Escherichia coli contains more than 250 known promoters that must be activated prior to transcription. cAMP receptor protein (crp), also known as cAMP regulatory protein and catabolite gene activator, is a 210 amino acid E. coli protein that complexes with cyclic AMP and binds to specific DNA sites near the promoter, resulting in homodimeric transcription activation of more than 100 E. coli promoters. Acting as a negative regulator against its own synthesis, crp regulates transcription of several catabolite-sensitive operons. Crp activates transcription by making multiple interactions with RNA polymerase. Containing one cyclic nucleotide binding domain and one HTH crp-type DNA binding domain, crp binds DNA as a dimer to induce a severe bend in the DNA. This formation is a regulatory mechanism of crp gene expression that plays a major role in determining the molecular mechanism in gene transcription.