High level expression of introduced chimaeric genes in regenerated transformed plants.

Promoter DNA sequences from a petunia chlorophyll a/b binding protein gene were fused to octopine synthase DNA sequences and the resulting chimaeric genes were introduced into petunia and tobacco cells. Populations of transformed regenerated petunia plants containing the chimaeric genes were examined so that the expression of any particular construction could be compared between independent transformants. Substantial variation was observed between transformants in the level of chimaeric gene expression. In general, transcriptional fusions in which a linker sequence interrupted the 5'-untranslated region gave rise to less chimaeric mRNA accumulation than a translational fusion. In the most actively expressing transformants the amount of mRNA from the introduced chimaeric genes was half that of the endogenous wild-type gene. Transcription initiated at the same place in the chimaeric and endogenous genes. Construction of the translational cab/ocs fusion caused three amino acid changes in the octopine synthase protein and functional octopine synthase enzyme was absent from plants in which mRNA for the chimaeric gene was abundantly expressed.