Question

The λ PE promoter is known to have a weak promoter. Explain the reason for this by comparing the λ PE promoter -35 element to a consensus -35 element.

Answer 1

Every gene contains a promoter region before the presence of the start sequence for transcription. The RNA polymerase binds to the gene by identifying the molecular sequence of the promoter region. The nature of interaction between the RNA polymerase and the promoter region has been evolved in a manner such that presence of a consensus and conserved sequence determines the extent of binding of RNA polymerase to the promoter. Any deviation from the consensus and conserved sequence of this upstream promoter region, i.e. the -35bp pCG sequence and the -10bp TATA box, lead to either loose binding of the RNA polymerase or altogether failure of binding.

Even under cases of loose binding of RNA polymerase, the gene expression is significantly down-regulated. This is because the loose binding leads to frequent disconnection between the gene sequence and the RNA polymerase thus preventing constant and un-interrupted movement of the RNA polymerase on the gene. This is why genetic engineering employs use of strong promoters before insertion of the gene of interest in a target organism.

Here, the lambda PE promoter seems to have low strength and weak binding. Thus, any gene placed post this promoter will be less expressed. Thus, the promoter region of lambda PE has been compared to the consensus and conserved sequence of the strong -35 element of strong promoters.