tac ( trp-lac ) promoter

The tac promoter was created by combining the tightly regulated lacUV5 operator with the trp promoter, which is three times as strong as the lac promoter. This hybrid promoter gives high-level expression utilizing the same induction and repression system as the lac promoter. IPTG or lactose functions as inducer, while the LacI repessor protein functions as repressor.The tac promoter is independent of cAMP regulation, therefore in contrast to the lac promoter media that contains glucose as carbon source can be used to express genes from this tac promoter. tac is a highly active promoter, it has incomplete repression in the uninduced state. To suppress transcription in the absence of induction the presence of the lac repressor (LacIq or overexpressed LacI) is necessary.