Cooperative non‐specific DNA binding of the N‐terminal core of the cyclic AMP receptor protein of Escherichia coli and its modulation by cyclic AMP

Abstract
The non‐specific DNA binding of CRP and its N‐terminal core, αCRP, to a 298 base pair DNA fragment, in the presence and absence of cAMP, has been studied using the nitrocellulose filter binding technique and analysed quantitatively using the theory of Clore et al. [J. Mol. Biol. (1982) 155, 447–466]. It is shown that both CRP and αCRP bind cooperatively to DNA. At an ionic strength of 100 mM and pH 7.5, the intrinsic equilibrium association constant for the binding of αCRP to DNA is ∼ 10‐times smaller than that for CRP, but the cooperativity parameter is ∼ 17‐times larger for αCRP than CRP. cAMP exerts its effect soley on the intrinsic equilibrium constant and does not alter the cooperativity. In the case of αCRP, cAMP reduces the intrinsic equilibrium association constant by a factor of 3, in contrast to the case of CRP where cAMP increases it by a factor of 3. The possible location of the DNA binding site present in the N‐terminal core of CRP is discussed in the light of crystallographic data on the cAMP·CRP complex [McKay (1982) J. Biol. Chem. 257, 9518–9524].

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