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4.1.2 Survey of the promoters used

The expression of a new phenotype is dependent on various factors, including the site of integration of the T-DNA whose location may have an impact on processes such as gene silencing (P. Meyer, 1995). Alteration of the codon usage is a method frequently used to optimise bacteria-derived transgenes for plant expression, based on the redundancy of the genetic code and the distinct prevalence of various codons and their respective t-RNAs in different phyla.

One of the most important factors for achieving the desired expression levels of a transgene is the choice of the promoter that regulates the transcription of the respective transgene. Many of the transgenes that are present in 22 out of these approved genetically modified products are regulated by the constitutive 35S promoter from the cauliflower mosaic virus (P-35S = P-CaMV 35S) or by derivatives of this promoter (e.g. with enhancer or duplicated). PCR-assays detecting the presence of these promoter sequences appear to be well suited for the development of detection methods (Figure 5, Promoters in approved genetically modified agricultural crops and Development of screening methods: Table 19).

The promoter from the nopaline synthase gene of Agrobacterium tumefaciens (P-nos) has been used in 7 genetically engineered products, whereas 4 products carry transgenes that are regulated by variants from the ribulose-1,5-bisphosphate carboxylase promoter derived from various plants. P-TA29 represents a tissue- and development- specific promoter isolated from tobacco that drives expression of one or several transgenes in 3 products. Seven genetically modified crops contain still other promoters, while ten harbour genes with bacterial promoters (Figure 5).

Some promoters until now have only been employed in single products; however, the detection of internal sequences of these promoters is generally not appropriate for detecting GMOs since many of these, such as the tissue-specific promoters P-PEPC and P-CDPK, originate from agricultural crops.

Figure 5: Number of occurrences of the most frequently used promoters introduced into the currently approved genetically engineered agricultural crops (in total 28 distinct products were approved; see text). In some cases (indicated by asterisks) distinct variants of the respective promoter were used (see Promoters in approved genetically modified agricultural crops).