Reversible toroidal compaction of DNA by aluminum.

Electron microscopy has been used to characterize the products of the reactions of aluminum with DNA under three different conditions, one of which, pH 5 and Al (III)/DNA(P) ratio of 0.4, has been previously shown to produce reversible interstrand crosslinking in double-stranded DNA molecules. Under this condition, aluminum produced macromolecular aggregates of DNA upon heating, with a distinctive ultrastructure reversible to double-stranded DNA after removal of the aluminum. These structures were toroidal in configuration and exhibited mean widths of 4.9 +/- 1.8 nm and were 18.6 +/- .4 nm in diameter with a toroidal internal diameter of 7.4 +/- 4.7 nm. Previous results have shown that these structures contain Al(III) - crosslinked DNA, the present data suggests that this intermolecular crosslinking is associated with the production of compacted structures.