The transcription factor Snf1p is involved in a Tup1p-independent manner in the glucose regulation of the major methanol metabolism genes of Hansenula polymorpha

The role of Mig1, Mig2, Tup1 and Hap4 transcription factors in regulation of xylose and glucose fermentation in the thermotolerant yeast Ogataea polymorpha

Glucose is a preferred carbon source for most living organisms. The metabolism and regulation of glucose utilization are well studied mostly for Saccharomyces cerevisiae. Xylose is the main pentose sugar released from the lignocellulosic biomass, which has a high potential as a renewable feedstock for bioethanol production. The thermotolerant yeast Ogataea (Hansenula) polymorpha, in contrast to S. cerevisiae, is able to metabolize and ferment not only glucose but also xylose. However, in non-conventional yeasts, the regulation of glucose and xylose metabolism remains poorly understood. In this study, we characterize the role of transcriptional factors Mig1, Mig2, Tup1 and Hap4 in the natural xylose-fermenting yeast O. polymorpha. The deletion of MIG1 had no significant influence on ethanol production either from xylose or glucose, however the deletion of both MIG1 and MIG2 reduced the amount of ethanol produced from these sugars. The deletion of HAP4-A and TUP1 genes resulted in increased ethanol production from xylose. Inversely, the overexpression of HAP4-A and TUP1 genes reduced ethanol production during xylose alcoholic fermentation. Thus, HAP4-A and TUP1 are involved in repression of xylose metabolism and fermentation in yeast O. polymorpha and their deletion could be a viable strategy to improve ethanol production from this pentose.

Glucose regulation in the methylotrophic yeast Hansenula (Ogataea) polymorpha is mediated by a putative transceptor Gcr1

The HpGcr1, a hexose transporter homologue from the methylotrophic yeast Hansenula (Ogataea) polymorpha, was previously identified as being involved in glucose repression. Intriguingly, potential HpGcr1 orthologues are found only in the genomes of a few yeasts phylogenetically closely related to H. polymorpha, but are absent in all other yeasts. The other closest HpGcr1 homologues are fungal high-affinity glucose symporters or putative transceptors suggesting a possible HpGcr1 origin due to a specific archaic gene retention or via horizontal gene transfer from Eurotiales fungi. Herein we report that, similarly to other yeast non-transporting glucose sensors, the substitution of the conserved arginine residue converts HpGcr1^R165K into a constitutively signaling form. Synthesis of HpGcr1^R165K in gcr1Δ did not restore glucose transport or repression but instead profoundly impaired growth independent of carbon source used. Simultaneously, gcr1Δ was impaired in transcriptional induction of repressible peroxisomal alcohol oxidase and in growth on methanol. Overexpression of the functional transporter HpHxt1 in gcr1Δ partially restored growth on glucose and glucose repression but did not rescue impaired growth on methanol. Heterologous expression of HpGcr1 in a Saccharomyces cerevisiae hxt-null strain did not restore glucose uptake due to protein mislocalization. However, HpGcr1 overexpression in H. polymorpha led to increased sensitivity to extracellular 2-deoxyglucose, suggesting HpGcr1 is a functional glucose carrier. The combined data suggest that HpGcr1 represents a novel type of yeast glucose transceptor functioning also in the absence of glucose.

Repression vs. activation of MOX, FMD, MPP1 and MAL1 promoters by sugars in Hansenula polymorpha: the outcome depends on cell's ability to phosphorylate sugar

A high-throughput approach was used to assess the effect of mono- and disaccharides on MOX, FMD, MPP1 and MAL1 promoters in Hansenula polymorpha. Site-specifically designed strains deficient for (1) hexokinase, (2) hexokinase and glucokinase, (3) maltose permease or (4) maltase were used as hosts for reporter plasmids in which β-glucuronidase (Gus) expression was controlled by these promoters. The reporter strains were grown on agar plates containing varied carbon sources and Gus activity was measured in permeabilized cells on microtitre plates. We report that monosaccharides (glucose, fructose) repress studied promoters only if phosphorylated in the cell. Glucose-6-phosphate was proposed as a sugar repression signalling metabolite for H. polymorpha. Intriguingly, glucose and fructose strongly activated expression from these promoters in strains lacking both hexokinase and glucokinase, indicating that unphosphorylated monosaccharides have promoter-derepressing effect. We also show that maltose and sucrose must be internalized and split into monosaccharides to exert repression on MOX promoter. We demonstrate that at yeast growth on glucose-containing agar medium, glucose-limitation is rapidly created that promotes derepression of methanol-specific promoters and that derepression is specifically enhanced in hexokinase-negative strain. We recommend double kinase-negative and hexokinase-negative mutants as hosts for heterologous protein production from MOX and FMD promoters.

Molecular characterization of Candida boidinii MIG1 and its role in the regulation of methanol-inducible gene expression

Methanol-inducible gene promoters in methanol-utilizing yeasts are used in high-level heterologous gene expression systems. Generally, expression of methanol-inducible genes is completely repressed by the presence of glucose. In this study we identified the MIG1 gene in Candida boidinii, which encodes a homologue of the glucose repressor Mig1p of Saccharomyces cerevisiae. Disruption of the CbMIG1 gene had no growth effect on various carbon sources. Activation of the methanol-inducible AOD1 gene, which encodes alcohol oxidase, was increased in the early stage of methanol induction when cells of the CbMIG1-disrupted strain were transferred from glucose medium to methanol medium. Furthermore, CbMig1p tagged with yellow fluorescent protein was primarily localized in the nucleus of glucose-grown cells, but was diffuse in the cytosol of methanol-grown cells. This cytosolic diffusion in methanol-grown cells occurred in a CbMsn5p-dependent manner. These results suggest that CbMig1p is involved in negative regulation of methanol-inducible gene expression in C. boidinii.

Trm2p-dependent derepression is essential for methanol-specific gene activation in the methylotrophic yeast Candida boidinii

We identified a gene, designated TRM2, responsible for methanol-inducible gene expression in the methylotrophic yeast Candida boidinii. The encoded protein Trm2p contains two C(2)H(2)-type zinc finger motifs near the N terminus and shows high similarity to Saccharomyces cerevisiae Adr1p and Pichia pastoris Mxr1p. A C. boidinii gene-disrupted strain (trm2Delta) could not grow on methanol or oleate, but could grow on glucose or ethanol. Trm2p was necessary for the activation of five methanol-inducible promoters tested. Trm2p was localized to the nucleus during growth on nonfermentable carbon sources, but to the cytosol during growth on glucose. A chromatin immunoprecipitation assay revealed that Trm2p specifically bound to the promoters of the alcohol oxidase gene (AOD1) and the dihydroxyacetone synthase gene in cells grown on methanol or oleate, but did not bind to these promoters in cells grown on glucose. The derepressed level of expression of AOD1, which was observed in the trm1Delta strain (the TRM1 gene encodes a transcription factor responsible for methanol-specific gene activation), was decreased in the trm1Deltatrm2Delta strain to a level similar to that observed in the trm2Delta strain. These results suggest that Trm2p-dependent derepression is essential for the Trm1p-dependent methanol-specific gene activation in C. boidinii.

The role of Hansenula polymorpha MIG1 homologues in catabolite repression and pexophagy

In the methanol-utilizing yeast Hansenula polymorpha, glucose and ethanol trigger the repression of peroxisomal enzymes at the transcriptional level, and rapid and selective degradation of methanol-induced peroxisomes by means of a process termed pexophagy. In this report we demonstrate that deficiency in the putative H. polymorpha homologues of transcriptional repressors Mig1 (HpMig1 and HpMig2), as well as HpTup1, partially and differentially affects the repression of peroxisomal alcohol oxidase by sugars and ethanol. As reported earlier, deficiency in HpTup1 leads to impairment of glucose- or ethanol-induced macropexophagy. In H. polymorpha mig1mig2 double-deletion cells, macropexophagy was also substantially impaired, whereas micropexophagy became a dominant mode of autophagic degradation. Our findings suggest that homologues of the elements of the Saccharomyces cerevisiae main repression pathway have pleiotropic functions in H. polymorpha.