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Promoter Characterization

By annealing PmerT-forward (5’-3’) and PmerT-reverse (5’-3’) primers, PTPCAD carrying a sticky end of EcoRI and SpeI was cloned upstream of BBa_E0840, a GFP generator. With exogenous expression of MerR in bacteria, GFP intensity could be induced by Hg (II) in a dose response manner. The PTPCAD-E0840 was then cloned into pSB3K3 backbone and BBa_J23103 (constitutive promoter)-merR into pSB1A3.

Protocols for promoter characterization

1.An overnight culture of bacteria carrying the two plasmids pSB3K3 and pSB1A3 were grown in LB broth with ampicillin and kanamycin at 37°C was reactivated by diluting the culture in a ratio of 1:100 with fresh LB.

2.When OD600 reached 0.4-0.6, the bacteria was disposed to several EP tubes, each owning 500uL, and different dose of Mercuric chloride solution was supplied with 3 duplicates and the final concentration varied from 0 to 1.0E-6 mol/L. 100uL of the 500uL was added to the black-96-well plates for GFP intensity measurement and another 100uL was pipetted to the transparent-96-well plates for OD600 measurement.

3.In-plate culture fluorescence and OD600 was recorded at 20min intervals from 0 to 275min and the GFP intensity of 20min before induction was also measured. Temperature was constant at 37°C.

The result is shown in Fig.5.

Fig.5. Time and dose response of GFP and OD 600. The data was measured every 20 minutes since 20 minutes before supplement of different dose of Hg(II) to the wells and the plates was incubated in the shaker at 37℃during the interval of measurement. For both plates, the volume of LB medium with bacteria was 100uL per well. A: GFP intensity was measured by Tecan Microplate Reader with excitation wavelength at 470nm and emission wavelength at 509nm. A black 96-well plate was used to minimize the interference of different well. B: OD 600 was also measured by Tecan Microplate Reader in a transparent 96-well plate, since the depth of 100uL in the well did not reach 1 centimeter, the OD 600 value here was smaller than that was measured by a spectrophotometer.

Construction of merOP Library

In our project, we planned to analyze the behavior of merOP thoroughly, and it could be expected that mutation at the dyad sequence would change the Ka of MerR-DNA interactions. Thus we mutated the semiconserved sites and left the conserved sites constant.

The approach was generally based upon degenerate primer PCR, with the combination of a ‘DNA shuffling’ procedure, that was performed on the target DNA sequence; the resulting library of variants was then screened for the desired feature, and selected isolates are subjected to a repeated procedure(Fig.6).

Fig.6. Library construction. We performed a degenerate primer PCR on the merTPCAD wild-type promoter region of the alterable sites. The resulting PCR fragments, each potentially contained one or more mutation sites at a restricted location.

Fig.7. Dose-response curves of mutants. Among all these candidates, mutant1, 3, 25, 44, 85, 88 (also shown in Fig 3) were selected for the final careful characterization during which a higher concentration resolution was exploited. Then the dose-response curves were fitted by Hill function.

Fig.8 Sequence comparison of the mutants

It can be observed that mutations at the semiconserved region of PmerT promoter significantly influenced the response behavior of MerR/PmerT pair(Fig.7). It is probably because that the mutations alter the binding affinity between MerR and PmerT promoter. As a result of this, different sensitivity (higher or lower than the wild-type) PmerT promoters were gained by us. More details about these mutants suffixed with GFP(BBa_E0840) were described in the parts BBa_K346020 - BBa_K346025.

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