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Abstract
BackgroundIn Saccharomyces cerevisiae, alpha-glucosidase (maltase) is a key enzyme in maltose metabolism. In addition, the overexpression of the alpha-glucosidase-encoding gene MAL62 has been shown to increase the freezing tolerance of yeast in lean dough. However, its cryoprotection mechanism is still not clear.ResultsRNA sequencing (RNA-seq) revealed that MAL62 overexpression increased uridine diphosphoglucose (UDPG)-dependent trehalose synthesis. The changes in transcript abundance were confirmed by quantitative reverse transcription–polymerase chain reaction (qRT-PCR) and enzyme activity assays. When the UDPG-dependent trehalose synthase activity was abolished, MAL62 overexpression failed to promote the synthesis of intracellular trehalose. Moreover, in strains lacking trehalose synthesis, the cell viability in the late phase of prefermentation freezing coupled with MAL62 overexpression was slightly reduced, which can be explained by the increase in the intracellular glycerol concentration. This result was consistent with the elevated transcription of glycerol synthesis pathway members.ConclusionsThe increased freezing tolerance by MAL62 overexpression is mainly achieved by the increased trehalose content via the UDPG-dependent pathway, and glycerol also plays an important role. These findings shed new light on the mechanism of yeast response to freezing in lean bread dough and can help to improve industrial yeast strains.
Highlights
In Saccharomyces cerevisiae, alpha-glucosidase is a key enzyme in maltose metabolism
The results showed that MAL62 overexpression caused significant changes in gene expression (Fig. 1a)
gene ontology (GO) analysis of the biological processes showed that several processes involving trehalose were affected by MAL62 overexpression (Fig. 1d)
Summary
In Saccharomyces cerevisiae, alpha-glucosidase (maltase) is a key enzyme in maltose metabolism. The overexpression of the alpha-glucosidase-encoding gene MAL62 has been shown to increase the freezing tolerance of yeast in lean dough. The system II trehalose synthetic pathway is adenosine-diphosphoglucose (ADPG) dependent and uses maltose, a disaccharide, to synthesize trehalose [12, 13]. In baker’s yeast, the MAL gene family, which regulates maltose metabolism, consists of five multigene complexes, including MAL1, MAL2, MAL3, MAL4, and MAL6. Each gene complex encodes a maltose permease, an alpha-glucosidase, and a transacting MAL-activator [14]. We have shown previously that overexpression of MAL62 enhances the cryotolerance of baker’s yeast [15] and speculated that multiple pathways may be involved in this phenomenon [16]. The mechanism for the enhanced freezing tolerance is still unknown
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