Low Level Expression

For our in vivo work, we needed to be able to express reflectin at low levels, and control the level of expression reliably. Therefore, we expressed reflectin under an arabinose inducible promoter (pBAD) on a low copy plasmid ([http://partsregistry.org/Part:pSB3K3 pSB3K3]) in cells with a titratable response to arabinose.

pBAD Promoter

The pBAD promoter [http://partsregistry.org/wiki/index.php?title=Part:BBa_I0500 I0500] is tightly controlled by two factors:

• L-arabinose monosaccharide taken up by the cell from the medium, which acts as an inducer.
• AraC protein included in the I0500 biobrick, which acts an a repressor.

Therefore, the araC-pBAD system offers regulatable control of gene expression in the presence of the inducer and highly repressed in the absence of the inducer.

AraC protein functions as a homodimer. The monomer possesses two domain - a dimerization domain that also binds arabinose and a DNA-binding domain.
In the absence of arabinose, the repressor protein AraC in a dimerized form binds to the AraI1 operator site of pBAD and the upstream operator site AraO2. The resulting DNA loop sterically blocks access of RNA polymerase to the pBAD promoter, inhibiting transcription of the downstream genes. In the presence of arabinose, the monosaccharide binds to AraC and change its conformation such that it interacts with the AraI1 and AraI2 operator sites permitting transcription.

Arabinose System

The native arabinose system is used by E.coli to:

1. take up L-arabinose from the growth medium by high-capacity and low-affinity trasporters: AraE and AraFGH;
2. convert intracellular arabinose into D-xylulose-5-phosphate in three reactions catalyzed by the products of the genes from the pBAD operon;

It is mainly controlled by the AraC repressor, which regulates expression of its own synthesis and the other genes of the arabinose system:

In the absence of arabinose:

• AraC represses transcription initiation at the pBAD promoter.
• AraC represses expression of its own.

In the presence of arabinose:

• AraC activates transcription from the pBAD promoter.
• AraC stimulates transcription of araE and araFGH genes.
• AraC represses expression of its own.

E.coli Strain

In order to obtain a linear titratable relation between the concentration of arabinose in the medium and the level of reflectin expression, a special strain of bacteria [http://cgsc.biology.yale.edu/Strain.php?ID=111773 BW27783] needs to be used.

The linear induction with increasing arabinose concentration is not achievable in the standard E.coli strain, because the endogenous promoter which controls the araE gene is upregulated by an increasing concentration of arabinose. Thus, with higher level of the monosaccharide in the medium, more arabinose transporters are present in the plasma membrane and therefore the rate of uptake rises accordingly.

This induction of arabinose transporter can be circumvented by deleting the chromosomal araE gene and replacing it with a plasmid-borne copy of the araE under the control of a constitutive promoter. This is what has been done in the [http://cgsc.biology.yale.edu/Strain.php?ID=111773 BW27783] strain of E.coli, which we used in our experiment.

Constructs

In this experiment we used confocal microscopy to compare distribution of Reflectin A1 when it is expressed at high and low levels in E.coli cells. In order to do so, we used four different plasmids with Reflectin A1 gene expressed under the control of the pBAD promoter, and their assembly is described in this section. The constructs we worked on are the following:

Observations

When reflectin was expressed on a low copy plasmid, we saw fewer inclusion bodies than when expressed on a high copy plasmid.

Induction

Using a plate reader, we measured the expression of reflectin-GFP over time after inducing with arabinose. We saw that reflectin does not appear to be particularly toxic to E. Coli.

References

1. Schleif R. (2000) Regulation of the l-arabinose operon of Escherichia coli. Trends in Genetics, 12:559-565.