Bt Expression

In order to get a transgene to work effectively in the new organism, geneticists have to engineer the right promoter and coding region combination.

Bt genes have a promoter, but it is a bacterial promoter that is not recognized by plants and will not induce expression when transformed into a plant. Therefore, genetic engineers must remove the bacterial promoter, and replace it with one that will be able to induce the proper gene expression in a plant. Currently, there are two types of Bt corn hybrids designed. One type has Bt expression in all of its cells, while the other has expression in only green tissue.

Bt expression in all cells

Those genetically engineered corn plants with expression in all cells have a combination of a Cry coding region with the 35S promoter. The plants will make the Bt protein in all their cells until the plant is dead. 

Bt Expression in Green Tissue

However, other genetic engineers have replaced the Bt promoter with the PEP carboxylase promoter or a pollen specific promoter. Bt genes with the pollen specific promoter express a lethal dose of the Bt protein in the pollen. However, once the pollen is shed, the tassle is no longer protected and some ECB feeding may occur. Bt genes with the PEP carboxylase promoter have the Bt toxin expressed only in green tissue. The plants also lose expression as the plant stops maintaining its photosynthetic activity. Therefore late season corn borers will not encounter the Bt endotoxin in these plants and some feeding damage may occur.

The 35S promoter gives expression of the Cry 1A(b) Bt protein in all cells while the PEP carboxylase promoter gives expression of the protein in green tissues only. (Image by P. Hain)