Safety assessment of genetically engineered yeast: elimination of mutagenicity of the yeast Saccharomyces cerevisiae by decreasing the activity of methylglyoxal synthase

Methylglyoxal synthase catalyses the transformation of dihydroxyacetonephosphate to methylglyoxal, a toxic 2-oxoaldehyde which is found to be present in the yeast Saccharomyces cerevisiae DKD-5D-H. Yeast cells in which genes for phosphoglucose isomerase, phosphofructokinase and triosephosphate isomerase had been extrachromosomally amplified by using a multi-copy plasmid showed an increased ability to induce mutagenesis in standard tests when compared to wild type yeast. This response is mainly due to the increased amount of methylglyoxal in the engineered cells. To decrease the mutagenic activity and make it possible to use genetically engineered yeasts for practical fermentation processes, a mutant having a decreased level of methylglyoxal synthase activity was isolated. When transformed with genes for the glycolytic enzymes, the mutant cells showed extremely low levels of methylglyoxal content and mutagenic activity, both levels being comparable with those of non-transformed DKD-5D-H cells.