Catalysis of DNA reassociation by the Escherichia coli DNA binding protein: A polyamine-dependent reaction.

The Escherichia coli DNA binding protein, which binds co-operatively to single-stranded DNA, has been found to catalyse the formation of the DNA double helix from complementary strands in specified conditions. These conditions are different from the ones in which Alberts &; Frey (1970) found catalysis of DNA reassociation by the binding protein coded by gene 32 of phage T4. Although a 300-fold catalysis is observed in 10 mm-Mg²⁺ at pH 5·5, the catalytic efficiency of the binding protein drops sharply above pH 6 and is negligible at pH 7. Substitution of Ca²⁺ for Mg²⁺ extends slightly the pH range of strong catalysis up to pH 6·4, but catalysis again is slight at pH 7. When only Na⁺ or K⁺ is present, no catalysis is observed.At pH 7 catalysis appears to be a polyamine-dependent reaction: at 2 mm-spermidine a 5000-fold catalysis is found over a broad pH range, and spermine gives even higher rates. The mechanism of catalysis has not yet been studied in detail, but four properties of the reaction are noted here. (1) The DNA reassociation reaction follows apparent second-order kinetics. (2) For effective catalysis, the DNA binding protein must be in excess. (3) The catalytic efficiency increases strongly with DNA length. (4) The complex dependence of catalysis on the type of counterion and on pH suggests that other factors are involved in addition to melting out hairpin helices in the DNA single strands.The effect of the DNA binding protein on the rate of joining of the bacteriophage λ cohesive ends has also been studied, using a gel electrophoresis assay, for the joining of the EcoRI restriction fragments from the left and right hand ends of λ DNA. No catalysis of this joining reaction has been found.