Genome-wide analysis of Kluyveromyces lactis in wild-type and rag2 mutant strains

The UPC2 gene in Kluyveromyces lactis stress adaptation

KlUpc2p, a transcription factor belonging to the fungal binuclear cluster family, is an important regulator of ergosterol biosynthesis and azole drug resistance in Kluyveromyces lactis. In this work, we show that the absence of KlUpc2p generates Rag^- phenotype and modulates the K. lactis susceptibility to oxidants and calcofuor white. The KlUPC2 deletion leads to increased expression of KlMGA2 gene, encoding an important regulator of hypoxic and lipid biosynthetic genes in K. lactis and also KlHOG1 gene. The absence of KlUpc2p does not lead to statistically significant changes in glycerol, corroborating the expression of KlGPD1 gene, encoding NAD⁺-dependent glycerol-3-phosphate dehydrogenase, that is similar in both the deletion mutant and the parental wild-type strain. Increased sensitivity of Klupc2 mutant cells to brefeldin A accompanied with significant increase in KlARF2 gene expression point to the involvement of KlUpc2p in intracellular signaling. Our observations highlight the connections between ergosterol and fatty acid metabolism to modulate membrane properties and point to the possible involvement of KlUpc2p in K. lactis oxidative stress response.

Quantitative physiology and elemental composition of Kluyveromyces lactis CBS 2359 during growth on glucose at different specific growth rates

The yeast Kluyveromyces lactis has received attention both from academia and industry due to some important features, such as its capacity to grow in lactose-based media, its safe status, its suitability for large-scale cultivation and for heterologous protein synthesis. It has also been considered as a model organism for genomics and metabolic regulation. Despite this, very few studies were carried out hitherto under strictly controlled conditions, such as those found in a chemostat. Here we report a set of quantitative physiological data generated during chemostat cultivations with the K. lactis CBS 2359 strain, obtained under glucose-limiting and fully aerobic conditions. This dataset serves [corrected] as a basis for the comparison of K. lactis with the model yeast Saccharomyces cerevisiae in terms of their elemental compositions, as well as for future metabolic flux analysis and metabolic modelling studies with K. lactis.

Identifying cis-regulatory changes involved in the evolution of aerobic fermentation in yeasts

Gene regulation change has long been recognized as an important mechanism for phenotypic evolution. We used the evolution of yeast aerobic fermentation as a model to explore how gene regulation has evolved and how this process has contributed to phenotypic evolution and adaptation. Most eukaryotes fully oxidize glucose to CO₂ and H₂O in mitochondria to maximize energy yield, whereas some yeasts, such as Saccharomyces cerevisiae and its relatives, predominantly ferment glucose into ethanol even in the presence of oxygen, a phenomenon known as aerobic fermentation. We examined the genome-wide gene expression levels among 12 different yeasts and found that a group of genes involved in the mitochondrial respiration process showed the largest reduction in gene expression level during the evolution of aerobic fermentation. Our analysis revealed that the downregulation of these genes was significantly associated with massive loss of binding motifs of Cbf1p in the fermentative yeasts. Our experimental assays confirmed the binding of Cbf1p to the predicted motif and the activator role of Cbf1p. In summary, our study laid a foundation to unravel the long-time mystery about the genetic basis of evolution of aerobic fermentation, providing new insights into understanding the role of cis-regulatory changes in phenotypic evolution.

Kluyveromyces lactis: a suitable yeast model to study cellular defense mechanisms against hypoxia-induced oxidative stress

Studies about hypoxia-induced oxidative stress in human health disorders take advantage from the use of unicellular eukaryote models. A widely extended model is the fermentative yeast Saccharomyces cerevisiae. In this paper, we describe an overview of the molecular mechanisms induced by a decrease in oxygen availability and their interrelationship with the oxidative stress response in yeast. We focus on the differential characteristics between S. cerevisiae and the respiratory yeast Kluyveromyces lactis, a complementary emerging model, in reference to multicellular eukaryotes.

Sugar metabolism, redox balance and oxidative stress response in the respiratory yeast Kluyveromyces lactis

A lot of studies have been carried out on Saccharomyces cerevisiae, an yeast with a predominant fermentative metabolism under aerobic conditions, which allows exploring the complex response induced by oxidative stress. S. cerevisiae is considered a eukaryote model for these studies. We propose Kluyveromyces lactis as a good alternative model to analyse variants in the oxidative stress response, since the respiratory metabolism in this yeast is predominant under aerobic conditions and it shows other important differences with S. cerevisiae in catabolic repression and carbohydrate utilization. The knowledge of oxidative stress response in K. lactis is still a developing field. In this article, we summarize the state of the art derived from experimental approaches and we provide a global vision on the characteristics of the putative K. lactis components of the oxidative stress response pathway, inferred from their sequence homology with the S. cerevisiae counterparts. Since K. lactis is also a well-established alternative host for industrial production of native enzymes and heterologous proteins, relevant differences in the oxidative stress response pathway and their potential in biotechnological uses of this yeast are also reviewed.

The role of glutathione reductase in the interplay between oxidative stress response and turnover of cytosolic NADPH in Kluyveromyces lactis

The phosphoglucose isomerase mutant of the respiratory yeast Kluyveromyces lactis (rag2) is forced to metabolize glucose through the oxidative pentose phosphate pathway and shows an increased respiratory chain activity and reactive oxygen species production. We have proved that the K. lactis rag2 mutant is more resistant to oxidative stress (OS) than the wild type, and higher activities of glutathione reductase (GLR) and catalase contribute to this phenotype. Resistance to OS of the rag2 mutant is reduced when the gene encoding GLR is deleted. The reduction is higher when, in addition, catalase activity is inhibited. In K. lactis, catalase activity is induced by peroxide-mediated OS but GLR is not. We have found that the increase of GLR activity is correlated with that of glucose-6-phosphate dehydrogenase (G6PDH) activity that produces NADPH. G6PDH is positively regulated by an active respiratory chain and GLR plays a role in the reoxidation of the NADPH from the pentose phosphate pathway in these conditions. Cytosolic NADPH is also used by mitochondrial external alternative dehydrogenases. Neither GLR overexpression nor induction of the OS response restores growth on glucose of the rag2 mutant when the mitochondrial reoxidation of cytosolic NADPH is blocked.

Reoxidation of cytosolic NADPH in Kluyveromyces lactis

Saccharomyces cerevisiae and Kluyveromyces lactis are considered to be the prototypes of two distinct metabolic models of facultatively-aerobic yeasts: Crabtree-positive/fermentative and Crabtree-negative/respiratory, respectively. Our group had previously proposed that one of the molecular keys supporting this difference lies in the mechanisms involved in the reoxidation of the NADPH produced as a consequence of the activity of the pentose phosphate pathway. It has been demonstrated that a significant part of this reoxidation is carried out in K. lactis by mitochondrial external alternative dehydrogenases which use NADPH, the enzymes of S. cerevisiae being NADH-specific. Moreover, the NADPH-dependent pathways of response to oxidative stress appear as a feasible alternative that might co-exist with direct mitochondrial reoxidation.

A comparative analysis of the GAL genetic switch between not-so-distant cousins: Saccharomyces cerevisiae versus Kluyveromyces lactis

Despite their close phylogenetic relationship, Kluyveromyces lactis and Saccharomyces cerevisiae have adapted their carbon utilization systems to different environments. Although they share identities in the arrangement, sequence and functionality of their GAL gene set, both yeasts have evolved important differences in the GAL genetic switch in accordance to their relative preference for the utilization of galactose as a carbon source. This review provides a comparative overview of the GAL-specific regulatory network in S. cerevisiae and K. lactis, discusses the latest models proposed to explain the transduction of the galactose signal, and describes some of the particularities that both microorganisms display in their regulatory response to different carbon sources. Emphasis is placed on the potential for improved strategies in biotechnological applications using yeasts.