The pest control gene produces a protein called tTAV (tetracycline repressible activator variant), which is able to act as a switch to control the activity of other genes. It’s a variant because this gene has been optimised to only work in insect cells. In the modified insects, when the tTAV gene is expressed, the non toxic protein ties up the cell’s machinery so it’s other genes aren’t expressed and the insect dies. Because no toxic proteins are produced in the insects, if any animals eat them it would be the same as eating a wild insect – they will be digested in just the same way that all other insects are digested.
But how do we produce insects if they die? There’s an antidote given to the insects in the rearing facility that acts like a switch to turn off the tTAV gene preventing the tTAV protein from working. This antidote, tetracycline, an antibiotic, binds to the tTAV protein and disables it. So in the presence of the antidote, the Oxitec insects are able to survive and reproduce in the rearing facility, but when the males are released into the wild, their offspring can’t access the antibiotic in the quantities needed to survive, so they die before reaching adulthood.
The self-limiting gene comprises tetO (tetracycline Operator) sites, which bind tTAV (tetracycline repressible Trans-Activating factor Variant) protein, a promoter and coding sequence for tTAV. Without the tetracycline antidote, tTAV protein is produced, which simultaneously binds to transcriptional machinery and tetO sites, thereby enhancing the expression of the self-limiting gene. This positive feedback system produces large amounts of tTAV, which binds to more and more transcriptional machinery, without the need to bind to the tetO, eventually making the transcriptional machinery unavailable for other essential gene expression. The inhibition of essential gene expression leads to cell death and the death of the insect before it reaches adulthood. When tetracycline is added to the larval aquatic diet, it binds and inactivates tTAV, switching off the positive feedback system. The transcriptional machinery is not depleted as only a small amount of tTAV is produced, which does not affect normal cell function, and the insects survive to reproduce naturally in the production facility.