Coding Region
The second region of a gene that is also often modified in transgenes is the coding region. This region contains the coded information which designates the amino acid sequence of the protein to be produced. The amino acid sequence of the protein determines its shape and thus the function of that protein. During transcription, it is the coding region that is copied into a complimentary mRNA strand that is able to travel from the nucleus to the cytoplasm. This allows the information encoded by the immobile DNA strand to be transported to the location of the amino acid pool and protein synthesis so a specific protein can be made.
Different Coding Regions
Many Bt genes have been cloned that have the potential to provide resistance to ECB and other insect pests. Three coding regions for ECB resistance have been used in commercial Bt corn; Cry 1A(b), Cry 1A(c), and the Cry 9c. Each region encodes crystaline proteins in the bacteria that are responsible for larvae toxicity. When eaten by the European corn borer, these crystaline, or Cry proteins, bind to the insects’ midgut causing those cells to burst from a water imbalance killing the corn borer.
Changing the coding region of a gene will result in the production of a different protein. (Image by P. Hain)
Transgenic plants containing a Bt gene produce these Cry proteins which the ECB ingest when feeding on the plants. There are hundreds of naturally occurring Bt proteins each with their own coding regions. However, not all are toxic to ECB.
Various Bt genes have been used in several crop species to provide resistance to pests. Below is a table of Bt events that have been approved in the United States.
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*Starlink corn has been removed from the market.Table of Bt events approved in the United States. Source www.agbiosafety.com
The nucleotide sequences of the Cry 1A(b) and Cry 1A(c) coding regions are very similar. The slight differences are not enough to cause a difference in protein toxicity. The nucleotide sequence of the Cry 9C coding region, on the other hand, is different enough that it produces a protein that is toxic to European corn borer, but binds to a different site in the midgut killing the corn borer larvae in a slightly different way. Therefore even if ECB become resistant to the Cry 1A(b) and Cry 1A(c) proteins they may still be susceptible to the Cry 9(c) protein.
This technique has potential for dealing with the development of insect resistance to the toxin. However, it is not the silver bullet. It is possible that when ECB develops resistance to one Bt protein, it will also develop resistance to other Bt proteins. This is called cross-resistance. The alterations of coding regions to avoid resistance requires an understanding of how the insecticidal proteins interact with the insect on a biochemical level.