Utilization of methanol by yeasts

Regulation of methanol utilisation pathway genes in yeasts

Methylotrophic yeasts such as Candida boidinii, Hansenula polymorpha, Pichia methanolica and Pichia pastoris are an emerging group of eukaryotic hosts for recombinant protein production with an ever increasing number of applications during the last 30 years. Their applications are linked to the use of strong methanol-inducible promoters derived from genes of the methanol utilisation pathway. These promoters are tightly regulated, highly repressed in presence of non-limiting concentrations of glucose in the medium and strongly induced if methanol is used as carbon source. Several factors involved in this tight control and their regulatory effects have been described so far. This review summarises available data about the regulation of promoters from methanol utilisation pathway genes. Furthermore, the role of cis and trans acting factors (e.g. transcription factors, glucose processing enzymes) in the expression of methanol utilisation pathway genes is reviewed both in the context of the native cell environment as well as in heterologous hosts.

Operational strategies, monitoring and control of heterologous protein production in the methylotrophic yeast Pichia pastoris under different promoters: a review

The methylotrophic yeast Pichia pastoris has been widely reported as a suitable expression system for heterologous protein production. The use of different phenotypes under PAOX promoter, other alternative promoters, culture medium, and operational strategies with the objective to maximize either yield or productivity of the heterologous protein, but also to obtain a repetitive product batch to batch to get a robust process for the final industrial application have been reported. Medium composition, kinetics growth, fermentation operational strategies from fed-batch to continuous cultures using different phenotypes with the most common PAOX promoter and other novel promoters (GAP, FLD, ICL), the use of mixed substrates, on-line monitoring of the key fermentation parameters (methanol) and control algorithms applied to the bioprocess are reviewed and discussed in detail.

Pectin utilization by the methylotrophic yeast Pichia methanolica

The methylotrophic yeast Pichia methanolica was able to grow on pectic compounds, pectin and polygalacturonate, as sole carbon sources. Under the growth conditions used, P. methanolica exhibited increased levels of pectin methylesterase, and pectin-depolymerizing and methanol-metabolizing enzyme activities. On the other hand, P. methanolica has two alcohol oxidase (AOD) genes, MOD1 and MOD2. On growth on pectin, the P. methanolica mod1Delta and mod1Deltamod2Delta strains showed a severe defect in the growth yield, although the mod2Delta strain could grow on polygalacturonate to the same extent as the wild-type strain. The expression of MOD1 was detected in pectin-grown cells, but the MOD2-gene expression detected by pectin was much lower than that of MOD1. Moreover, pectin could induce peroxisome proliferation in P. methanolica, like methanol and oleic acid. These findings showed that P. methanolica was able to utilize the methylester moiety of pectin by means of methanol-metabolic enzymes in peroxisomes, and that the functional AOD subunit for pectin utilization was Mod1p in P. methanolica.

Identification of four alcohol oxidases from methylotrophic yeasts

Three yeast strains capable of utilizing methanol as sole carbon and energy source were isolated. Two were classified as Candida boidinii, while the third belonged in the genus Pichia. From these three strains, four alcohol oxidases genes were identified and the sequences of the coding regions were determined: one from each Candida boidinii (aox0673 and aox0680) and two from Pichia sp. 159 (aoxA and aoxB). Methanol induces both alcohol oxidases in Pichia sp. 159: the levels of aoxA and aoxB mRNA reach about 100% and 300%, respectively, of that of his4 mRNA. aoxA, but not aoxB, is expressed at a low level in the presence of glucose. The newly described alcohol oxidases have proper dinucleotide binding sites and PTS1-like C-terminal tripeptides, identified as important elements for peroxisomal localization.

Production of metabolites by methylotrophic yeasts

Recent advances in biotechnology of methanol-utilizing yeasts are briefly summarized. The emphasis is given to production of some fine and commercial chemicals such as formaldehyde, formate, hydrogen peroxide, dihydroxyacetone, ATP, FAD as well as proteins, specifically alcohol oxidase. The advantages of mutants and recombinants derived from methylotrophic yeasts for efficient production of various useful materials are demonstrated.

A methylotrophic pathway participates in pectin utilization by Candida boidinii

The methylotrophic yeast Candida boidinii S2 was found to be able to grow on pectin or polygalacturonate as a carbon source. When cells were grown on 1% (wt/vol) pectin, C. boidinii exhibited induced levels of the pectin-depolymerizing enzymes pectin methylesterase (208 mU/mg of protein), pectin lyase (673 mU/mg), pectate lyase (673 mU/mg), and polygalacturonase (3.45 U/mg) and two methanol-metabolizing peroxisomal enzymes, alcohol oxidase (0.26 U/mg) and dihydroxyacetone synthase (94 mU/mg). The numbers of peroxisomes also increased ca. two- to threefold in cells grown on these pectic compounds (3.34 and 2.76 peroxisomes/cell for cells grown on pectin and polygalacturonate, respectively) compared to the numbers in cells grown on glucose (1.29 peroxisomes/cell). The cell density obtained with pectin increased as the degree of methyl esterification of pectic compounds increased, and it decreased in strains from which genes encoding alcohol oxidase and dihydroxyacetone synthase were deleted and in a peroxisome assembly mutant. Our study showed that methanol metabolism and peroxisome assembly play important roles in the degradation of pectin, especially in the utilization of its methyl ester moieties.

Biosynthesis and regulation of the peroxisomal methanol oxidase from the methylotrophic yeast Hansenula polymorpha

The biosynthesis of methanol oxidase, a peroxisomal enzyme in the methanol-utilizing yeast Hansenula polymorpha, was studied in vitro. Translation of Hansenula mRNA in a rabbit reticulocyte lysate yields methanol oxidase protein in high amounts. The apparent molecular mass of the protein was found to be identical to the subunit of the functional multimeric enzyme, which indicates the absence of an N-terminal extension typical of most transported proteins. The regulation of methanol oxidase by glucose repression and depression as well as by induction of methanol was shown to be controlled at the level of transcription. Two mutants of Hansenula polymorpha, unable to grow on methanol as a carbon and energy source were shown to be affected in methanol oxidase synthesis.

Substructure of crystalline peroxisomes in methanol-grown Hansenula polymorpha: evidence for an in vivo crystal of alcohol oxidase

The substructural organization of completely crystalline peroxisomes present in Hansenula polymorpha cells grown under methanol limitation in a chemostat was investigated by different cytochemical and ultrastructural techniques. Time-dependent cytochemical staining experiments indicated that activities of the two main constituents of these organelles, namely, alcohol oxidase and catalase, were present throughout the crystalline matrix. Catalase was completely removed from isolated peroxisomes by osmotic shock treatment. After such treatment, the ultrastructure of the crystalline matrix of the organelles remained virtually intact. Because alcohol oxidase activity was still present in this matrix, it was concluded that alcohol oxidase protein is the only structural element of the peroxisomal crystalloids. The molecular architecture of the crystalloids was investigated in ultrathin cryosections which permitted recognition of individual molecules in the crystalline matrix. Depending on the plane of sectioning, different crystalline patterns were observed. Tilting experiments indicated that these images were caused by superposition of octameric alcohol oxidase molecules arranged in a tetragonal lattice. A three-dimensional model of the crystalloid is presented. The repeating unit of this structure is composed of four alcohol oxidase molecules. The crystalloid represents an open structure, which may explain the observed free mobility of catalase molecules.

Substructure of crystalline peroxisomes in methanol-grown Hansenula polymorpha: evidence for an in vivo crystal of alcohol oxidase

The substructural organization of completely crystalline peroxisomes present in Hansenula polymorpha cells grown under methanol limitation in a chemostat was investigated by different cytochemical and ultrastructural techniques. Time-dependent cytochemical staining experiments indicated that activities of the two main constituents of these organelles, namely, alcohol oxidase and catalase, were present throughout the crystalline matrix. Catalase was completely removed from isolated peroxisomes by osmotic shock treatment. After such treatment, the ultrastructure of the crystalline matrix of the organelles remained virtually intact. Because alcohol oxidase activity was still present in this matrix, it was concluded that alcohol oxidase protein is the only structural element of the peroxisomal crystalloids. The molecular architecture of the crystalloids was investigated in ultrathin cryosections which permitted recognition of individual molecules in the crystalline matrix. Depending on the plane of sectioning, different crystalline patterns were observed. Tilting experiments indicated that these images were caused by superposition of octameric alcohol oxidase molecules arranged in a tetragonal lattice. A three-dimensional model of the crystalloid is presented. The repeating unit of this structure is composed of four alcohol oxidase molecules. The crystalloid represents an open structure, which may explain the observed free mobility of catalase molecules.

Development of amine oxidase-containing peroxisomes in yeasts during growth on glucose in the presence of methylamine as the sole source of nitrogen

The metabolism of methylamine as the nitrogen source for growth of the non-methylotrophic yeast Candida utilis and the methylotrophic yeast Hansenula polymorpha was investigated. Growth of both organisms in media with glucose and methylamine was associated with the presence of an amine oxidase in these cells. The enzyme catalyses the oxidation of methylamine by molecular oxygen into ammonia, formaldehyde and hydrogen peroxide and it is considered to be the key enzyme in methylamine metabolism in the organisms studied. In addition to synthesis of amine oxidase, derepression of catalase, formaldehyde and formate dehydrogenase was also observed upon transfer of cells of the two organisms from media containing ammonium ions into media containing methylamine as the nitrogen source. The synthesis of enzymes was paralleled by the development of a number of large microbodies in the cells. Cytochemical staining experiments indicated that the amine oxidase activity was located in the microbodies in both organisms. Catalase-activity was also demonstrated in these organelles, which can therefore be considered as peroxisomes. The present contribution is the first description of a peroxisomal amine oxidase.

Ultrastructure of methanotrophic yeasts

The cellular structure of two yeast strains capable of growth on methane was investigated by electron microscopy. Microbodies were observed in cells of Sporobolomyces roseus strain Y and Rhodotorula glutinis strain CY when grown on methane but rarely when grown on glucose. The size of the microbodies and the number observed per cell in a thin section did not increase with culture age. No crystalline organization was observed within these organelles. Similar microbodies were also observed in cells of R. glutinis CY grown on hexadecane. The plasma membranes of both methane and hexadecane-grown cells exhibited increased invagination compared to that of glucose-grown cells. Catalase activity was detected in the microbodies of alkane-grown cells by using 3,3'-diaminobenzidine as a cytochemical stain. The data presented suggest that microbodies, and the catalase contained within them, play a role in eucaryotic methane metabolism.