PBAD Promoter
PBAD (systematically araBp) is a promoter found in bacteria and especially as part of plasmids used in laboratory studies. The promoter is a part of the arabinose operon whose name derives from the genes it regulates transcription of: araB, araA, and araD. In E. coli, the PBAD promoter is adjacent to the PC promoter (systematically araCp), which transcribes the araC gene in the opposite direction. araC encodes the AraC protein, which regulates activity of both the PBAD and PC promoters. The cyclic AMP receptor protein CAP binds between the PBAD and PC promoters, stimulating transcription of both when bound by cAMP.
Transcription initiation at the PBAD promoter occurs in the presence of high arabinose and low glucose concentrations. Upon arabinose binding to AraC, the N-terminal arm of AraC is released from its DNA binding domain via a “light switch” mechanism. This allows AraC to dimerize and bind the I1 and I2operators. The AraC-arabinose dimer at this site contributes to activation of the PBAD promoter. Additionally, CAP binds to two CAP binding sites upstream of the I1 and I2 operators and helps activate the PBAD promoter. In the presence of both high arabinose and high glucose concentrations however, low cAMP levels prevent CAP from activating the PBAD promoter. It is hypothesized that PBAD promoter activation by CAP and AraC is mediated through contacts between the C-terminal domain of the α-subunit of RNA polymerase and the CAP and AraC proteins.
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