ANTISENSE EXPRESSION AND OVEREXPRESSION OF BIOTIN CARBOXYLASE IN TOBACCO-LEAVES

The plastid acetyl-coenzyme A carboxylase (ACCase) catalyzes the first committed step of fatty acid synthesis and in most plants is present as a heteromeric complex of at least four different protein subunits: the biotin carboxylase (BC), the biotin carboxyl carrier protein, and the alpha and beta subunits of the carboxyltransferase. To gain insigh t into the subunit organization of this heteromeric enzyme complex and to further evaluate the role of ACCase in regulating fatty acid synth esis, BC expression was altered in transgenic plants. Tobacco (Nicotia na tabacum) was transformed with antisense-expression and overexpressi on tobacco BC constructs, which resulted in the generation of plants w ith BC levels ranging from 20 to 500% of wild-type levels. Tobacco pla nts containing elevated or moderate decreases in leaf BC were phenotyp ically indistinguishable from wild-type plants. However, plants with l ess than 25% of wild-type BC levels showed severely retarded growth wh en grown under low-light conditions and a 26% lower leaf fatty acid co ntent than wild-type plants. A comparison of leaf BC and biotin carbox yl carrier protein levels in plants with elevated and decreased BC exp ression revealed that these two subunits of the plastid ACCase are not maintained in a strict stoichiometric ratio.