gusA (also called uidA[1]) encodes β-Glucuronidase, an intracellular enzyme that catalyzes the hydrolysis of β-D-glucuronides.
3D representation of the β-Glucuronidase from [http://www.rcsb.org/pdb/explore/explore.do?structureId=3K46 RCSB]
A form of this protein with added TEV and poly-HIS tags can be found here.
Usage and Biology
β-Glucuronidase is used as a fusion protein marker in higher plants, due to them lacking intrinsic β-Glucuronidase activity[2].
Generally it can be used as reporter enzyme with detection by biochemical activity assays, immunological assays or by histochemical staining of tissue sections or cells[3].
Sequence and Features
Assembly Compatibility:
10
COMPATIBLE WITH RFC[10]
12
COMPATIBLE WITH RFC[12]
21
INCOMPATIBLE WITH RFC[21]
Illegal BamHI site found at 538
23
COMPATIBLE WITH RFC[23]
25
COMPATIBLE WITH RFC[25]
1000
COMPATIBLE WITH RFC[1000]
Characterization
The final construct was sequenced.
Colorimetric response
Figure 1. Liquid culture from E.Coli overexpressing GusA after reacting with Salmon-Gluc (left). The negative control shows an Escherichia coli liquid culture without substrate added.
[http://2013.igem.org/Team:ETH_Zurich ETH Zurich 2013] used GusA in their project as reporter enzyme.
To test the functionality of the enzyme, liquid culture of E.Coli overexpressing GusA was incubated with Salmon-Gluc.
Figure 2. Enzymatic reaction of GusA with Salmon-Gluc.
Figure 3. Cell lysate from E.Coli overexpressing GusA after reacting with 4-MU-β-D-Glucuronide.
Cell lysate for the assay described below was tested for active enzyme in the same way, but with the fluorescent substrate 4-MU-β-D-Glucuronide. The picture was taken with a common single lens reflex camera mounted on a dark hood at λEx 365 nm.
Figure 4. Enzymatic reaction of GusA with 4-MU-β-D-Glucuronide.
Substrate
Hydrolase
Color, Absorption λmax
Stock
Liquid culture
Colonies
Response time
6-Chloro-3-indolyl-β-D-glucuronide (Salmon-Glc)
GusA
Salmon, 540 nm
0.3 M in DMSO
1.5 mM
0.1 M
Kinetics
To characterize the enzyme they conducted fluorometric assays to obtain Km values. To this end bacterial cells were grown until in exponential growth phase. Upon reaching this, gene expression was induced by AHL (see [http://http://2013.igem.org/Team:ETH_Zurich/Infoproc ETHZ system 2013]). After another 4-5 h of growth, cells were harvested and lysed, the cell free extract (CFX) used for the fluorometric assay. The properly diluted CFX was measured on a 96 well plate in triplicates per substrate concentration. A plate reader took measurements at λEx 365 nm and λEm 445 nm.
The obtained data was evaluated and finally fitted to Michaelis-Menten-Kinetics with SigmaPlot™.
See the resulting graph below.
Figure 5. Michaelis-Menten-Kinetics of GusA cell lysate from E.Coli overexpressing GusA: plots velocity versus substrate concentration (8 μL, 16 μL, 32 μL, 65 μL, 130 μL, 260 μL, 520 μL)) in 20 mM Tris buffer of pH 8. A kinetic value for Km obtained by fitting the raw data to standard the Michaelis Menten equation; Km = 141.1 ± 5.3 μM. All assays were carried out in triplicates, results are presented as means.
The experimental procedure was as following:
Prepare buffers
Lysis buffer: 10 mg/ml Lysozyme, 20 mM Tris buffer, pH 8
Reaction buffer: 20 mM Tris buffer, pH 8
NOTE: For other enzymes than the ones we tested (Aes,GusA,NagZ,PhoA) you might need different buffers
Cell culture
Inoculate bacteria in 20 mL of LB with antibiotics
Let grow at 37°C shaking(200 rpm) to an OD600 of 0.6
Induce enzyme expression (100nM AHL in our case)
Let grow at 37°C shaking(200 rpm) for 4-5h
Cell lysis
Transfer to 50 mL Falcon™ tube
Spin down at 4°C for 5 min with 4 rcf
Resuspend in lysis buffer, 1 μL/mg pellet
Transfer to eppendorf tubes
Incubate at room temperature for 10 min at 220 rpm
Spin down at 4°C for 10 min with max. speed
Transfer the supernatant to new tubes, discard pellets
Cell free extract can be stored at -20°C or continue processing
Dilution
The following values were provided by Johannes Haerle
Aes: Dilute CFX 1:100 in reaction buffer
GusA: Dilute CFX 1:100 in reaction buffer
NagZ: Use pure
PhoA: Dilute CFX 1:10 in reaction buffer
Hydrolysis reaction
Perform this measurement in a 96 well plate or similar
Perform 3 replicates for each substrate concentration
Present 41.6 μL reaction buffer in each well
Add 8 μL diluted CFX (the further dilution ocurring here is intended)
Add 30.4 μL of corresponding substrate
Detection of fluorescence in suitable plate reader (λEx 365 nm, λEm 445 nm)
Crosstalk
Figure 26. Liquid cultures of the ΔaesΔgusAΔnagZEscherichia coli strain overexpressing Aes, GusA, NagZ, PhoA or none in a 96-well plate, with substrates indicated on the left added horizontally.
To ensure orthogonality between the enzyme-substrate reactions used in multi-reporter systems, a crosstalk test was done to make sure that all overexpressed enzymes specifically cleave their assigned substrate.
Jefferson A R, "GUS fusions: beta-glucuronidase as a sensitive and versatile gene fusion marker in higher plants.", EMBO J. 1987 December 20; 6(13): 3901–3907 [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC553867/]
