GENOMIC ORGANIZATION OF 251 KDA ACETYL-COA CARBOXYLASE GENES IN ARABIDOPSIS - TANDEM GENE DUPLICATION HAS MADE 2 DIFFERENTIALLY EXPRESSED ISOZYMES

Acetyl-CoA carboxylase (ACCase) catalyzes the carboxylation of acetyl- CoA, forming malonyl-CoA a key intermediate in the biosynthesis of fat ty acids and a variety of secondary metabolites. Based upon amino acid sequences conserved among rat, chicken, and E. coli ACCases, PCR-prim ers were used to amplify a genomic fragment which codes for an ACCase of Arabidopsis. The resulting fragment was used for isolation of genom ic and cDNA clones. We have determined the complete cDNA sequence codi ng for an Arabidopsis ACCase consists of 2,254 amino acids with the mo lecular mass of 251 kDa. This enzyme contains no recognizable plastid transit-peptide sequence. Therefore, this ACCase is presumably the cyt osolic isozyme. Southern analysis indicates that there are two ACCase genes in the Arabidopsis genome. Surprisingly, the results of RFLP ana lysis and physical mapping of the isolated genomic clones demonstrate that these two genes, acc1 and acc2, are contiguously located within a 25-kbp genomic region near the middle of chromosome 1. Both genes are transcriptionally active, as transcripts from each gene were detected by reverse transcription-PCR analysis using gene-specific primers. Th e acc1 and acc2 transcripts accumulate in leaves and seedlings but onl y the acc1 transcript accumulates in developing siliques, unexpectedly . The differences in the expression patterns may be indicative of the differential role of the two genes.