TetR (Tetracycline Repressor)
- Origin: From the E. coli Tn10 tetracycline resistance operon.
- Function: Binds to tet operator (tetO) DNA sequences and blocks transcription of downstream genes.
- Response to Tetracycline/Doxycycline:
- Without doxycycline: TetR binds tetO → transcription OFF.
- With doxycycline: Doxycycline binds TetR, changes its conformation → TetR releases DNA → transcription ON.
- Used in: Tet-Off systems (gene turns off when doxycycline is added).
rTetR (reverse TetR)
- Origin: A mutant version of TetR with several amino acid substitutions (notably E15A, L17G, L25V, etc.) that reverse its doxycycline response.
- Function: Also binds tetO sequences but in the opposite manner to TetR.
- Response to Tetracycline/Doxycycline:
- Without doxycycline: rTetR does not bind tetO → transcription ON.
- With doxycycline: rTetR binds tetO → transcription OFF.
- Used in: Tet-On systems (gene turns on when doxycycline is added).
Summary Table
| Feature | TetR | rTetR (reverse TetR) |
|---|---|---|
| Binds tetO (no doxycycline) | Yes | No |
| Binds tetO (with doxycycline) | No | Yes |
| Gene expression (no doxycycline) | OFF | ON |
| Gene expression (with doxycycline) | ON | OFF |
| Common system name | Tet-Off | Tet-On |
| Typical modern use | Rare | Widely used (e.g., Tet-On 3G, rtTA) |
In Practice
- TetR (Tet-Off): Expression is high without doxycycline and repressed when doxycycline is added.
- rTetR / rtTA (Tet-On): Expression is induced by adding doxycycline, making it more convenient for precise temporal control.
- Modern systems often use optimized versions like rtTA2S-M2 or Tet-On 3G, derived from rTetR, for tighter control and lower background expression.