DEVELOPMENTAL SPECIFIC EXPRESSION AND ORGANELLE TARGETING OF THE ESCHERICHIA-COLI FABD GENE, ENCODING MALONYL COENZYME A-ACYL CARRIER PROTEIN TRANSACYLASE IN TRANSGENIC RAPE AND TOBACCO SEEDS

In both plants and bacteria, de novo fatty acid biosynthesis is cataly sed by a type II fatty acid synthetase (FAS) system which consists of a group of eight discrete enzyme components. The introduction of heter ologous, i.e. bacterial, FAS genes in plants could provide an alternat ive way of modifying the plant lipid composition. In this study the Es cherichia coli fabD gene, encoding malonyl CoA-ACP transacylase (MCAT) , was used as a model gene to investigate the effects of over-producin g a bacterial FAS component in the seeds of transgenic plants. Chimeri c genes were designed, so as not to interfere with the household activ ities of fatty acid biosynthesis in the earlier stages of seed develop ment, and introduced into tobacco and rapeseed using the Agrobacterium tumefaciens binary vector system. A napin promoter was used to expres s the E. coli MCAT in a seed-specific and developmentally specific man ner. The rapeseed enoyl-ACP reductase transit peptide was used success fully, as confirmed by immunogold labelling studies, for plastid targe ting of the bacterial protein. The activity of the bacterial enzyme re ached its maximum (up to 55 times the maximum endogenous MCAT activity ) at the end of seed development, and remained stable in mature transg enic seeds. Significant changes in fatty acid profiles of storage lipi ds and total seed lipid content of the transgenic plants were not foun d. These results are in support of the notion that MCAT does not catal yse a rate-limiting step in plant fatty acid biosynthesis.