Part:BBa_K082034:Experience
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Characterization of BBa_K082034 by ETH Zurich 2010 iGEM Team
Introduction
The iGEM 2010 Team of ETH Zurich considered using this part as a reporter system to evaluate the [http://2010.igem.org/Team:ETHZ_Basel/Biology/Cloning cloning strategy]. Since the part contains a LacI binding site on the operator the expression behaviour under different concentrations of LacI and binding sites were examined. Two plasmids for expression of BBa_K082034 and one for lacI were examined.
Plasmids
plasmid | purpose | origin | resistance | additional information |
---|---|---|---|---|
pSB1A2 | expression of BBa_K082034 | pMB1; 100-300 copies/cell | amp | |
pSEVA132 | expression of BBa_K082034 | pBBR1; approx. 75 copies/cell | kan | Victor de Lorenzo's lab; analysis of copy number (pSEVA132 = wv1) |
pKQV4 | expression of lacI | pBR322; high copy | tet, amp | [1] |
Cloning
Since the part BBa_K082034 was distributed in the plasmid pSB1A2 it could readily be used for the experiments and did not have to be cloned further.
pSEVA132 required some preparation. First pSB1A2_BBa_K082034 and pSEVA132 were digested according to the protocol found [http://openwetware.org/wiki/Engineering_BioBrick_vectors_from_BioBrick_parts/Restriction_digest here]. The restriction enzymes used were EcoRI and PstI. The part BBa_K082034 was then isolated from pSB1A2 with an agarose gel and ligated into pSEVA132 according to the [http://www.neb.com/nebecomm/products/protocol2.asp quick ligation protocol] of New England Biolabs to give rise to the plasmid pSEVA132_BBa_K082034.
control digest of pSB1A2. lane 1: [http://www.neb.com/nebecomm/products/productn0468.asp 1kb ladder]; lane 2: digested pSB1A2, part at 1.1kb, vector at 2kb.
control digest of pSEVA132. lane 1 and 6: [http://www.neb.com/nebecomm/products/productn0468.asp 1kb ladder]; lane 2 and 4: pSEVA132_BBa_K082034 not digested; lane 3 and 5: digested pSEVA132_BBa_K082034, part at 1.1kb, vector at 4.5kb.
Expression Behaviour of BBa_K082034 in pSB1A2
Methods
An initial culture of E. coli DH5α (5 ml LB in 15 ml Falcon tube) was incubated overnight on a shaker (37°C, 220rpm). From this initial culture 1 ml were transferred to 25 ml Falcon tubes containing 4 ml LB. After one hour of incubation induction was initiated by 5uM, 50uM, 500uM and 5 mM Isopropyl-β-D-thiogalactopyranosid (IPTG) respectively. Fluorescence (excitation at 485nm and emission at 530nm) and optical density at 595 nm were measured after two hours of incubation with a PerkinElmer Victor3 Fluorometer.
From the measured fluorescence the fluorescence of an LB blank was substracted and then divided by the difference in optical density between the sample and the LB blank. The obtained values were normalized by the control (E. coli DH5α cells not carrying the plasmid).
Results
E. coli DH5α cells harboring pSB1A2_BBa_K082034 showed an increase of fluorescence by a factor of around 6 compared to E. coli DH5α cells not containing the plasmid (see picture on the right). Inducer concentration did not have an influence on fluorescence which would be distinguishable from noise. As a representative the culture of E. coli DH5α harboring pSB1A2_BBa_K082034 induced at 5mM is shown.
Conclusion
It seems that the cytosolic level of LacI arising from chromosomally encoded lacI is not sufficient to repress the high copy plasmid pSB1A2_BBa_K082034. Thus, the fluorescence observed resulted from "leaky" expression, while the effect of the inducer was probably hidden by noise. The experiment would need to be repeated in order to obtain significant results. Background fluorescence could be minimized by using M9 supplemented minimal medium.
Expression Behaviour of BBa_K082034 in pSEVA132
Methods
From an initial culture of E. coli DH5α cells (5 ml LB in 15 ml Falcon tube, incubation overnight at 37°C, 220rpm) containing either only pSEVA132_BBa_K082034 or pSEVA132_BBa_K082034 and pKQV4_lacIq, cultures (10 ml LB in 100 ml Erlenmayer flask) were inoculated to an OD (at 600 nm, measured with Eppendorf Biophotometer) of 0.05. Fluorescence (excitation at 485nm and emission at 530nm) and optical density at 595 nm were measured with a PerkinElmer Victor3 Fluorometer at time intervals of 15min. After 1 hour of incubation (37°C, 220rpm) expression was initiated by 1mM IPTG. The obtained values for fluorescence and optical density were corrected by the values of an LB blank.
Results
Conclusion
In contrast to pSB1A2_BBa_K082034, cells harboring pSEVA132_BBa_K082034 seem to produce enough LacI in order to repress the load of BBa_K082034 brought to them, at least to some extent. Some leaky expression could still be observed.
Cells containing pSEVA132_BBa_K082034 and pKQV4_lacIq did not show any expression even at an inducer concentration of 1 mM. The reason for this might be the elevated cytosolic level of LacI provoked by the additional pKQV4_lacIq plasmid.
Our goal to use BBa_K082034 as a reporter for presence or absence looks quite easy. Either a high copy plasmid like pSB1A2 or a medium copy plasmid like pSEVA132 accompanied by IPTG would work. However, if a tightly regulated expression of BBa_K082034 is required levels of BBa_K082034 and of LacI would need to be carefully adjusted. Too much BBa_K082034 leads to leaky expression, as seen with the high copy plasmid pSB1A2_BBa_K082034 and also with the medium copy plasmid pSEVA132_BBa_K082034. Too much LacI, on the other hand, might completely block expression even if induced, as seen with pSEVA132_BBa_K082034 in combination with pKQV4_lacIq.
We could observe a delay between induction and the fluorescence gain of approximately 120min. The reason might be the time needed for correct folding of the GFP.
Reference
[1] [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC400994/pdf/emboj00129-0314.pdf Strauch, M. A.; Spiegelman, G. B.; Perego, M.; Johnson, W. C.; Burbulys, D.; Hoch, J. A. The transition state transcription regulator abrB of Bacillus subtilis is a DNA binding protein. EMBO J. 1989, 8, 1615-1621.]
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