DNA binding of cAMP receptor protein and its N‐terminal core stabilizes the double helix and is modulated by the allosteric effector cAMP

3’3’~Cyclic AMP (CAMP) receptor protein (CRP) binds sequence specitically to a site at or near promotors of catabolite-sensitive operons in Escherichia coli, stimulating the initiation of mBNA synthesis in the presence of CAMP [l-7]. The smaller carboxyterminal domain of both subunits of CBP may bind to a left-handed helix, resulting in destabilisation of the helix and facilitation of binding and of initiation of mBNA synthesis by RNA polymerase [8]. This model was suggested on the basis of computer modelling on the interactions between DNA and the polypeptide acarbon backbone of CBP, based on the 2.9 A resolution crystal structure of the CAMP CBP complex [8]. If this hypothesis is correct, the N-terminal core of CBP, aCBP, produced by subtilisin digestion of the CAMP CBP complex, in which the small carboxy-terminal domain has been removed [9], should not bind to DNA. Electron microscopy has shown that the binding of CBP non-specifically to DNA results in the formation of a complex with regular striations along the DNA whose length is -× shorter than that of free DNA [lo]. This observation suggests that the binding of CRP alone might result in stabilisation of double stranded DNA. Here, we present a thermal denaturation study on double stranded poly [d(AT)] complexed with CBP and aCW in the presence and absence of CAMP. We demonstrate that both CRP and aCRF’stabilise the double stranded structure of poly [d(AT)], and that this effect is modulated by CAMP. 2. Experimental