The lethal gene produces a protein called tTA, which is able to act as a switch to control the activity of other genes. In the modified insects, the presence of high levels of this gene causes the machinery in the insect’s cells to go into over-drive. The gene doesn’t produce any toxic proteins, but it ties up some of the cell’s essential machinery and disrupts its normal function – causing the insects to die. Because no toxic proteins are produced in the insects, when any other animals eat them they will be digested in just the same way that all other insects are digested, so natural predators won’t suffer any harmful effects from consumption of a modified insect.
The key to the RIDL system is that we are able to use a supplement in the lab which acts like an antidote, preventing the tTA protein from working. This supplement – the antibiotic, tetracyline – binds to the tTA protein and stops it from switching on other genes. So in the presence of the supplement, the RIDL insects survive; when released into the wild, their offspring can’t access the antibiotic in the quantities needed to survive, so they die before reaching adulthood.
Different molecular constructs are used in the different products to achieve the desired target profiles. Sometimes, the tTA protein on its own can cause the insect to die. In other cases, the protein can switch on other genes which cause death.
In diagram (a), the positive feedback system is composed of a tetO binding domain, a minimal promoter and the tTA gene. Without the repressor (b), the minimal promoter induces production of a small amount of tTA, which then binds to the tetO binding sites. The binding of tTA enhances expression of more tTA, which in turn binds to more tetO sites. This positive feedback loop results in the production of tTA, which, at high enough levels, are deleterious to cells. (c) The repressor is able to bind to tTA and block it from binding to tetO, turning off the positive feedback loop and only a basal amount of tTA is produced, which is harmless to the cells.