The nucleotide sequence of the yeast ILV5 gene, which codes for the branched-chain amino acid biosynthesis enzyme acetohydroxyacid reductoisomerase, has been determined. The ILV5 coding region is 1,185 nucleotides, corresponding to a polypeptide with a molecular weight of 44,280. Transcription of the ILV5 mRNA initiates at position -81 upstream from the ATG translation start codon and terminates between 218 and 222 bases downstream from the stop codon. Consensus sequences have been identified for initiation and termination of transcription, and for general control of amino acid biosynthesis, as well as repression by leucine. The ILV5 gene is regulated slightly by general amino acid control. Codon usage of the ILV5 gene has the strong bias observed in yeast genes that are highly expressed. In agreement with this, the reductoisomerase monomer, with an apparent molecular weight of 40,000, has been identified in an SDS polyacrylamide gel pattern of total soluble yeast proteins as a gene dosage dependent band.