
of the DNA, This strand-strand stabilization is further indicated
by the formation of very large (>500 nm) multimolecular aggregates,
at 150 mu M spermidine, composed predominantly of flowers and disks.
These aggregates are initially planar with a monomolecular thickness
and few crossover points. At the highest spermidine concentrations
examined growth in the third dimension is seen as additional layers;
of condensates formed. These results suggest that there are several
intermediates early in Spermidine-induced DNA condensation on mica,
with well-defined characteristics. Some of these intermediates have
novel intra- and intermolecular contacts.
