Difference between revisions of "Part:BBa K4167666"

Revision as of 08:02, 6 October 2022

Toehold switch-LacZ

Toehold switch-LacZ is designed to express β-galactosidase triggered by miRNA 34a-5p. It is used to detect the amount of miRNA 34a-5p in samples.

Usage and Biology

Sequence and Features

To construct the standard part, toehold switch-LacZ was amplified and checked the restriction enzyme information, which is shown as follows:

Fig.1 The map of toehold switch-LacZ described with SnapGene Viewer, showing the restriction enzyme information (no EcoRI and PstI sites).


After detecting the restriction enzyme information of toehold switch-LacZ using SnapGene software, it was inserted into the pSB1C3 plasmid to construct the standard part pSB1C3-toehold switch-LacZ with PCR method. Then it was identified as follows:

Fig.2 Identification of standard part pSB1C3-toehold switch-LacZ using PCR and digestion with EcoRI and PstI. M: Marker; 1: PCR result; Digestion result.


Toehold switch-LacZ plasmid was designed to express β-galactosidase controlled by the toehold switch and miRNA 34a-5p. It comprises the antisense sequence of miRNA 34a-5p, RBS, Linker and part sequence of miRNA 34a-5p, which form a toehold switch, as well as the gene of β-galactosidase. At the presence of miRNA 34a-5p, it binds to its antisense sequence, opening the toehold switch to trigger the expression of β-galactosidase which catalyzes the substrate X-gal to produce 5-bromo-4-chloro Indigo (blue color). The mechanism is shown as Fig.3.

Fig.3 The mechanism of toehold switch-LacZ.


To express β-galactosidase in BL21 bacteria, the recombined plasmid pET-28a-toehold switch-LacZ controlled by miRNA 34a-5p was constructed using PCR method. For identification, the restriction endonuclease digestion and PCR assays were performed, which showed that the fragment length of lacZ was consistent with the expected results (Fig.4)

Fig.4 Identification of pET-28a-toehold switch-lacZ plasmid. M: Marker, 1: The plasmid of pET-28a-toehold switch-LacZ, 2: The pET-28a-toehold switch-LacZ plasmid was digested by EcoRⅠ and Hind Ⅲ restriction endonuclease, 3: The LacZ gene amplified by PCR method.


pET-28a-toehold switch-lacZ plasmid was transfected into BL21ΔlacZ strain（LacZ deleted）. Under the optimal conditions, the cell-free expression system was prepared by mixing the cell extract with other components such as ATP, PEP, amino acid, etc. (see protocol section for details). After the filter paper was blocked with bovine serum albumin (BSA), washed and dried, a drop of the cell-free reaction system mentioned above fell onto the filter paper strip which was followed by putting it into the ultra-low temperature refrigerator and frozen dryer to form a paper strip sensor.

In order to obtain sensitive and fast detection effects, the reaction conditions that X-gal is converted to 5-bromo-4-chloro Indigo (blue color) catalyzed by β-galactosidase in the cell-free expression system was optimized under different temperature, reaction time and miRNA concentration, which were shown as follows:

Fig.5 The optimization of reaction temperature at which X-gal is converted to 5-bromo-4-chloro Indigo (blue color) catalyzed by β-galactosidase in the cell-free expression system. (A): OD570 value, (B): Photograph of paper strip sensor reaction in cell-free system.

Fig.6 The optimization of reaction time for β-galactosidase enzyme reaction in cell-free system. (A):OD570 value, (B): Photograph of paper strip sensor reaction in cell-free system.

Fig.7 The optimization of miRNA concentration for β-galactosidase enzyme reaction after 1h. (A):OD570 value, (B): Photograph of paper strip sensor reaction in cell-free system.

The optimization results showed that the best temperature is 30°C, as shown in Fig.5. When the reaction lasts for 1h, the reaction is almost over, so 1h is chosen as the best reaction time (Fig.6). For miR-34a-5p target sensor, the lowest limit of visible color development is 500fM (Fig.7).