araC

araC is a gene that encodes the AraC transcriptional regulator in Escherichia coli and related bacteria. The AraC protein belongs to the AraC family of transcriptional regulators and typically consists of about 280 amino acids. It functions as a dimer and acts as both a repressor and an activator of transcription, depending on the presence of L-arabinose.

The primary role of AraC is to control the arabinose operon, especially the araBAD genes that enable utilization of arabinose as a carbon source. In the absence of arabinose, AraC binds to two operator sites near the araBAD promoter, promoting DNA looping that represses transcription. When arabinose is available, the sugar binds to AraC, triggering a conformational change that enables the protein to activate transcription by facilitating RNA polymerase recruitment at the P_BAD promoter.

Structurally, AraC has an N-terminal regulatory domain that binds arabinose and senses its presence, and a C-terminal DNA-binding domain containing helix-turn-helix motifs that attach to DNA. Dimerization of AraC is important for its regulatory activity and for the switch between repression and activation.

Because AraC regulates the arabinose-responsive promoter, the AraC–P_BAD system is widely used in molecular biology and synthetic biology as an inducible gene expression system. The promoter is activated by arabinose and tightly controlled by AraC, allowing graded expression and easy modulation of gene expression in bacteria. Variants and homologs of AraC are found across diverse bacteria, reflecting the broad utility of this regulatory family.