- The Ptac promoter is a synthetic hybrid promoter created by combining elements of the trp and lac promoters, designed to offer strong, IPTG-inducible expression in Escherichia coli.
- Like the lacUV5 promoter, Ptac is regulated by the LacI repressor, which binds to the operator sequence and inhibits transcription in the absence of an inducer. When IPTG is added, it binds to LacI, causing it to release from the operator and thereby allowing transcription to proceed.
- The Ptac promoter retains the high-level transcription initiation strength of the trp promoter while preserving the inducibility of the lac system, resulting in a promoter that is stronger than the native lac promoter but still controllable.
- Ptac operates using the host’s native E. coli RNA polymerase, eliminating the need for T7 RNA polymerase or specialized strains like BL21(DE3). This makes it useful for more general expression needs, especially when working with proteins that do not require extremely high yields or when simpler host strains are preferred.
- However, the Ptac promoter can exhibit leaky expression—some basal transcription even without IPTG—which may be undesirable when expressing toxic proteins. To minimize this, researchers often co-express LacI at higher levels or use host strains with tighter repressor control.
- Overall, the Ptac promoter offers a balanced option between expression strength and ease of use, and is widely used in vectors like pGEX for producing tagged recombinant proteins.
