Disruption of the gene enhances protein secretion in and

Identification of genetic variants of the industrial yeast Komagataella phaffii (Pichia pastoris) that contribute to increased yields of secreted heterologous proteins

The yeast Komagataella phaffii (formerly called Pichia pastoris) is used widely as a host for secretion of heterologous proteins, but only a few isolates of this species exist and all the commonly used expression systems are derived from a single genetic background, CBS7435 (NRRL Y-11430). We hypothesized that other genetic backgrounds could harbor variants that affect yields of secreted proteins. We crossed CBS7435 with 2 other K. phaffii isolates and mapped quantitative trait loci (QTLs) for secretion of a heterologous protein, β-glucosidase, by sequencing individual segregant genomes. A major QTL mapped to a frameshift mutation in the mannosyltransferase gene HOC1, which gives CBS7435 a weaker cell wall and higher protein secretion than the other isolates. Inactivation of HOC1 in the other isolates doubled β-glucosidase secretion. A second QTL mapped to an amino acid substitution in IRA1 that tripled β-glucosidase secretion in 1-week batch cultures but reduced cell viability, and its effects are specific to this heterologous protein. Our results demonstrate that QTL analysis is a powerful method for dissecting the basis of biotechnological traits in nonconventional yeasts, and a route to improving their industrial performance.

Modulation of the cell wall protein Ecm33p in yeast Saccharomyces cerevisiae improves the production of small metabolites

The cell wall is a dynamic organelle that determines the shape and provides the cell with mechanical strength. This study investigated whether modulation of cell wall composition can influence the production or secretion of small metabolites by yeast cell factories. We deleted and upregulated several cell wall-related genes KRE2, CWP1, CWP2, ECM33, PUN1, and LAS21 in yeast Saccharomyces cerevisiae engineered for p-coumaric acid or β-carotene production. Deletions of las21∆ and ecm33∆ impaired the yeast growth on medium with cell wall stressors, calcofluor white, and caffeine. Both overexpression and deletion of ECM33 significantly improved the specific yield of p-coumaric acid and β-carotene. We observed no change in secretion in any cell wall altered mutants, suggesting the cell wall is not a limiting factor for small molecule secretion at the current production levels. We evaluated the cell wall morphology of the ECM33 mutant strains using transmission electron microscopy. The ecm33∆ mutants had an increased chitin deposition and a less structured cell wall, while the opposite was observed in ECM33-overexpressing strains. Our results point at the cell wall-related gene ECM33 as a potential target for improving production in engineered yeast cell factories.

Exploiting strain diversity and rational engineering strategies to enhance recombinant cellulase secretion by Saccharomyces cerevisiae

Consolidated bioprocessing (CBP) of lignocellulosic material into bioethanol has progressed in the past decades; however, several challenges still exist which impede the industrial application of this technology. Identifying the challenges that exist in all unit operations is crucial and needs to be optimised, but only the barriers related to the secretion of recombinant cellulolytic enzymes in Saccharomyces cerevisiae will be addressed in this review. Fundamental principles surrounding CBP as a biomass conversion platform have been established through the successful expression of core cellulolytic enzymes, namely β-glucosidases, endoglucanases, and exoglucanases (cellobiohydrolases) in S. cerevisiae. This review will briefly address the challenges involved in the construction of an efficient cellulolytic yeast, with particular focus on the secretion efficiency of cellulases from this host. Additionally, strategies for studying enhanced cellulolytic enzyme secretion, which include both rational and reverse engineering approaches, will be discussed. One such technique includes bio-engineering within genetically diverse strains, combining the strengths of both natural strain diversity and rational strain development. Furthermore, with the advancement in next-generation sequencing, studies that utilise this method of exploiting intra-strain diversity for industrially relevant traits will be reviewed. Finally, future prospects are discussed for the creation of ideal CBP strains with high enzyme production levels.Key Points• Several challenges are involved in the construction of efficient cellulolytic yeast, in particular, the secretion efficiency of cellulases from the hosts.• Strategies for enhancing cellulolytic enzyme secretion, a core requirement for CBP host microorganism development, include both rational and reverse engineering approaches.• One such technique includes bio-engineering within genetically diverse strains, combining the strengths of both natural strain diversity and rational strain development.

Yeast synthetic biology for designed cell factories producing secretory recombinant proteins

Yeasts are prominent hosts for the production of recombinant proteins from industrial enzymes to therapeutic proteins. Particularly, the similarity of protein secretion pathways between these unicellular eukaryotic microorganisms and higher eukaryotic organisms has made them a preferential host to produce secretory recombinant proteins. However, there are several bottlenecks, in terms of quality and quantity, restricting their use as secretory recombinant protein production hosts. In this mini-review, we discuss recent developments in synthetic biology approaches to constructing yeast cell factories endowed with enhanced capacities of protein folding and secretion as well as designed targeted post-translational modification process functions. We focus on the new genetic tools for optimizing secretory protein expression, such as codon-optimized synthetic genes, combinatory synthetic signal peptides and copy number-controllable integration systems, and the advanced cellular engineering strategies, including endoplasmic reticulum and protein trafficking pathway engineering, synthetic glycosylation, and cell wall engineering, for improving the quality and yield of secretory recombinant proteins.

Improved cellulase production in recombinant Saccharomyces cerevisiae by disrupting the cell wall protein-encoding gene CWP2

Budding yeast Saccharomyces cerevisiae has been widely used for heterologous protein production. However, low protein production titer and secretion levels continue to challenge its practical applications. The yeast cell wall plays important roles in yeast cell growth and environmental responses. Nevertheless, the effects of yeast cell wall proteins on heterologous protein production and secretion remain unclear. CWP2 encodes a mannoprotein that is the major component of the yeast cell wall. So far, studies on its function have been very limited. Here we show that CWP2 disruption improved extracellular cellobiohydrolase activity by 85.9%. A calcofluor white hypersensitivity assay revealed increased sensitivity of the mutant compared to the parental strain, indicating impaired cell wall integrity. However, no changes were observed in normal cell growth or growth stressed by tunicamycin and dithiothreitol, suggesting that the unfolded protein response pathway was not affected by the gene disruption. Comparative transcriptome analysis revealed changes in multiple genes involved in cell wall structure, biosynthesis, and cell wall integrity induced by CWP2 disruption, suggesting a pivotal role of Cwp2p in yeast cell wall organization. Notably, CWP2 disruption also led to elevated transcription of a large number of genes involved in ribosome biogenesis, which indicated that CWP2 is not only in yeast cell wall biosynthesis, but also in protein translation. This work reveals novel insights into the functions of CWP2 and also presents a new strategy to increase heterologous protein production in yeast strains by manipulating cell wall-related proteins.

Mutations of Kluyveromyces lactis dolichol kinase enhances secretion of recombinant proteins

Although there are similarities in the core steps of the secretion pathway from yeast to higher eukaryotes, significant functional differences exist even among diverse yeast species. Here, we used next-generation sequencing to identify two mutations in the Kluyveromyces lactis KlSEC59 gene, encoding dolichol kinase (DK), which are responsible for an enhanced secretion phenotype in a previously isolated mutant, MD2/1-9. Compared with the temperature-sensitive Saccharomyces cerevisiae sec59-1 mutant, which exhibits reduced N-glycosylation and decreased secretory efficacy, the identified K. lactis DK mutations had fewer effects on glycosylation, as well as on survival at high temperature and cell wall integrity. Moreover, despite some glycosylation defects, double DK mutations (G405S and I419S) in the K. lactis mutant strain demonstrated three times the level of recombinant α-amylase secretion as the wild-type strain. Overexpression of potential suppressors KlMNN10, KlSEL1, KlERG20, KlSRT1, KlRER2, KlCAX4, KlLPP1 and KlDPP1 in the DK-mutant strain restored carboxypeptidase Y glycosylation to different extents and, with the exception of KISRT1, reduced α-amylase secretion to levels observed in wild-type cells. Our results suggest that enhanced secretion related to reduced activity of mutant DK in K. lactis results from mild glycosylation changes that affect activity of other proteins in the secretory pathway.

N-hypermannose glycosylation disruption enhances recombinant protein production by regulating secretory pathway and cell wall integrity in Saccharomyces cerevisiae

Saccharomyces cerevisiae is a robust host for heterologous protein expression. The efficient expression of cellulases in S. cerevisiae is important for the consolidated bioprocess that directly converts lignocellulose into valuable products. However, heterologous proteins are often N-hyperglycosylated in S. cerevisiae, which may affect protein activity. In this study, the expression of three heterologous proteins, β-glucosidase, endoglucanase and cellobiohydrolase, was found to be N-hyperglycosylated in S. cerevisiae. To block the formation of hypermannose glycan, these proteins were expressed in strains with deletions in key Golgi mannosyltransferases (Och1p, Mnn9p and Mnn1p), respectively. Their extracellular activities improved markedly in the OCH1 and MNN9 deletion strains. Interestingly, truncation of the N-hypermannose glycan did not increase the specific activity of these proteins, but improved the secretion yield. Further analysis showed OCH1 and MNN9 deletion up-regulated genes in the secretory pathway, such as protein folding and vesicular trafficking, but did not induce the unfolded protein response. The cell wall integrity was also affected by OCH1 and MNN9 deletion, which contributed to the release of secretory protein extracellularly. This study demonstrated that mannosyltransferases disruption improved protein secretion through up-regulating secretory pathway and affecting cell wall integrity and provided new insights into glycosylation engineering for protein secretion.

Mnn10 Maintains Pathogenicity in Candida albicans by Extending α-1,6-Mannose Backbone to Evade Host Dectin-1 Mediated Antifungal Immunity

The cell wall is a dynamic structure that is important for the pathogenicity of Candida albicans. Mannan, which is located in the outermost layer of the cell wall, has been shown to contribute to the pathogenesis of C. albicans, however, the molecular mechanism by which this occurs remains unclear. Here we identified a novel α-1,6-mannosyltransferase encoded by MNN10 in C. albicans. We found that Mnn10 is required for cell wall α-1,6-mannose backbone biosynthesis and polysaccharides organization. Deletion of MNN10 resulted in significant attenuation of the pathogenesis of C. albicans in a murine systemic candidiasis model. Inhibition of α-1,6-mannose backbone extension did not, however, impact the invasive ability of C. albicans in vitro. Notably, mnn10 mutant restored the invasive capacity in athymic nude mice, which further supports the notion of an enhanced host antifungal defense related to this backbone change. Mnn10 mutant induced enhanced Th1 and Th17 cell mediated antifungal immunity, and resulted in enhanced recruitment of neutrophils and monocytes for pathogen clearance in vivo. We also demonstrated that MNN10 could unmask the surface β-(1,3)-glucan, a crucial pathogen-associated molecular pattern (PAMP) of C. albicans recognized by host Dectin-1. Our results demonstrate that mnn10 mutant could stimulate an enhanced Dectin-1 dependent immune response of macrophages in vitro, including the activation of nuclear factor-κB, mitogen-activated protein kinase pathways, and secretion of specific cytokines such as TNF-α, IL-6, IL-1β and IL-12p40. In summary, our study indicated that α-1,6-mannose backbone is critical for the pathogenesis of C. albicans via shielding β-glucan from recognition by host Dectin-1 mediated immune recognition. Moreover, our work suggests that inhibition of α-1,6-mannose extension by Mnn10 may represent a novel modality to reduce the pathogenicity of C. albicans.

Mannan plays a crucial role in cell wall structure and virulence of the opportunistic pathogen Candida albicans. Both the invasive ability of the pathogen and the host defense against the pathogen contribute to the outcome of invasive infection. In the present study, we identified a novel α-1,6-mannosyltransferase, which was responsible for cell wall α-1,6-mannose backbone extension in C. albicans. We determined that α-1,6-mannose backbone is necessary for the pathogenesis of C. albicans due to its ability to shield β-(1,3)-glucan from the host Dectin-1 recognition and Th1/Th7 response. Our study highlights a novel strategy to enhance the host immune response towards C. albicans.

Production of secretory cutinase by recombinant Saccharomyces cerevisiae protoplasts

Background

During heterologous protein production using recombinant microbes, the protein tends to accumulate in the cell and may not be secreted. Here, we studied the production of secretory cutinase (heterologous protein) by recombinant Saccharomyces cerevisiae protoplasts.

Findings

Recombinant S. cerevisiae cells (i.e., cells into which the cutinase gene was transferred) secreted trace amounts of cutinase into the broth. Approximately 28 % of the cutinase produced in the cells localized to the cell walls and/or between cell wall and cell membrane (CW). In comparison with cell culture, protoplasts in a static culture secreted measurable amounts of cutinase into the broth. Protoplasts were protected from physical and osmotic stresses by entrapping them in a membrane capsule with a low-viscous liquid-core of 1.92 % w/v calcium-alginate. To increase secretory cutinase production, the entrapped protoplasts were cultivated in a shake flask at low osmotic pressure without disruption. During 60 h of cultivation, the extracellular cutinase activity of the free protoplasts at 29.3 atm and protoplasts entrapped in the capsule at 17.2 atm were 0.13 and 0.39 U/mL, respectively.

Conclusions

This is the first report which demonstrates that the efficient production of a secretable enzyme by using protoplasts isolated from recombinant microbes. This system described here is useful to produce products that accumulate in the CW.

Electronic supplementary material

The online version of this article (doi:10.1186/s40064-016-1806-4) contains supplementary material, which is available to authorized users.

Identification of Candida glabrata genes involved in pH modulation and modification of the phagosomal environment in macrophages

Candida glabrata currently ranks as the second most frequent cause of invasive candidiasis. Our previous work has shown that C. glabrata is adapted to intracellular survival in macrophages and replicates within non-acidified late endosomal-stage phagosomes. In contrast, heat killed yeasts are found in acidified matured phagosomes. In the present study, we aimed at elucidating the processes leading to inhibition of phagosome acidification and maturation. We show that phagosomes containing viable C. glabrata cells do not fuse with pre-labeled lysosomes and possess low phagosomal hydrolase activity. Inhibition of acidification occurs independent of macrophage type (human/murine), differentiation (M1-/M2-type) or activation status (vitamin D₃ stimulation). We observed no differential activation of macrophage MAPK or NFκB signaling cascades downstream of pattern recognition receptors after internalization of viable compared to heat killed yeasts, but Syk activation decayed faster in macrophages containing viable yeasts. Thus, delivery of viable yeasts to non-matured phagosomes is likely not triggered by initial recognition events via MAPK or NFκB signaling, but Syk activation may be involved. Although V-ATPase is abundant in C. glabrata phagosomes, the influence of this proton pump on intracellular survival is low since blocking V-ATPase activity with bafilomycin A1 has no influence on fungal viability. Active pH modulation is one possible fungal strategy to change phagosome pH. In fact, C. glabrata is able to alkalinize its extracellular environment, when growing on amino acids as the sole carbon source in vitro. By screening a C. glabrata mutant library we identified genes important for environmental alkalinization that were further tested for their impact on phagosome pH. We found that the lack of fungal mannosyltransferases resulted in severely reduced alkalinization in vitro and in the delivery of C. glabrata to acidified phagosomes. Therefore, protein mannosylation may play a key role in alterations of phagosomal properties caused by C. glabrata.

Systematic screening of glycosylation- and trafficking-associated gene knockouts in Saccharomyces cerevisiae identifies mutants with improved heterologous exocellulase activity and host secretion

Background

As a strong fermentator, Saccharomyces cerevisiae has the potential to be an excellent host for ethanol production by consolidated bioprocessing. For this purpose, it is necessary to transform cellulose genes into the yeast genome because it contains no cellulose genes. However, heterologous protein expression in S. cerevisiae often suffers from hyper-glycosylation and/or poor secretion. Thus, there is a need to genetically engineer the yeast to reduce its glycosylation strength and to increase its secretion ability.

Results

Saccharomyces cerevisiae gene-knockout strains were screened for improved extracellular activity of a recombinant exocellulase (PCX) from the cellulose digesting fungus Phanerochaete chrysosporium. Knockout mutants of 47 glycosylation-related genes and 10 protein-trafficking-related genes were transformed with a PCX expression construct and screened for extracellular cellulase activity. Twelve of the screened mutants were found to have a more than 2-fold increase in extracellular PCX activity in comparison with the wild type. The extracellular PCX activities in the glycosylation-related mnn10 and pmt5 null mutants were, respectively, 6 and 4 times higher than that of the wild type; and the extracellular PCX activities in 9 protein-trafficking-related mutants, especially in the chc1, clc1 and vps21 null mutants, were at least 1.5 times higher than the parental strains. Site-directed mutagenesis studies further revealed that the degree of N-glycosylation also plays an important role in heterologous cellulase activity in S. cerevisiae.

Conclusions

Systematic screening of knockout mutants of glycosylation- and protein trafficking-associated genes in S. cerevisiae revealed that: (1) blocking Golgi-to-endosome transport may force S. cerevisiae to export cellulases; and (2) both over- and under-glycosylation may alter the enzyme activity of cellulases. This systematic gene-knockout screening approach may serve as a convenient means for increasing the extracellular activities of recombinant proteins expressed in S. cerevisiae.

Random and direct mutagenesis to enhance protein secretion in Ashbya gossypii

To improve the general secretion ability of the biotechnologically relevant fungus Ashbya gossypii, random mutagenesis with ethyl methane sulfonate (EMS) was performed. The selection and screening strategy followed revealed mutants with improved secretion of heterologous Trichoderma reesei endoglucanase I (EGI), native α-amylase and/or native β-glucosidase. One mutant, S436, presented 1.4- to 2-fold increases in all extracellular enzymatic activities measured, when compared with the parent strain, pointing to a global improvement in protein secretion. Three other mutants exhibited 2- to 3-fold improvements in only one (S397, B390) or two (S466) of the measured activities.

 

A targeted genetic approach was also followed. Two homologs of the Saccharomyces cerevisiae GAS1, AgGAS1A (AGL351W) and AgGAS1B (AGL352W), were deleted from the A. gossypii genome. For both copies deletion, a new antibiotic marker cassette conferring resistance to phleomycin, BLE3, was constructed. GAS1 encodes an β-1,3-glucanosyltransglycosylase involved in cell wall assembly. Higher permeability of the cell wall was expected to increase the protein secretion capacity. However, total protein secreted to culture supernatants and secreted EGI activity did not increase in the Aggas1AΔ mutants. Deletion of the AgGAS1B copy affected cellular morphology and resulted in severe retardation of growth, similarly to what has been reported for GAS1-defficient yeast. Thus, secretion could not be tested in these mutants.

Proper protein glycosylation promotes mitogen-activated protein kinase signal fidelity

The ability of cells to sense and respond appropriately to changing environmental conditions is often mediated by signal transduction pathways that employ mitogen-activated protein kinases (MAPKs). In the yeast Saccharomyces cerevisiae, the high-osmolarity glycerol (HOG) and filamentous growth (FG) pathways are activated following hyperosmotic stress and nutrient deprivation, respectively. Whereas the HOG pathway requires the MAPK Hog1, the FG pathway employs the MAPK Kss1. We conducted a comprehensive screen of nearly 5000 gene deletion strains for mutants that exhibit inappropriate cross-talk between the HOG and FG pathways. We identified two novel mutants, mnn10Δ and mnn11Δ, that allow activation of Kss1 under conditions that normally stimulate Hog1. MNN10 and MNN11 encode mannosyltransferases that are part of the N-glycosylation machinery within the Golgi apparatus; deletion of either gene results in N-glycosylated proteins that have shorter mannan chains. Deletion of the cell surface mucin Msb2 suppressed the mnn11Δ phenotype, while mutation of a single glycosylation site within Msb2 was sufficient to confer inappropriate activation of Kss1 by salt stress. These findings reveal new components of the N-glycosylation machinery needed to ensure MAPK signaling fidelity.

Global functional map of the p23 molecular chaperone reveals an extensive cellular network

In parallel with evolutionary developments, the Hsp90 molecular chaperone system shifted from a simple prokaryotic factor into an expansive network that includes a variety of cochaperones. We have taken high-throughput genomic and proteomic approaches to better understand the abundant yeast p23 cochaperone Sba1. Our work revealed an unexpected p23 network that displayed considerable independence from known Hsp90 clients. Additionally, our data uncovered a broad nuclear role for p23, contrasting with the historical dogma of restricted cytosolic activities for molecular chaperones. Validation studies demonstrated that yeast p23 was required for proper Golgi function and ribosome biogenesis, and was necessary for efficient DNA repair from a wide range of mutagens. Notably, mammalian p23 had conserved roles in these pathways as well as being necessary for proper cell mobility. Taken together, our work demonstrates that the p23 chaperone serves a broad physiological network and functions both in conjunction with and sovereign to Hsp90.

Cloning of the Zygosaccharomyces bailii GAS1 homologue and effect of cell wall engineering on protein secretory phenotype

Background

Zygosaccharomyces bailii is a diploid budding yeast still poorly characterized, but widely recognised as tolerant to several stresses, most of which related to industrial processes of production. Because of that, it would be very interesting to develop its ability as a cell factory. Gas1p is a β-1,3-glucanosyltransglycosylase which plays an important role in cell wall construction and in determining its permeability. Cell wall defective mutants of Saccharomyces cerevisiae and Pichia pastoris, deleted in the GAS1 gene, were reported as super-secretive. The aim of this study was the cloning and deletion of the GAS1 homologue of Z. bailii and the evaluation of its deletion on recombinant protein secretion.

Results

The GAS1 homologue of Z. bailii was cloned by PCR, and when expressed in a S. cerevisiae GAS1 null mutant was able to restore the parental phenotype. The respective Z. bailii Δgas1 deleted strain was obtained by targeted deletion of both alleles of the ZbGAS1 gene with deletion cassettes having flanking regions of ~400 bp. The morphological and physiological characterization of the Z. bailii null mutant resulted very similar to that of the corresponding S. cerevisiae mutant. As for S. cerevisiae, in the Z. bailii Δgas1 the total amount of protein released in the medium was significantly higher. Moreover, three different heterologous proteins were expressed and secreted in said mutant. The amount of enzymatic activity found in the medium was almost doubled in the case of the Candida rugosa lipase CRL1 and of the Yarrowia lipolytica protease XPR2, while for human IL-1β secretion disruption had no relevant effect.

Conclusions

The data presented confirm that the engineering of the cell wall is an effective way to improve protein secretion in yeast. They also confirmed that Z. bailii is an interesting candidate, despite the knowledge of its genome and the tools for its manipulation still need to be improved. However, as already widely reported in literature, our data confirmed that an "always working" solution to the problems related to recombinant protein production can be hardly, if never, found; instead, manipulations have to be finely tuned for each specific product and/or combination of host cell and product.

Yeast-secreted recombinant extracellular domain of human CD105 antigen is able to bind TGF-beta type II receptor in vitro

Human CD105 antigen, a type I integral membrane glycoprotein, is expressed as homodimer and oligomer by human endothelial cells, and forms a heteromeric association with TGF-beta signaling receptors I and II. Several mutations of CD105 antigen gene are involved in a vascular disorder known as hereditary hemorrhagic telangiectasia type 1. The proposed mechanism by which CD105 is involved in said disorder is haploinsufficiency. We report expression and characterization of human CD105 antigen extracellular domain in yeast Saccharomyces cerevisiae. Different strategies to influence the release of heterologous proteins in the medium, such as alteration of cell wall integrity or coexpression of protein disulfide isomerase, were addressed. Purified extracellular domain of human CD105 antigen retains capacity to bind human TGF-beta receptor II in vitro.

Endless versatility in the biotechnological applications of Kluyveromyces LAC genes

Most microorganisms adapted to life in milk owe their ability to thrive in this habitat to the evolution of mechanisms for the use of the most abundant sugar present on it, lactose, as a carbon source. Because of their lactose-assimilating ability, Kluyveromyces yeasts have long been used in industrial processes involved in the elimination of this sugar. The identification of the genes conferring Kluyveromyces with a system for permeabilization and intracellular hydrolysis of lactose (LAC genes), along with the current possibilities for their transfer into alternative organisms through genetic engineering, has significantly broadened the industrial profitability of lactic yeasts. This review provides an updated overview of the general properties of Kluyveromyces LAC genes, and the multiple techniques involving their biotechnological utilization. Emphasis is also made on the potential that some of the latest technologies, such as the generation of transgenics, will have for a further benefit in the use of these and related genes.

Production of glucoamylase in pyruvate decarboxylase deletion mutants of the yeast Kluyveromyces lactis

Yeasts are widely used as hosts for the production of diverse heterologous proteins ranging from laboratory scale to industrial scale. The aim of this work is to provide new tools for the production of heterologous proteins in the yeast Kluyveromyces lactis. The promoter of the single gene (KlPDC1) encoding pyruvate decarboxylase is strong, inducible, and responsive to the presence of fermentable sugars and anoxic conditions in this yeast. Expression of KlPDC1 is repressed by ethanol and by autoregulation, a mechanism that involves protein KlPdc1. We constructed a heterologous gene expression cassette for a secreted protein (glucoamylase, GAM) under the control of the KlPDC1 promoter on a stable multicopy plasmid. GAM production by wild-type transformed strains was compared with that of klpdc1-deleted transformants. We obtained higher GAM production in the latter strains, which was due to continued expression of the GAM gene during the stationary phase rather than due to GAM transcription levels higher than the wild-type strains during growth phase. This finding opens new perspectives on the physiology of the stationary phase in K. lactis and suggests the possibility of using high-cell-density approaches for the efficient production of heterologous proteins with this yeast.