Team:USTC Software/Toggle-Switch


Toggle Switch

The toggle switch is composed of two repressors and two promoters, each of which is inhibited by the repressor transcribed by the other promoter. We choose pLacI and pTetR as the two promoters, and the repressors are TetR and LacI, respectively. Theoretically, if ratio of LacI to TetR is greater than 1, strong binding of LacI to pLacI repress transcription initiation from pLacI and therefore quantities of TetR will decrease, which in turn relieves its repression to pTet leading to increase of quantities of LacI itself. In this situation, concentration of LacI is far greater than that of tetR, whose expression is repressed at an extream low level and the system enters into its 'LacI' state. Similar analysis will do for TetR and the system will enter into 'TetR' state if its quantity dominates. To show transition between the two states, IPTG is added in 1s after the system enters into 'LacI' state completely. The assembling of parts are construcuted as follows:

Parts assembling: the top assembling is the construction used in our software and the bottom one is the toggle switch design in Construction of a genetic toggle switch in Escherichia coli.

where we use two consecutive terminators to indicate a complete termination. Time course simulation was performed to generate the state transition:

State of system has changed to 'TetR' state from 'LacI' state after adding inducer IPTG at 40000s.

The legend used in the figure is not clear. 13:DNA;(lacI153)4, 11:DNA;(tetR154)2, and 14:DNA;(lacI153)4;(tetR154)2 means binding products of initial plasmid with lacI tetramer at site pLacI gene, with tetR dimer at site pTet gene, and with lacI tetramer and tetR dimer both, respectively. At the beginning, initial plasmids are all bound with lacI tetramer and the system is in its 'LacI' state. After adding inducer IPTG at 40000s, concentration of tetR dimer increases and plasmids bound with lacI tetramer start to bind with tetR dimer and form complex of plasmids, lacI tetramer, and tetR dimer. At about 70000s, all plasmids bound with lacI tetramer are further bound with tetR dimer. At about 85000s, due to repression of expression of lacI, plasmids bound with LacI tetramer and tetR dimer both start to unbind to form plasmids bound with tetR dimer only and the system steps into its 'TetR' state gradually. Details of modeling is available here.