The first region of a gene is the promoter. It acts as a 'genetic dimmer switch' in that it turns the gene on and off and specifies how many mRNA copies of the coding region to make. When more copies of mRNA are made, there are more mRNA strands available to be read by ribosomes during translation, and thus more protein can be produced. Two promoters currently used in transgenic crops are the 35S promoter and the PEP carboxylase promoter. Each promoter is turned on differently in plants.
The 35S promoter originated from a cauliflower mosaic virus gene encoding the 35S subunit of the ribosome. Since ribosomes are continuously needed in every cell of a plant, this promoter turns on the gene in every cell of a plant that is metabolically active. Therefore, when genetic engineers use this promoter in a transgene, the protein encoded by the gene will be produced in every cell all of the time until the cell dies.
The phosphoenolpyruvate (PEP) carboxylase promoter is from a plant gene encoding a photosynthetic enzyme. As a result, any transgene with this promoter will produce the protein only in cells that are actively making photosynthetic proteins. Genetic engineers use this promoter to limit gene expression to cells that make up green tissue. This would not include root, tassel, or ear tissue. Expression also begins to slow and eventually stop toward the end of the season when the plant is completing its lifecycle and photosynthesizing less.
A good example of these two promoters in action is in Bt corn. Some Bt hybrids are designed to have resistance to European corn borer all season long in every part of the plant. Those lines have a transgene with the 35S promoter. Other Bt hybrids are designed to have resistance only in green tissue but not in other parts of the plant, such as the seed or roots. These lines contain a transgene with the PEP carboxylase promoter. These plants also have reduced resistance later in the growing season.