Preliminary chemical and X-ray studies on the interactions of E. coli DNA with putrescine.

: The interactions of putrescine, the major diamine in E. coli, with E. coli DNA as a model of phage DNA were studied by melting temperature analysis, equilibrium dialysis and X-ray diffraction with the aid of molecular model building. The chemical analysis of the DNA-putrescine complex shows that the molar binding ratio of putrescine to DNA (phosphate) is nearly 1 to 2. The equilibrium (or reversible) binding of putrescine to DNA was suggested by the fact that the melting temperature increased according to the concentration of added putrescine, and its elevation was not saturated even at the molar ratio of 6 to 1. The equilibrium dialysis experiments indicate that the association constant for the complex is a little smaller than, but in the same order (10(3) liter/mol) as, that of DNA-spermine complex. The binding of putrescine stabilizes the B-form of DNA fiber, which is well preserved even at 66% relative humidity. The distance between the neighboring DNA helices in the wet fiber increased with the increasing degree of hydration, as in the case of native DNA. Unlike spermine, putrescine does not form precipitate upon mixing with DNA in the concentration range for UV measurement, suggesting that the cross-bridge formed by putrescine is intra-double helical. The equilibrium binding of putrescine to DNA, seems to be important for the life cycle of lambda-phage.