Enhancing volatile phenol concentrations in wine by expressing various phenolic acid decarboxylase genes in Saccharomyces cerevisiae
Phenolic acids, which are generally esterified with tartaric acid, are natural constituents of grape must and wine and can be released as free acids (principally p-coumaric, caffeic, and ferulic acids) by certain cinnamoyl esterase activities during the wine-making process. Some of the microorganisms present in grape can metabolize the free phenolic acids into 4-vinyl and 4-ethyl derivatives. These volatile phenols contribute to the aroma of wine. The Saccharomyces cerevisiae phenyl acrylic acid decarboxylase gene (PAD1) is steadily transcribed, but its encoded product, Pad1p, shows low activity. In contrast, the phenolic acid decarboxylase (PADC) from Bacillus subtilis and the p-coumaric acid decarboxylase (PDC) from Lactobacillus plantarum display substrate-inducible decarboxylating activity in the presence of phenolic acids. In an attempt to develop wine yeasts with optimized decarboxylation activity on phenolic acids, the padc, pdc, and PAD1 genes were cloned under the control of S. cerevisiae's constitutive phosphoglyceratekinase I gene promoter (PGK1P) and terminator (PGK1T) sequences. These gene constructs were integrated into the URA3 locus of a laboratory strain of S. cerevisiae, Σ1278b. The overexpression of the two bacterial genes, padc and pdc, in S. cerevisiae showed high enzyme activity. However, this was not the case for PAD1. The padc and pdc genes were also integrated into an industrial wine yeast strain, S. cerevisiae VIN13. As an additional control, both alleles of PAD1 were disrupted in the VIN13 strain. In microvinification trials, all of the laboratory and industrial yeast transformants carrying the padc and pdc gene constructs showed an increase in volatile phenol formation as compared to the untransformed host strains (Σ1278b and VIN13). This study offers prospects for the development of wine yeast starter strains with optimized decarboxylation activity on phenolic acids and the improvement of wine aroma in the future.