Regulatory

Part:BBa_K1847008

Designed by: Anna Fomitcheva Khartchenko   Group: iGEM15_ETH_Zurich   (2015-08-26)
Revision as of 22:28, 27 September 2015 by Milhene (Talk | contribs)

lldRO1-J23117-lldRO2

Collection of promoters regulated by LldR.

Sequence and Features


Assembly Compatibility:
  • 10
    COMPATIBLE WITH RFC[10]
  • 12
    INCOMPATIBLE WITH RFC[12]
    Illegal NheI site found at 78
    Illegal NheI site found at 101
  • 21
    COMPATIBLE WITH RFC[21]
  • 23
    COMPATIBLE WITH RFC[23]
  • 25
    COMPATIBLE WITH RFC[25]
  • 1000
    COMPATIBLE WITH RFC[1000]

Usage and Biology

Figure 1. Theoretical model for the dual function as repressor and activator of LldR.

The natural promoter of LldR (Part:BBa_K822000) consists of two operators (O1 and O2) and a promoter which is intercalated between the operators. It regulates the expression of the lldPRD operon, and it is involved in L-lactate metabolism. This promoter is repressed by a dimer of LldR, possibly by forming a DNA loop that does not allow the RNA polymerase to bind to the promoter. LldR can also have a function as an activator [1]. The repression of the promoter can be removed by lactate.

Characterization of the promoter

We wanted to know how the promoter would respond to the lactate presence and how important is the architecture of the promoter in this response. For this regard, we built several promoters. Part:BBa_K1847002, Part:BBa_K1847003, Part:BBa_K1847004, Part:BBa_K1847005, Part:BBa_K1847006, Part:BBa_K1847007, Part:BBa_K1847008, and Part:BBa_K1847009.

Genetic design for characterization with LldR

To test our promoter we designed a plasmid containing the promoter with sfGFP in a medium copy plasmid pSEVA261 and transformed it into Escherichia coli TOP10. Then, we added a second plasmid containing lldR with a medium strong promoter (Part:BBa_J23118) and a strong RBS (Part:BBa_B0034) in pSEVA371 backbone. According to literature [1], LldR will repress the promoter and by the addition of lactate the production of GFP will start, as the repressor will be removed.

Genetic design for characterization with LldP-LldP

The second step for our characterization was to add an L-lactate permease (LldP), which despite its name is an active transporter of L-lactate, D-lactate and glycolate. To understand the effect of LldP (L-lactatep permease) in our system, we tried the following conditions:

  • Medium copy plasmid with the promoter and sfGFP+ J23114-B0032-lldP-lldR (LldP and LldR are under the control of the same weak promoter) in low copy number plasmid
  • Medium copy plasmid with the promoter and sfGFP + J23118-B0032-lldP-lldR (LldP and LldR are under the control of the same medium promoter) in low copy number plasmid

Characterization in a different medium

DMEM and RPMI mediums for mammalian cells where used to characterize this promoter. Two conditions were tested:

  • Plasmid with the promoter and sfGFP + J23118-B0032-lldR
  • Plasmid with the promoter and sfGFP+ J23118-B0034-lldP-lldR

Plate reader

E. coli TOP10 strains were grown overnight in Lysogeny Broth (LB) containing kanamycin (50 µg/mL) and chloramphenicol (12.5 µg/mL) at 37°C and 200 rpm. Cultures were diluted 1:50 in fresh LB with the corresponding antibiotic and transferred to a 96-well plate (200 µL/well). Cultures were grown for 90 min to arrive to exponential phase and then different concentrations of lactate were added. Samples were always made in triplicates and a blank of LB with the corresponding lactate concentration was done. During 7 h the absorbance at OD600 and fluorescence (excitation 488 nm and emission 530 nm) were measured with intervals of 7 min. The plate was always kept at 37°C in a Tecan Infinite M200 Pro Plate Reader. We calculated dose-response curves from the exponential phase of the bacteria after normalizing fluorescence by optical density.

Modeling

Each experimental data set was fitted to a Hill function using the Least Absolute Residual method (Fitting Toolbox in MatLab).

ETH15 equation.png

Where:

  • al: leakiness in µM/min
  • mu: constant in µM/min
  • KM: half-maximum effective concentration (a representation of sensitivity) in µM
  • n: Hill coefficient (a representation of cooperativity), unitless


J23118-B0034-lldR J23118-B0034-lldP-lldR J23114-B0032-lldP-lldR
Coefficient values KM=1075 (889.5, 1261) KM=1751 (918.5, 2584) KM=337.7 (152.4, 522.9)
al = 479.3 (-142.2, 1101) al = 550 (-1635, 2735) al = 3444 (2571, 4317)
mu = 7311 (6583, 8039) mu = 1.318e+04 (1.03e+04, 1.607e+04) mu = 4886 (3817, 5954)
n1 = 1.326 (1.063, 1.588) n1 = 0.9388 (0.5614, 1.316) n1 = 1.09 (0.531, 1.649)
Goodness to fit sse: 4.2545e+07 sse: 6.4285e+08 sse: 2.7027e+08
rsquare: 0.9964 rsquare: 0.9879 rsquare: 0.9743
fe: 7 dfe: 7 dfe: 7
adjrsquare: 0.9949 adjrsquare: 0.9828 adjrsquare: 0.9633
rmse: 2.4653e+03 rmse: 9.5831e+03 rmse: 6.2137e+03
Figure 2. Comparison of the fluorescence levels obtained using the synthetic lldR promoter alone and in combination with LldR and LldP-LldR. Mean fluorescence obtained with n=3 ± SD.

As can be seen in the picture above, when PlldR GFP is alone, the fluorescence levels do not vary with lactate concentration. However, if PlldR GFP is expressed in a system with constitutive expression of LldR, at low concentrations of lactate there is repression of the promoter, while at high concentrations there is activation. Activation is higher when there is constitutive expression of LldP-LldR. KM is shifted regarding the wild-type promoter, which makes this promoter more sensitive.

The promoter has higher ON/OFF ratios than the wild-type promoter (see Table 1). Its KM is higher than the one from the wild-type promoter, meaning that is has lower affinity for the substrate.

This promoter is one from our designed and tested synthetic promoters based on the wild-type PlldR. In the following table, one can see the ON/OFF ratios and the KM of the wild-type promoter and the three more prominent members of the library.

Table 1: ON/OFF ratio

LldR Additional LldP
J23118-B0034-lldR J23118-B0034-lldP-lldR J23114-B0032-lldP-lldR
Part:BBa_K822000 (wild-type) 10.35 8.04 1.16
Part:BBa_K1847008 15.26 23.96 1.42 Increasing promoter strength
Part:BBa_K1847009 2.5 24.34 0.96
Part:BBa_K1847007 2.15 3.85 1.29

Table 2: KM (µM)

J23118-B0034-lldR J23118-B0034-lldP-lldR J23114-B0032-lldP-lldR
Part:BBa_K822000 (wild-type) 955 1930 720.2
Part:BBa_K1847008 1075 1751 337.5
Part:BBa_K1847009 977.5 2361 459.8
Part:BBa_K1847007 697.7 1977 1337

The graph below shows the behavior of our promoter in two mediums usually used for mammalian cell cultures. The promoter does not respond to lactate concentrations in DMEM while in RPMI it shows a behavior similar to the one in LB.

Figure 2. Comparison of the fluorescence levels obtained using DMEM and RPMI with LldR and LldP-LldR.
RPMI DMEM
J23118-B0034-lldR J23118-B0034-lldP-lldR J23118-B0034-lldR J23118-B0034-lldP-lldR
Coefficient values K=3.389e+04 (-1.7e+05, 2.378e+05) K=173.6 (15.19, 331.9) K=5e+04 (-2.563e+05, 3.563e+05) K = 448.4 (52.45, 844.3)
al = 136.2 (130.8, 141.7) al = 253 (-584.6, 1091) al = 230.3 (202.7, 258) al = 314.1 (138.6, 489.5)
mu = 92.3 (-659.7, 844.3) mu = 1735 (863.1, 2607) mu = 253.2 (-1098, 1605) mu = 1085 (907.3, 1263)
n1 = 3.165 (-13.87, 20.2) n1 = 1.7 (0.05844, 3.341) n1 = 1.5 (fixed at bound) n1 = 0.7 (fixed at bound)
Goodness to fit sse: 939.5028 sse: 1.6713e+07 sse: 1.8073e+05 sse: 5.1609e+06
rsquare: 0.9640 rsquare: 0.8846 rsquare: 0.7522 rsquare: 0.9608
fe: 7 dfe: 7 dfe: 8 dfe: 8
adjrsquare: 0.9485 adjrsquare: 0.8351 adjrsquare: 0.6902 adjrsquare: 0.9510
rmse: 11.5851 rmse: 1.5452e+03 rmse: 150.3044 rmse: 803.1891

References

  1. J Bacteriol. 2008 Apr; 190(8): 2997–3005.
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