Cloning the Gene for the Malolactic Fermentation of Wine from Lactobacillus delbrueckii in Escherichia coli and Yeasts.

The gene responsible for the malolactic fermentation of wine was cloned from the bacterium Lactobacillus delbrueckii into Escherichia coli and the yeast Saccharomyces cerevisiae. This gene codes for the malolactic enzyme which catalyzes the conversion of l-malate to l-lactate. A genetically engineered yeast strain with this enzymatic capability would be of considerable value to winemakers. L. delbrueckii DNA was cloned in E. coli on the plasmid pBR322, and two E. coll clones able to convert l-malate to l-lactate were selected. Both clones contained the same 5-kilobase segment of L. delbrueckii DNA. The DNA segment was transferred to E. coli-yeast shuttle vectors, and gene expression was analyzed in both hosts by using enzymatic assays for l-lactate and l-malate. When grown nonaerobically for 5 days, E. coli cells harboring the malolactic gene converted about 10% of the l-malate in the medium to l-lactate. The best expression in S. cerevisiae was attained by transfer of the gene to a shuttle vector containing both a yeast 2-mum plasmid and yeast chromosomal origin of DNA replication. When yeast cells harboring this plasmid were grown nonaerobically for 5 days, ca. 1.0% of the l-malate present in the medium was converted to l-lactate. The L. delbrueckii controls grown under these same conditions converted about 25%. A laboratory yeast strain containing the cloned malolactic gene was used to make wine in a trial fermentation, and about 1.5% of the l-malate in the grape must was converted to l-lactate. Increased expression of the malolactic gene in wine yeast will be required for its use in winemaking. This will require an increased understanding of the factors governing the expression of this gene in yeasts.