The Brassica napus nuclear gene, AHAS3R, codes for a mutant chloroplast enzyme acetohydroxyacid synthase (AHAS, EC 4.1.3.18) that is resistant to inhibition by the sulfonylurea herbicide, chlorsulfuron. It was introduced into Nicotiana tabacum under control of the cauliflower mosaic virus 35S promoter and leader sequences. The relative mRNA levels of the AHAS3R gene and the resident tobacco AHAS genes, SURA and SURB, were determined by RNAase protection assays. The AHAS3R mRNA level varied by more than 6-fold among different transformants, depending on the position of insertion and on gene dosage. The expression level was generally consistant with the percentage of AHAS activity that was chlorsulfuron-resistant. In contrast, the specific activity of AHAS, which includes the resident tobacco AHAS and introduced AHAS3R activities, was shown to be independent of AHAS3R gene dosage, mRNA level and proportion of chlorsulfuron-resistant enzyme activity. These observations reveal the presence of a post-transcriptional mechanism that governs the total amount of AHAS activity in the transgenic plants. When AHAS3R mRNA levels were at their highest, SURA and SURB activities appeared to be completely replaced by AHAS3R activity.
