GFP controlled by TetR+LVA and pTet.
This composite part has GFP controlled by a TetR, p(TetR) expression system. The TetR repressor protein binds to the Tet promoter repressing expression.  The tet repressor protein is inhibited by ATc (anhydrotetracycline) or the analogue dox (doxycycline) hereby inducing the expression of GFP.
Please note that this part contains the TetR repressor with LVA rapid degradation tag, for a similar part containing TetR repressor without LVA tag please see Part:BBa_K1475005. The TetR without LVA tag can be found here as a basic part: Part:BBa_ K1475003.
Usage and Biology
This part can be used to test the expression from the TetR p(TetR) system.
Sequence and Features
- 10COMPATIBLE WITH RFC
- 12Illegal NheI site found at 7
Illegal NheI site found at 30
- 21COMPATIBLE WITH RFC
- 23COMPATIBLE WITH RFC
- 25COMPATIBLE WITH RFC
- 1000Illegal BsaI.rc site found at 1485
It has been investigated if the Tet promoter could be fine-tuned, and what influence the LVA tag on TetR has on the expression.
The ligation of TetR(no LVA) construct with the pTet-GFP construct was cloned done successfully and can be found as BBa_K1475005.
The promoters in the TetR-pTet constructs are supposed to be inhibited by TetR. By induction with doxycycline, the repressor is inhibited, and thus pTet will be active. In this case, GFP will be expressed after induction with doxycycline. 
To test if the Tet promoter could be fine-tuned using different concentrations of doxycycline, FACS (Fuorescence-activated Cell Sorting) was made on E. coli expressing GFP controlled by pTet, regulated by TetR with and without LVA tag. A wild-type was used as control.
Comparing only the strains expressing either variants of TetR, the results of the FACS illustrates that without induction with doxycycline, GFP is still expressed. Most likely because the promoter is leaky. Despite 100% of the cells being fluorescent in the absence of doxycycline one can see that the fluorescence intensity is makedly reduced in the constructs containing TetR repressor. There is a very little variation in expression of GFP upon induction with low concentration of doxycycline. At high concentration of doxycycline (2000 ng/mL) it can be seen that TetR (+LVA) inhibits pTet at a weaker extent than TetR without LVA.
Although the FACS results indicates that the pTet inhibited by TetR with LVA tag is the most responsive upon induction by doxycycline, it can be argued that the effect seen is due to overexpressing of TetR repressor. The hypothesis is based on the poor median fluorescence compared to un-regulated pTet promoter, even at doxycycline concentrations inhibitory of cell growth. pSB1C3 being a high copy plasmid leads to a high number of repressors, thus a higher concentration of doxycycline in needed to induce the expression from pTet. The LVA tag destabilizes TetR thus lovering the number of TetR proteins. This could explain the better response from induction of TetR with LVA. It can be seen from the Coomassie stain below that there is less TetR repressor with LVA than without, supporting this hypothesis.
By using a strain, constitutively expressing tetR with pTet on a low copy plasmid UNIPV-Pavia iGEM 2011 shows here: Part:BBa_R0040:Experience that pTet can be induced by aTc. Thus less TetR repressors in comparison to pTet sites increases the response to inducer, futher supporting the hypothesis.
To analyse the amount of TetR with and without LVA tag present in the cell, Coomassie stain was made on a SDS-PAGA with E. coli K12 (induced by 0 ng/mL, 50 ng/mL, 100 ng/mL, 200 ng/mL, 500 ng/mL, 1000 ng/mL and 2000 ng/mL doxycycline) expressing pTet-GFP, pTet-TetR (no LVA)-GFP and pTet-TetR (+LVA)-GFP, respectively.
The Coomassie stain shows that the construct expressing TetR(+LVA) expresses more GFP than the construct expressing TetR(no LVA). In addition to this, the stain shows a higher amount of TetR(no LVA) in the cell than of TetR(+LVA). This is consistent with the FACS results that illustrates that pTet-TetR(+LVA) expresses more GFP than pTet-TetR(no LVA). The Coomassie stain indicates that the reason for the higher expression of GFP by pTet-TetR (+ LVA) is because the cell contains less inhibitor. This must be due to the LVA tag making TetR unstable and tagging it for degredation.
Because pTet is leaky, all cells express GFP. It can be difficult to tell if the pTet has been induced and to what extent, however, plates containing the corresponding concentrations of doxycycline as used in FACS clearly show an induction.
Duplicates of plates with doxycycline were made with 0 ng/mL, 50 ng/mL, 100 ng/mL, 200 ng/mL, 500 ng/mL, 1000 ng/mL and 2000 ng/mL doxycycline. On the plates, TetR-pTet construct with LVA, TetR-pTet construct with no LVA, pTet-GFP without TetR construct and wild-type were plated.
The plating of TetR-GFP constructs on plates with doxycycline shows that GFP is expressed at different levels at different concentrations of doxycycline. Expression increases with an increase in doxycycline concentrations. The plates also show that GFP, to some extent, is expressed without doxycycline. This indicates that the Tet promoter is leaky and is not fully inhibited by TetR as it was also seen from the FACS results. Furthermore, the plating assay proves that the bricks are functional, however slowly responding to induction (continuous induction over 24 hours compared to induction over 1 hour).
To see how the growth of the bacteria expressing GFP controlled by pTet are affected, OD600 was measured over 8 hours. The experiment was made with biological triplicates of bacteria with an empty vector, pTet-GFP, pTet (no LVA)- GFP, pTet (+LVA)-GFP and a wild-type.
Figure 3 shows the growth of bacteria expressing GFP constitutiely, are attenuated the most with most comprised growth. Removing the LVA tag from TetR also has a negative effect on the growth of the bacteria. This could be because TetR without LVA stresses the metabolism of the bacteria more than TetR with LVA or because LVA tags TetR for degradation and thus TetR with LVA stresses the cell less than TetR without LVA.
1. C. Krafft, et al.: Interaction of Tet Repressor with Operator DNA and with Tetracycline Studied by Infrared and Raman Spectroscopy. Biophysical Journal, Volume 74, Issue 1, January 1998, Pages 63–71. http://www.sciencedirect.com/science/article/pii/S0006349598777677
2. Tetsystems, 2008: Principles and Components Description. http://www.tetsystems.com/science-technology/principles-components
3. Cormack, B.P., Valdivia, R.H., and S. Falkow. FACS-optimized mutants of green fluorescent protein (GFP). Gene 173: 33-38 (1996). http://www.sciencedirect.com/science/article/pii/0378111995006850
4. Aagaard, L., et al.: A Facile Lentiviral Vector System for Ekspression of Doxycycline-Inducible dhRNAs: Knockdown of the Pre-miRNA Processing Enzyme Drosha. Molecular Therapy, 2007. 15:5, p. 938-945. http://www.nature.com/mt/journal/v15/n5/full/6300118a.html