CLONING OF THE YEAST FAS3 GENE AND PRIMARY STRUCTURE OF YEAST ACETYL-COA CARBOXYLASE

We have isolated and determined the nucleotide sequence of the yeast FAS3 gene, which encodes acetyl-CoA carboxylase (EC 6.4.1.2). The sequence has an open reading frame of 67 11 bases coding for a protein of 2237 amino acids with a calculated molecular weight of 250,593. The presence of the unique biotin-binding site, Met-Lys-Met, and the known CNBr peptide and COOH-terminal sequences confirmed the nucleotide-derived amino add sequence. The yeast, chicken, and rat carboxylases have an overall sequence identity of 34%, suggesting that the eukaryotic carboxylase evolved from a single ancestral gene. The amino acid sequences of yeast fatty acid synthase subunits are least homologous with the animal synthase sequences, whereas carboxylase sequences are highly conserved. The sequences of the ATP, HCO3-, and CoA binding sites of the carboxylases are also well conserved (almost-equal-to 50% identical). The sequences surrounding the biotin binding site are poorly conserved, suggesting that this sequence may not be critical as long as the biotin is available for carboxylase reactions. On the basis of this sequence identity, we have defined the putative biotin carboxylase and transcarboxylase domains.