Identification of SMM as the precursor for DMS production SMM degradation pathway

Abstract

The unpleasant, asparagus odour in wine, associated with dimethyl sulfide (DMS), can be produced by yeast during wine fermentation. DMS is found to be responsible for premature wine aging in Sauvignon blanc and causes significant losses to the wine industry throughout the world. In this project, S-methyl methionine (SMM), was studied a putative precursor to DMS, on 26 yeast strains up to the concentrations of 100 mM. SMM was found to be an efficient sulfur source for yeast growth and non-toxic. Bioscreen C analyses were carried out to study SMM efficiency on Saccharomyces cerevisiae. The prime goal of this research was to test SMM conversion to DMS. SMM was added in SGM to prepare ferments, followed by DMS quantification using Head Space Solid-Phase Micro-Extraction Gas Chromatography coupled with Mass Spectrophotometer (HS-SPME GC/MS). SMM conversion to DMS is consistent and is confirmed to produce massive DMS peaks at higher levels such as 100 mM. The growth of mutants; SMM permease, mmp1, and methyltransferase, mht1 were investigated with different sulfur sources at 0.15 mM plus SMM up to 100 mM. At low SMM concentrations, both deletion strains blocked SMM influx and transportation, but at high concentrations, 50 and 100 mM both were ineffective. Growth tests using auxotrophic yeast strains with BY4743 background were carried out to test by-products of SMM on degradation. Methionine auxotrophs; met2 and met6, and homoserine auxotrophs; hom2, hom3, and hom6, were able to synthesize the required amino acids by utilizing SMM. However, cysteine auxotrophs; cys3 and cys4 could not utilize SMM as shown in results obtained using Bioscreen C analysis. Four c-s lyases deletion mutants; yhr112c, yml082w, ygr012w, and yll058w and three aromatic mutants; aro8, aro9, and aro10 were studied for their involvement in SMM pathway. The four c-s lyases do not have a known biological function, and aromatic genes were thought to be potential candidates due to SMM structure similarity with methionine. Seven candidate genes listed were used to prepare ferments in SGM with added SMM and analysed using HS-SPME GC/MS. None of the yeast strains produced distinguishable DMS, but SMM concentrations produced significant DMS peaks.