Part:BBa_K4167001

Toehold switch-mRFP

Toehold switch-mRFP is designed to express mRFP protein triggered by miRNA 221-3p. It is used to detect the amount of miRNA 221-3p in samples.

Sequence and Features

Assembly Compatibility:

• 10
  COMPATIBLE WITH RFC[10]
• 12
  COMPATIBLE WITH RFC[12]
• 21
  COMPATIBLE WITH RFC[21]
• 23
  COMPATIBLE WITH RFC[23]
• 25
  COMPATIBLE WITH RFC[25]
• 1000
  COMPATIBLE WITH RFC[1000]

To construct the standard part, toehold switch-mRFP was synthesized and checked the restriction enzyme information, which is showed as follows:

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


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

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


Toehold switch-mRFP plasmid is designed to express the mRFP protein controlled by the toehold switch and miRNA 221-3p. It comprises the antisense sequence of miRNA 221-3p, RBS, Linker and part sequence of miRNA 221-3p, which form a toehold switch, as well as the gene of marker protein mRFP. At the presence of miRNA 221-3p, it binds to its antisense sequence, opening the toehold switch to trigger the expression of mRFP, which is easily measured. The mechanism is showed as Fig.3.

Fig.3 The mechanism of toehold switch-mRFP.


Toehold switch-mRFP was also cloned into pET-28a expression vector, constructing the recombined plasmid pET-28a-toehold switch-mRFP. After it was transfected into BL21 strain, no mRFP protein (red color) could be observed with naked eyes, indicating that the toehold switch was effective. However, after transfection with miRNA 221-3p into the BL21 strain transfected with pET-28a-toehold switch-mRFP, some transfected clones appeared red color, which were showed in Fig.4.

Fig.4 The effectiveness of toehold switch-mRFP. Bacteria clones only transfected with toehold switch-mRFP appeared white color, while bacteria clones transfected with both toehold switch-mRFP and miRNA 221-3p appeared red color (miRNA 221-3p switched on the expression of mRFP).


To increase the yielding of marker protein mRFP, some different culture conditions were optimized, including the pH value, temperature, fermentation time, and the concentration of transfected miRNA. BL21 strain containing toehold switch plasmid were cultured under different conditions. Since reporter protein mRFP has color, we can easily intuitively find the optimal conditions through the change of color. The optimization experiment results indicated that pH7.2, 37°C, fermentation 18h, and 1.5uM miRNA are the best culture conditions for higher reporter protein production in E. coli.

Fig.5 Optimization of culture conditions of BL21 strain with toehold switch-mRFP plasmid and miRNA221-3p.