Aggregation of DNA by analogs of spermidine; enzymatic and structural studies

A homologous series of spermidine analogs, with defined abilities to replace the natural polyamine in supporting cell growth, was examined for its influence on the structure of supercoiled, aggregated DNA and on the ability of the DNA aggregates to act as substrates for various enzymes. The concentration of amine necessary to aggregate negatively supercoiled Col E1 DNA was progressively increased as the diaminobutane moiety of spermidine was extended beyond 5 methylene groups. 1H- and 31P-NMR spectroscopy suggested that less rigid DNA aggregates were formed by spermidine analogs than by spermidine itself. Spermidine and its analogs differentially modulated the activities of bacterial and mammalian type I topoisomerases and EcoRI restriction endonuclease on aggregated DNA in a manner reminiscent of the abilities of the amines to stimulate cell growth. When DNA was not aggregated, the influence of the various amines on these reactions was almost identical. These results are discussed in relation to the structures of the DNA aggregates in the presence of the various triamines.