Kluyveromyces marxianus, prominent potential yeast in the future

Kluyveromyces marxianus is a budding yeast that has a prominent potential for production of bioethanol, hydrolytic enzymes, food biomass, and food additives (Limtong et al. 2007; Fonseca et al. 2008; Gethins et al. 2014). This yeast can also produce volatile compounds and flavor metabolites (Gethins et al. 2014), enzymes such as β-galactosidase (Fonseca et al. 2008; Zhou et al. 2013), β-glucosidase and polygalacturonase (Fonseca et al. 2008), and inulinase (Fonseca et al. 2008; Kango and Jain 2011; Lertwattanasakul et al. 2011; Zhou et al. 2014) in addition to ethanol from various primary biomasses (Limtong et al. 2007; Fonseca et al. 2008; Goshima et al. 2013) including cellulosic biomass (Matsuzaki et al. 2012; Goshima et al. 2013). K. marxianus DMKU3-1042, which was isolated in Thailand, is one of the most thermotolerant yeast strains that can efficiently produce ethanol at a high temperature (Limtong et al. 2007) and can utilize various sugars such as glucose, galactose, sucrose, arabinose, and xylose (Rodrussamee et al. 2011; Lertwattanasakul et al. 2013; Lertwattanasakul et al. 2015). However, like other microbes, K. marxianus has an intrinsic system of glucose repression, which prevents utilization of other sugars.

Unlike S. cerevisiae, which exhibits a strong glucose repression on sucrose utilization, K. marxianus DMKU3-1042 utilizes sucrose resistantly to glucose repression (Lertwattanasakul et al. 2011). In S. cerevisiae, Mig1 and Hxk2 play key roles as a regulator complex in glucose repression (Ahuatzi et al. 2004), and Mig1 represses the expression of several genes, including GAL83, SUC2, MAL62, LAC4, and LAC12, when glucose is present (Gancedo and Gancedo 1986; Nehlin and Ronne 1990; Sun et al. 2012; Lin et al. 2014; Zou et al. 2015). K. marxianus with MIG1 mutation exhibited increased inulinase production (Zhou et al. 2014) and increased lactose hydrolysis (Zhou et al. 2013). The hexokinase protein Hxk2 in S. cerevisiae, which is an ortholog of Rag5 in K. lactis (Prior et al. 1993) and K. marxianus, is a hexokinase in the glycolytic pathway and also a transcriptional regulator as a complex with Mig1 and other proteins (Ahuatzi et al. 2004; Ahuatzi et al. 2007). Hxk2 is thus a bi-functional protein that is localized in both the cytoplasm and the nucleus (Pelaez et al. 2010; Bergdahl et al. 2013). Recently, Zhang et al. (2017) reported that HXK1-disrupted mutant in K. marxianus exhibited enhancements of inulin hydrolysis and fructose production.

The intracellular location of Hxk2 in S. cerevisiae is changed by the concentration of glucose available. In the presence of a high concentration of glucose, Hxk2 in the cytoplasm moves to the nucleus and forms a complex with dephosphorylated Mig1 and a general co-repressor complex of Cyc8-Tup1 (Ahuatzi et al. 2004). This complex then binds to the upstream regions of promoters of glucose-repressible genes including respiratory and gluconeogenic genes. However, in the presence of a low concentration or absence of glucose, Hxk2 and Mig1 remain in the cytoplasm, where neither Mig1 nor Hxk2 can repress Mig1- regulated genes (Ahuatzi et al. 2004).

Functional analysis of regulators involved in glucose repression in S. cerevisiae has been extensively performed, but the corresponding regulators of K. marxianus have not been investigated well. In particular, no comparison between MIG1 null mutant and RAG5 null mutant has been reported. In this study, to elucidate the roles of MIG1 and RAG5 in glucose repression in K. marxianus, we constructed disrupted mutants of both genes and compared their effects on growth or sugar metabolism with those of disrupted mutants of the corresponding genes, MIG1 and HXK2, respectively, in S. cerevisiae. The first extensive study on the two regulators in K. marxianus suggests that the negative regulation by Mig1 and Rag5 is shared with that by Mig1 and Hxk2 in S. cerevisiae. Positive regulation by Mig1 and Rag5 was discovered, the corresponding one of which has not been reported in S. cerevisiae. Our results also suggest that Mig1 is essential, probably as a positive regulator, for the His biosynthesis pathway in K. marxianus.


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