The design of these recombinant genes was based on the goal of getting the Bt protein made at levels that are toxic to the corn borer in tissues that the corn borer will be feeding on. All promoters used allow for expression of the gene at relatively low levels in the plant. This is important because only low levels of these proteins are needed to kill the ECB larvae compared to the numbers of photosynthetic protein needed to maximize plant production.
Making more protein than is needed would result in the Bt protein competing for amino acids with other proteins the plant needs to make. If excessive Bt protein synthesis in photosynthetic cells resulted in a decrease in the production of other photosynthetic proteins, the plant could lose productivity and a loss of yield called yield drag could be the result. In this case making a little of the protein was all that was needed for ECB control and the promoters chosen worked well for that.
The timing and tissue specificity of expression was another choice made by the genetic engineers who developed the Bt transgenes. The genetic engineers who used the 35S promoter made a plant that would turn on the gene in all living cells (a constitutive promoter). The decision by the genetic engineers to use the green tissue specific or pollen specific promoters resulted in a genetically engineered plant that had more control over when and where it made the Bt protein. These plants quit making the protein late in the season by design. This strategy works if the corn borer feeding on the plants at that time cannot do damage that results in significant yield loss. The gene design strategy of the genetic engineer thus influences the trait expression in the transgenic plant. Examples of gene design for herbicide resistance are described in the next section.