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Last Document: 4.2  Development of screening methods

Table of Contents  Foods derived from genetically modified organisms and detection methods

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4.3 Product-specific detection methods and available sequence information

Product-specific detection methods based on PCR technology ideally employ target sequences uniquely found in the respective genetically engineered organism. This can be accomplished by choosing primers that bind to two different adjacent genetic elements combined in the respective product but which are found neither naturally in that combination nor in any other known approved transgenic product. These primers result in amplification products containing interfaces between unique¹ combinations of:

• regulatory sequences and structural genes,
• leader sequences (e.g. chloroplast transit peptide sequences) and structural genes,
• different regulatory elements, or
• the interface between the genomic sequence of the host plant and the DNA that was introduced.

Specifically altered sequences of structural genes to allow plant-specific codon usage (here, termed 'synthetic' genes) or chimeric constructs may also allow specific identification of the respective products.

The availability of precise and comprehensive sequence information is an important prerequisite for the development of such product-specific DNA-based detection methods. If products have been approved that may have originated from various transformation events, the data should also assess differences in the content of genetic elements between these lines. Ideally, the data should include complete sequences of the vectors used for plant transformation, knowledge as to which parts of the plasmid are stably integrated into the host genome as well as the sequences of the sites of integration, when possible. Minimal requirements are specific sequence information on a unique expression cassette (promoter-structural gene-terminator) or the sequence of a transgene with altered codon usage.

Some of the sequence information that has been disclosed on the currently approved genetically modified agricultural crops is presented in  Released sequence information on genetic elements introduced into genetically engineered crops.

¹ In order to distinguish different genetically modified products containing, for example, the same (trans-) gene and promoter, but divergent untranslated sequences in between, the length of the amplicon may also be considered as a factor to achieve a 'unique' combination.