Development of bottom-fermenting Saccharomyces strains that produce high SO₂ levels, using integrated metabolome and transcriptome analysis

Sulfite plays an important role in beer flavor stability. Although breeding of bottom-fermenting Saccharomyces strains that produce high levels of SO ₂ is desirable, it is complicated by the fact that undesirable H ₂S is produced as an intermediate in the same pathway. Here, we report the development of a high-level SO₂-producing bottom-fermenting yeast strain by integrated metabolome and transcriptome analysis. This analysis revealed that O-acetylhomoserine (OAH) is the rate-limiting factor for the production of SO₂ and H₂S. Appropriate genetic modifications were then introduced into a prototype strain to increase metabolic fluxes from aspartate to OAH and from sulfate to SO ₂, resulting in high SO₂ and low H₂S production. Spontaneous mutants of an industrial strain that were resistant to both methionine and threonine analogs were then analyzed for similar metabolic fluxes. One promising mutant produced much higher levels of SO₂ than the parent but produced parental levels of H₂S.