Properties of cyclic AMP-independent catabolite gene activator proteins of Escherichia coli.

The protein products of two crp alleles encoding mutationally altered catabolite gene activator proteins CAP and CAPc, which are functionally active in vivo in the absence of cAMP, were purified by an immunoaffinity purification procedure. These proteins bind cAMP with the same affinity as does the wild-type catabolite gene activator protein. From their susceptibility to the proteolytic enzyme subtilisin, we conclude that the two mutationally altered proteins adopt structural features adequate for biological activity and similar to the conformation that cAMP elicits or stabilizes in wild-type catabolite gene activator protein. We note, however, that their conformation is not unique and can be modulated by cAMP. The two altered proteins, CAP and CAPc, bind to the lactose promoter, giving rise to specific DNA-protein complexes in the absence of cAMP and promote initiation of specific lac messenger RNA synthesis.