The heat-treatment induced reduction of the pat gene encoded herbicide resistance in Nicotiana tabacum is influenced by the transgene sequence

After 10 days of cultivation at 37°C, the herbicide resistance encoded by the chimaeric pat41 gene (coding region from Streptomyces viridochromogenes fused to the 823 bp CaMV35S promoter) was strongly reduced in all of the 27 independent transgenic Nicotiana tabacum SRI lines analyzed. This reversible reduction occurred in sterile and unsterile culture in the first and second generation and even when the overnight temperature was reduced to 24 °C. Neither the enzyme activity, the protein nor the pat41 specific RNA could be detected in the heat treated plants, regardless of the number of copies and the hemi- or homozygous state. In contrast to this, the expression of the synthetic pats coding region fused to the 534 bp CaMV35S promoter and coding for essentially the same protein, was stable in heat treated plants. The exchange of the GC rich coding region of the pat41 gene by the AT rich synthetic DNA fragment carrying the pats coding region led to the stabilization of the specific RNA steady state level. However, the presence of the transgene-encoded protein at 37 °C could only be achieved by using specific 5' and 3' untranslated regions of the synthetic pats gene.