Characterization of the Arxula adeninivorans AHOG1 gene and the encoded mitogen-activated protein kinase

Yeasts of the Blastobotrys genus are promising platform for lipid-based fuels and oleochemicals production

Strains of the yeast genus Blastobotrys (subphylum Saccharomycotina) represent a valuable biotechnological resource for basic biochemistry research, single-cell protein, and heterologous protein production processes. Species of this genus are dimorphic, non-pathogenic, thermotolerant, and can assimilate a variety of hydrophilic and hydrophobic substrates. These can constitute a single-cell oil platform in an emerging bio-based economy as oleaginous traits have been discovered recently. However, the regulatory network of lipogenesis in these yeasts is poorly understood. To keep pace with the growing market demands for lipid-derived products, it is critical to understand the lipid biosynthesis in these unconventional yeasts to pinpoint what governs the preferential channelling of carbon flux into lipids instead of the competing pathways. This review summarizes information relevant to the regulation of lipid metabolic pathways and prospects of metabolic engineering in Blastobotrys yeasts for their application in food, feed, and beyond, particularly for fatty acid-based fuels and oleochemicals. KEY POINTS: • The production of biolipids by heterotrophic yeasts is reviewed. • Summary of information concerning lipid metabolism regulation is highlighted. • Special focus on the importance of diacylglycerol acyltransferases encoding genes in improving lipid production is made.

Plasma membrane proteome analysis identifies a role of barley membrane steroid binding protein in root architecture response to salinity

Although the physiological consequences of plant growth under saline conditions have been well described, understanding the core mechanisms conferring plant salt adaptation has only started. We target the root plasma membrane proteomes of two barley varieties, cvs. Steptoe and Morex, with contrasting salinity tolerance. In total, 588 plasma membrane proteins were identified by mass spectrometry, of which 182 were either cultivar or salinity stress responsive. Three candidate proteins with increased abundance in the tolerant cv. Morex were involved either in sterol binding (a GTPase-activating protein for the adenosine diphosphate ribosylation factor [ZIGA2], and a membrane steroid binding protein [MSBP]) or in phospholipid synthesis (phosphoethanolamine methyltransferase [PEAMT]). Overexpression of barley MSBP conferred salinity tolerance to yeast cells, whereas the knock-out of the heterologous AtMSBP1 increased salt sensitivity in Arabidopsis. Atmsbp1 plants showed a reduced number of lateral roots under salinity, and root-tip-specific expression of barley MSBP in Atmsbp1 complemented this phenotype. In barley, an increased abundance of MSBP correlates with reduced root length and lateral root formation as well as increased levels of auxin under salinity being stronger in the tolerant cv. Morex. Hence, we concluded the involvement of MSBP in phytohormone-directed adaptation of root architecture in response to salinity.

Comparative analysis of the grain proteome fraction in barley genotypes with contrasting salinity tolerance during germination

In the present paper, we based a search for candidates underlying different levels of salinity tolerance during germination in the Oregon Wolfe Barley mapping population (DOM x REC) by proteomic profiling of the mature grain of lines showing differing levels of salinity tolerance. By contrasting the parents DOM and REC, displaying divergent stress responses, and two tolerant and two sensitive segregants, six protein spots were identified that showed a differential abundance between the tolerant and the sensitive lines. The tolerant lines expressed a higher level of 6-phosphogluconate dehydrogenase and glucose/ribitol dehydrogenase (Glc/RibDH). Both proteins were heterologously over-expressed in an osmo-sensitive yeast strain and over-expression of Glc/RibDH resulted in an enhanced ability of yeast transformants to grow on salt containing media. A quantitative trait locus (QTL) analysis of the population germinating at different salt concentrations led to the identification of two chromosome regions on 5H and one on 7H associated with salt stress response. A dense barley transcript map was employed to map the genomic region of all identified proteins. Two of these, heat-shock protein 70 and Glc/RibDH, co-localized with the identified QTL on chromosome 5H. The putative functional role of the candidates is discussed.

Characterization and expression analysis of a gene cluster for nitrate assimilation from the yeast Arxula adeninivorans

In Arxula adeninivorans nitrate assimilation is mediated by the combined actions of a nitrate transporter, a nitrate reductase and a nitrite reductase. Single-copy genes for these activities (AYNT1, AYNR1, AYNI1, respectively) form a 9103 bp gene cluster localized on chromosome 2. The 3210 bp AYNI1 ORF codes for a protein of 1070 amino acids, which exhibits a high degree of identity to nitrite reductases from the yeasts Pichia anomala (58%), Hansenula polymorpha (58%) and Dekkera bruxellensis (54%). The second ORF (AYNR1, 2535 bp) encodes a nitrate reductase of 845 residues that shows significant (51%) identity to nitrate reductases of P. anomala and H. polymorpha. The third ORF in the cluster (AYNT1, 1518 bp) specifies a nitrate transporter with 506 amino acids, which is 46% identical to that of H. polymorpha. The three genes are independently expressed upon induction with NaNO(3). We quantitatively analysed the promoter activities by qRT-PCR and after fusing individual promoter fragments to the phytase (phyK) gene from Klebsiella sp. ASR1. The AYNI1 promoter was found to exhibit the highest activity, followed by the AYNT1 and AYNR1 elements. Direct measurements of nitrate and nitrite reductase activities performed after induction with NaNO(3) are compatible with these results. Both enzymes show optimal activity at around 42 degrees C and near-neutral pH, and require FAD as a co-factor and NADPH as electron donor.

The MAPk ASTE11 is involved in the maintenance of cell wall integrity and in filamentation in Arxula adeninivorans, but not in adaptation to hypertonic stress

In many fungal species, cell growth and morphology, thermo- and osmotolerance are regulated by mitogen-activated protein kinase (MAPk) cascades. Ste11p is a MAP kinase kinase kinase, which plays a central role in various pathways in Saccharomyces cerevisiae. Here we describe ASTE11, an STE11 homologue from Arxula adeninivorans, an imperfect, dimorphic, but nonpathogenic and extremophilic yeast. ASTE11 lacks introns, and codes for a protein of 824 amino acids with a predicted molecular weight of 91.6 kDa. The gene is constitutively expressed at low levels, but is induced by high-salt stress. To facilitate functional analysis of ASTE11, disruption and overexpressing mutants were constructed. The phenotypes of these strains indicate that Aste11p is involved in regulating aspects of cell wall structure and form, but is dispensable for adaptation to hypertonic stress. Despite its structural homology to STE11, ASTE11 cannot complement the mating defect of S. cerevisiae ste11 mutants. These findings emphasize that, although components of MAPk pathways are conserved among yeasts, they often operate in different contexts in different species.

The Arxula adeninivorans ATAL gene encoding transaldolase-gene characterization and biotechnological exploitation

The yeast Arxula adeninivorans provides an attractive expression platform and can be exploited as gene source for biotechnologically interesting proteins. In the following study, a striking example for the combination of both aspects is presented. The transaldolase-encoding A. adeninivorans ATAL gene, including its promoter and terminator elements, was isolated and characterized. The gene includes a coding sequence of 963 bp encoding a putative 321 amino acid protein of 35.0 kDa. The enzyme characteristics analyzed from isolates of native strains and recombinant strains overexpressing the ATAL gene revealed a molecular mass of ca. 140 kDa corresponding to a tetrameric structure, a pH optimum of ca. 5.5, and a temperature optimum of 20 degrees C. The preferred substrates for the enzyme include D-erythrose-4-phosphate and D-fructose-6-phosphate, whereas D-glyceraldehyde is not converted. The ATAL expression level under salt-free conditions was observed to increase in media supplemented with 5% NaCl rendering the ATAL promoter attractive for moderate heterologous gene expression under high-salt conditions. Its suitability was assessed for the expression of a human serum albumin (HSA) reporter gene.

Construction of an Arxula adeninivorans host-vector system based on trp1 complementation

A host/vector expression system based on an Arxula adeninivorans Delta atrp1 gene disruption mutant has been constructed. For this purpose the ATRP1 gene encoding a phosphoribosyl anthranilate isomerase was isolated from the yeast A. adeninivorans and its genome locus was characterized. The Delta atrp1 mutant was generated applying an amplified DNA fragment containing the ALEU2m gene flanked by ATRP1 gene sequences of some 750 bp. The generated auxotrophic host strain was transformed with the plasmid pAL-ATRP1-amyA, which contains the ATRP1 gene as selection marker and the 25S rDNA for targeting. For expression assessment, the plasmid was equipped with an expression cassette consisting of the Bacillus amyloliquefaciens-derived amyA gene fused to the constitutive A. adeninivorans-derived TEF1 promoter and Saccharomyces cerevisiae-derived PHO5 terminator. Transformants contained a single chromosomal copy of the heterologous DNA and were found to be mitotically stable. In initial fermentation trials on a 200 ml shake flask scale maximal alpha-amylase product levels of ca. 300 nkat ml(-1) were observed after 72 h of cultivation with more than 95% of the recombinant alpha-amylase accumulated in the culture medium.

Baker's yeast as a tool for the development of antifungal kinase inhibitors—targeting protein kinase C and the cell integrity pathway

Today, the yeast Saccharomyces cerevisiae is probably the best-studied eukaryotic organism. This review first focuses on the signaling process which is mediated by the unique yeast protein kinase C (Pkc1p) and a downstream mitogen-activated protein kinase (MAPK) cascade. This pathway ensures cellular integrity by sensing cell surface stress and controlling cell wall biosynthesis and progression through the cell cycle. The domain structure of Pkc1p is conserved from yeast to humans. A yeast system for heterologous expression of specific domains in a chimeric yeast/mammalian PKC enzyme ("domain shuffling") is depicted. It is also proposed how this system could be employed for the study of protein kinase inhibitors in high-throughput screens. Moreover, a reporter assay that allows a quantitative readout of the activity of the cell integrity signaling pathway is introduced. Since a variety of protein kinases take part in the signal transduction, this broadens the range of targets for potential inhibitors.

Debaryomyces hansenii, a highly osmo-tolerant and halo-tolerant yeast, maintains activated Dhog1p in the cytoplasm during its growth under severe osmotic stress

The HOG pathway is an important mitogen-activated protein kinase (MAPK) signal transduction pathway in Saccharomyces cerevisiae that mediates adaptation of cells to hyper-osmotic stress. Activation of this pathway causes rapid but transient, phosphorylation of the MAPK Hog1p. Phosphorylated Hog1p is rapidly transported to the nucleus that results in the transcription of target genes. The HOG pathway appears to be ubiquitous in yeast. Components of HOG pathway have also been identified in Debaryomyces hansenii, a highly osmotolerant and halotolerant yeast. We have studied activation of HOG pathway in D. hansenii under different stress conditions. Our experiments demonstrated that the pathway is activated by high osmolarity, oxidative and UV stress but not by heat stress. We have provided evidence, for the first time, that D. hansenii maintains phosphorylated Dhog1p in the cytoplasm during its growth under severe osmotic stress.

Assessment of Hansenula polymorpha and Arxula adeninivorans-derived rDNA-targeting elements for the design of Arxula adeninivorans expression vectors

Different targeting sequences derived from the Arxula adeninivorans and Hansenula polymorpha rDNA clusters were tested in A. adeninivorans integration/expression vectors. For element identification, the rDNA unit of A. adeninivorans (accession number ) was first isolated and characterized in addition to the known H. polymorpha unit. The rDNA is a cluster of some forty 7653-bp units without the 5S rDNA gene. The selected elements were integrated into a set of A. adeninivorans expression/integration vectors harbouring a TEF1 promoter - amyA ORF - PHO5 terminator sequence as reporter gene. No differences in mitotic stability, copy number and transformation frequency were observed. All transformants harboured a single copy integrated into the rDNA by a homologous recombination. In contrast, the choice of the rDNA targeting sequence was found to be of impact on productivity. Use of ETS-18S-5.8S fragments from both organisms resulted in a more than 50% increase in comparison to the use of other elements, independent of the orientation within the vector.