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T7 promoter variants

We made two families of T7 promoter variants. One family has mutations on the T7 promoter consensus sequence while the other has the same set of mutations on the consensus sequence but also has a lac operator downstream of the T7 promoter (but upstream of the reporter RFP or Lysis). In each family, we made six designed variants with different predicted promoter strengths compared to the wildtype and also made three sets of randomer variants which we wanted to use to check the overall range of promoter strength.

The Making Of A Variant

To produce these T7 promoter variants, we used a two-step PCR process. The first PCR, which we call "gene-specific" PCR, is a typical PCR that adds a ribosome-binding site (rbs) in front of either RFP or the Lysis operon and adds a terminator downstream.

With this PCR product now serving as the DNA template, we start a second PCR which we call the "extension" PCR. It extends the product by adding the T7 promoter, with or without a lac operator downstream. In these illustrations, we have substituted the lysis operon for the RFP gene but the same process is done for RFP.

In the last stage, we run another PCR with a set of primers that will add Gibson overhangs for the Gibson assembly that will add this promoter construct into the desired plasmid.

Characterization with RFP

With T7 and T7-lac promoter variants in hand, we want to characterize their relative strengths. To obtain the right set-up, we transform the promoter variant plasmids into a different strain of E. coli called BL21. These cells have a mutation in the promoter for the lacI gene that makes LacI protein. As a result of this mutation, LacI protein is overproduced and is found abundantly in these cells. In this same strain, the gene for the T7 RNA polymerase is preceded by a lac operator. Since LacI is a repressor and LacI is strongly present in BL21 cells, the production of T7 RNA polymerases is severely repressed. With very few T7 RNA polymerases available,

DNA Recovery with Lysis