2. The trp-lac (tac) promoter- it is a hybrid promoter where -10 region from lac UV5 promoter is fused with the -35 region of trp promoter. Ex. of plasmid is pKK223-3

3. The trp-lac (trc) promoter- it is similar to tac promoter except that distance separating -10 and -35 region of promoter is different from the tac prommoter. Ex. of plasmid pTrc 99A.

Bacteriophage λ P L promoter- This promoter keeps the tight control over the protein production. It is regulated by the presence of repressor cIts857 to either repress the transcription or not. cIts857 is temperature sensitive and degraded at high temperature and consequently in a temperature depenent fasion it represses the transcription at low temp but not at high temperature. This promoter is useful in the cases where the protein is toxic in nature.

Bacteriophage T7 Promoter- Similar to Bacteriophage λ P_L promoter, T7 RNA polymerase promoter is used to design plasmid with tight control on the protein production. These vectors contains most of the structural blocks from pBR322 and MCS is in front of the T7 promoter to drive the transcription of the insert. Hence, vector contains foreign gene in front of the T7 promoter for expression. The host E.coli also need modification to suits the T7 promoter expression system. Host E.coli is either transformed with a plasmid which carries the T7 RNA polymerase gene or the T7 RNA polymerase gene is integrated into the bacterial chromosome. In few host strain T7 RNA polymerase is placed under the control of IPTG inducible lacUV5 promoter to tightly control the production of T7 RNA polymerase. A schematic is given in Figure 24.3 to explain the control mechanism in T7 promoter based expression system. After induction with IPTG, the inducer binds the lac repressor and stimulate the production of T7 RNA polymerase using E.coli RNA polymerase. The T7 RNA polymerase binds to the T7 promoter and drive the transcription of the target gene to eventually give large amount of protein.