Coordinate Regulation of the Nuclear and Plastidic Genes Coding for the Subunits of the Heteromeric Acetyl-Coenzyme A Carboxylase

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Abstract
Plastidic acetyl-coenzyme A (CoA) carboxylase (ACCase) catalyzes the first committed reaction of de novo fatty acid biosynthesis. This heteromeric enzyme is composed of one plastid-coded subunit (β-carboxyltransferase) and three nuclear-coded subunits (biotin carboxy-carrier, biotin carboxylase, and α-carboxyltransferase). We report the primary structure of the Arabidopsis α-carboxyltransferase and β-carboxyltransferase subunits deduced from nucleotide sequences of the respective genes and/or cDNA. Co-immunoprecipitation experiments confirm that the α-carboxyltransferase and β-carboxyltransferase subunits are physically associated. The plant α-carboxyltransferases have gained a C-terminal domain relative to eubacteria, possibly via the evolutionary acquisition of a single exon. This C-terminal domain is divergent among plants and may have a structural function rather than being essential for catalysis. The four ACCase subunit mRNAs accumulate to the highest levels in tissues and cells that are actively synthesizing fatty acids, which are used either for membrane biogenesis in rapidly growing tissues or for oil accumulation in developing embryos. Development coordinately affects changes in the accumulation of the ACCase subunit mRNAs so that these four mRNAs maintain a constant molar stoichiometric ratio. These data indicate that the long-term, developmentally regulated expression of the heteromeric ACCase is in part controlled by a mechanism(s) that coordinately affects the steady-state concentrations of each subunit mRNA.