Isolation and Characterization of Barosensitive Mutants of Saccharomyces cerevisiae Obtained by UV Mutagenesis

Mutagenesis and fluorescence-activated cell sorting of oleaginous Saccharomyces cerevisiae and the multi-omics analysis of its high lipid accumulation mechanisms

Acquiring lipid-producing strains of Saccharomyces cerevisiae is necessary for producing high-value palmitoleic acid. This study sought to generate oleaginous S. cerevisiae mutants through a combination of zeocin mutagenesis and fluorescence-activated cell sorting, and then to identify key mutations responsible for enhanced lipid accumulation by multi-omics sequencing. Following three consecutive rounds of mutagenesis and sorting, a mutant, MU310, with the lipid content of 44%, was successfully obtained. Transcriptome and targeted metabolome analyses revealed that a coordinated response involving fatty acid precursor biosynthesis, nitrogen metabolism, pentose phosphate pathway, ethanol conversion, amino acid metabolism and fatty acid β-oxidation was crucial for promoting lipid accumulation. The carbon fluxes of acetyl-CoA and NADPH in lipid biosynthesis were boosted in these pathways. Certain transcriptional regulators may also play significant roles in modulating lipid biosynthesis. Results of this study provide high-quality resource for palmitoleic acid production and deepen the understanding of lipid synthesis in yeast.

Importance of Intracellular Energy Status on High-Hydrostatic-Pressure Inactivation of sake Yeast Saccharomyces cerevisiae

The HHP inactivation behaviors of Niigata sake yeast Saccharomyces cerevisiae strain S9arg and its aerobic respiratory-deficient mutant strains were investigated after cultivating them in a YPD media containing 2% to 15% glucose, as well as in moromi mash, in a laboratory-scale sake brewing process. The piezotolerance of strain S9arg, shown after cultivation in a YPD medium containing 2% glucose, decreased to become piezosensitive with increasing glucose concentrations in YPD media. In contrast, the piezosensitivity of a mutant strain UV1, shown after cultivation in the YPD medium containing 2% glucose, decreased to become piezotolerant with increasing glucose concentrations in the YPD medium. The intracellular ATP concentrations were analyzed for an S. cerevisiae strain with intact aerobic respiratory ability, as well as for strain UV1. The higher concentration of ATP after cultivation suggested a higher energy status and may be closely related to higher piezotolerance for the yeast strains. The decreased piezotolerance of strain S9arg observed after a laboratory-scale sake brewing test may be due to a lower energy status resulting from a high glucose concentration in moromi mash during the early period of brewing, as well as a lower aeration efficiency during the brewing process, compared with cultivation in a YPD medium containing 2% glucose.

Genomic and Metabolomic Analyses of a Piezosensitive Mutant of Saccharomyces cerevisiae and Application for Generation of Piezosensitive Niigata-Sake Yeast Strains

A sparkling-type draft cloudy sake (Japanese rice wine), AWANAMA, was recently developed using high hydrostatic pressure (HHP) treatment as a non-thermal pasteurization method. This prototype sake has a high potential market value, since it retains the fresh taste and flavor similar to draft sake while avoiding over-fermentation. From an economic point of view, a lower pressure level for HHP pasteurization is still required. In this study, we carried out a genome analysis of a pressure-sensitive (piezosensitive) mutant strain, a924E1, which was generated by UV mutagenesis from a laboratory haploid Saccharomyces cerevisiae strain, KA31a. This mutant strain had a deletion of the COX1 gene region in the mitochondrial DNA and had deficient aerobic respiration and mitochondrial functions. A metabolomic analysis revealed restricted flux in the TCA cycle of the strain. The results enabled us to use aerobic respiration deficiency as an indicator for screening a piezosensitive mutant. Thus, we generated piezosensitive mutants from a Niigata-sake yeast strain, S9arg, which produces high levels of ethyl caproate but does not produce urea and is consequently suitable for brewing a high-quality sake. The resultant piezosensitive mutants showed brewing characteristics similar to the S9arg strain. This study provides a screening method for generating a piezosensitive yeast mutant as well as insight on a new way of applying HHP pasteurization.

Comparative analysis on inactivation kinetics of between piezotolerant and piezosensitive mutant strains of Saccharomyces cerevisiae under combinations of high hydrostatic pressure and temperature

We previously obtained a pressure-tolerant (piezotolerant) and a pressure sensitive (piezosensitive) mutant strain, under ambient temperature, from Saccharomyces cerevisiae strain KA31a. The inactivation kinetics of these mutants were analyzed at 150 to 250MPa with 4 to 40°C. By a multiple regression analysis, the pressure and temperature dependency of the inactivation rate constants k values of both mutants, as well as the parent strain KA31a, were well approximated with high correlation coefficients (0.92 to 0.95). For both mutants, as well as strain KA31a, the lowest k value was shown at a low pressure levels with around ambient temperature. The k value approximately increased with increase in pressure level, and with increase and decrease in temperature. The piezosensitive mutant strain a924E1 showed piezosensitivity at all pressure and temperature levels, compared with the parent strain KA31a. In contrast, the piezotolerant mutant strain a2568D8 showed piezotolerance at 4 to 20°C, but did not show significant piezotolerance at 40°C. These results of the variable influence of temperature on pressure inactivation of these strains would be important for better understanding of piezosensitive and piezotolerant mechanisms, as well as the pressure inactivation mechanism of S. cerevisiae.

Barosensitivity in Saccharomyces cerevisiae is Closely Associated with a Deletion of the COX1 Gene

High hydrostatic pressure causes physical stress to microorganisms; therefore, this technology may be applied to food pasteurization without introducing the unfavorable effects of thermal denaturation. However, its application is limited to high-value foods because the treatment requires a robust steel vessel and expensive pressurization equipment. To reduce these costs, we studied the pasteurization of Saccharomyces cerevisiae using relatively moderate high-pressure levels. A mutant strain isolated by ultraviolet mutagenesis showed significant loss of viability under high-pressure conditions. Gene expression analysis of the mutant strain revealed that it incurred a deletion of the COX1 gene. Our results suggest that the pressure-sensitivity can readily be introduced into industrial/food microorganisms by complementing a COX1 deleted mitochondria.

High-throughput method for a kinetics analysis of the high-pressure inactivation of microorganisms using microplates

Using microplates as pressure and cultivation vessels, a high-throughput method was developed for analyzing the high-pressure inactivation kinetics of microorganisms. The loss of viability from a high-pressure treatment, measured based on the growth delay during microplate cultivation, showed reproducibility with the conventional agar plate method and was applicable for the kinetics analysis.