Acetyl-CoA carboxylase (ACCase) is a biotin-containing enzyme that catalyzes the formation of malonyl-CoA from acetyl-CoA. Dicot plants such as Arabidopsis contain two structurally distinct types of ACCases that are located in different subcellular compartments. The ACCase located in the cytosol of plant cells generates malonyl-CoA for the synthesis of plant secondary metabolites, including a variety of polyketides and derivatives, and malonylated phytochemicals. The ACCase that is located in chloroplasts generates malonyl-CoA for de novo fatty acid biosynthesis. This enzyme has a heteromeric structure consisting of at least four subunits: the biotin-carrier subunit, coded by the CAC1 gene, the biotin carboxylase subunit coded by the CAC2 gene, and the carboxytransfease subunit coded by the CAC3 and accD genes. Whereas CAC1, CAC2 and CAC3 are nuclear encoded genes, accD is chloroplast encoded. I have molecularly cloned and sequenced the Arabidopsis CAC1 gene and its cDNA product. Immuno-chemical assays with anti-CAC1 antibody show that the gene encodes a subunit of plastidic ACCase (i.e., the 37kDa biotin-containing subunit). Northern blot analysis indicates that the CAC1 mRNA accumulates to higher levels in organs that would be expected to be actively synthesizing fatty acids. In situ hybridization of the developing siliques shows that the CAC1 mRNA accumulates in the developing embryos at around 7 days after flower opening. The CAC1 gene is interrupted by six intronic sequences and the putative promoter region upstream of the gene contains several conserved motifs which have been confirmed in other plant genes to be required for seed specific expression (G-box and E-box) and light-responsiveness (GT-1 site and I-box) and for enhancing transcriptional activity. The distribution of acetyl-CoA carboxylase between the epidermal and mesophyll tissues of leek leaves was determined. The 240-kDa biotinylated polypeptide, the subunit of the homomeric ACCase was found to accumulate exclusively in the epidermis of leek leaves. The 37-kDa and 35-kDa biotinylated polypeptides, the biotin carboxylase, and the accD subunit of the heteromeric acetyl-CoA carboxylase were equally abundant in leek mesophyll and epidermal tissues. These data indicate that leek leaves contain two structurally distinct isozymes of acetyl-CoA carboxylase, one of which, the homomeric form is restricted to the epidermis.