Functional roles of the two cyclic AMP-dependent forms of cyclic AMP receptor protein from Escherichia coli

The cyclic AMP receptor protein activates transcription in Escherichia coli , only when complexed with cyclic AMP. The cyclic AMP receptor protein-cycl ic AMP complex formed at low concentrations of cyclic AMP has a different c onformation from either cyclic AMP receptor protein alone or its complex wi th cyclic AMP formed at high cyclic AMP concentrations. Various biophysical data suggest that the latter complex resembles free cyclic AMP receptor pr otein. We have examined the conformational and biological properties of cyc lic AMP receptor protein as a function of cyclic AMP concentrations, using the gal operon of E, coli. A biphasic behavior is observed. it is shown tha t only the complex formed at lower concentrations of cyclic AMP is the tran scriptionally active form, This difference between the complexes at differe nt levels of cyclic AMP arises from a decreased ability of the cyclic AMP r eceptor protein-cyclic AMP complex at high cyclic AMP concentrations to bin d to DNA at specific sites, (C) 1999 Federation of European Biochemical Soc ieties.