Heterologous gene expression in Saccharomyces cerevisiae

Membrane potential dynamics unveil the promise of bioelectrical antimicrobial susceptibility Testing (BeAST) for anti-fungal screening

Membrane potential is a useful marker for antimicrobial susceptibility testing (AST) due to its fundamental roles in cell function. However, the difficulties associated with measuring the membrane potential in microbes restrict its broad application. In this study, we present bioelectrical AST (BeAST) using the model fungus Saccharomyces cerevisiae. Using fluorescent indicators [DiBAC4(3), ThT, and TMRM], we measured plasma and mitochondrial membrane-potential dynamics upon electric stimulation. We find that a 2.5 second electric stimulation induces hyperpolarization of plasma membrane lasting 20 minutes in vital S. cerevisiae, but depolarization in inhibited cells. The numerical simulation of FitzHugh-Nagumo model successfully recapitulates vitality-dependent dynamics. The model also suggests that the magnitude of plasma-membrane potential dynamics (PMD) correlates with the degree of inhibition. To test this prediction and to examine if BeAST can be used for assessing novel anti-fungal compounds, we treat cells with biogenic silver nanoparticles (bioAgNPs) synthesized using orange fruit flavonoids and Fusarium oxysporum. Comparing BeAST with optical density assay alongside various stressors, we show that PMD correlates with the magnitude of growth inhibitions. These results suggest that BeAST holds promise for screening anti-fungal compounds, offering a valuable approach to tackling antimicrobial resistance.

IMPORTANCE

Rapid assessment of the efficacy of antimicrobials is important for optimizing treatments, avoiding misuse and facilitating the screening of new antimicrobials. The need for rapid antimicrobial susceptibility testing (AST) is growing with the rise of antimicrobial resistance. Here, we present bioelectrical AST (BeAST). Combining time-lapse microscopy and mathematical modeling, we show that electrically induced membrane potential dynamics of yeast cells correspond to the strength of growth inhibition. Furthermore, we demonstrate the utility of BeAST for assessing antimicrobial activities of novel compounds using biogenic silver nanoparticles.

Organelle Engineering in Yeast: Enhanced Production of Protopanaxadiol through Manipulation of Peroxisome Proliferation in Saccharomyces cerevisiae

Isoprenoids, which are natural compounds with diverse structures, possess several biological activities that are beneficial to humans. A major consideration in isoprenoid production in microbial hosts is that the accumulation of biosynthesized isoprenoid within intracellular membranes may impede balanced cell growth, which may consequently reduce the desired yield of the target isoprenoid. As a strategy to overcome this suggested limitation, we selected peroxisome membranes as depots for the additional storage of biosynthesized isoprenoids to facilitate increased isoprenoid production in Saccharomyces cerevisiae. To maximize the peroxisome membrane storage capacity of S.cerevisiae, the copy number and size of peroxisomes were increased through genetic engineering of the expression of three peroxisome biogenesis-related peroxins (Pex11p, Pex34p, and Atg36p). The genetically enlarged and high copied peroxisomes in S.cerevisiae were stably maintained under a bioreactor fermentation condition. The peroxisome-engineered S.cerevisiae strains were then utilized as host strains for metabolic engineering of heterologous protopanaxadiol pathway. The yields of protopanaxadiol from the engineered peroxisome strains were ca 78% higher than those of the parent strain, which strongly supports the rationale for harnessing the storage capacity of the peroxisome membrane to accommodate the biosynthesized compounds. Consequently, this study presents in-depth knowledge on peroxisome biogenesis engineering in S.cerevisiae and could serve as basic information for improvement in ginsenosides production and as a potential platform to be utilized for other isoprenoids.

Current state and recent advances in biopharmaceutical production in Escherichia coli, yeasts and mammalian cells

Almost all of the 200 or so approved biopharmaceuticals have been produced in one of three host systems: the bacterium Escherichia coli, yeasts (Saccharomyces cerevisiae, Pichia pastoris) and mammalian cells. We describe the most widely used methods for the expression of recombinant proteins in the cytoplasm or periplasm of E. coli, as well as strategies for secreting the product to the growth medium. Recombinant expression in E. coli influences the cell physiology and triggers a stress response, which has to be considered in process development. Increased expression of a functional protein can be achieved by optimizing the gene, plasmid, host cell, and fermentation process. Relevant properties of two yeast expression systems, S. cerevisiae and P. pastoris, are summarized. Optimization of expression in S. cerevisiae has focused mainly on increasing the secretion, which is otherwise limiting. P. pastoris was recently approved as a host for biopharmaceutical production for the first time. It enables high-level protein production and secretion. Additionally, genetic engineering has resulted in its ability to produce recombinant proteins with humanized glycosylation patterns. Several mammalian cell lines of either rodent or human origin are also used in biopharmaceutical production. Optimization of their expression has focused on clonal selection, interference with epigenetic factors and genetic engineering. Systemic optimization approaches are applied to all cell expression systems. They feature parallel high-throughput techniques, such as DNA microarray, next-generation sequencing and proteomics, and enable simultaneous monitoring of multiple parameters. Systemic approaches, together with technological advances such as disposable bioreactors and microbioreactors, are expected to lead to increased quality and quantity of biopharmaceuticals, as well as to reduced product development times.

Introduction and expression of genes for metabolic engineering applications in Saccharomyces cerevisiae

Metabolic pathway engineering in the yeast Saccharomyces cerevisiae leads to improved production of a wide range of compounds, ranging from ethanol (from biomass) to natural products such as sesquiterpenes. The introduction of multienzyme pathways requires precise control over the level and timing of expression of the associated genes. Gene number and promoter strength/regulation are two critical control points, and multiple studies have focused on modulating these in yeast. This MiniReview focuses on methods for introducing genes and controlling their copy number and on the many promoters (both constitutive and inducible) that have been successfully employed. The advantages and disadvantages of the methods will be presented, and applications to pathway engineering will be highlighted.

Overview of protein expression in Saccharomyces cerevisiae

This overview presents vectors and host strains that are available to direct gene expression in S. cerevisiae, including information on promoters, vector maintenance and copy number, transcription terminators, and selectable markers. Challenges to the expression of foreign proteins are also covered, including attainment of desired production yield, production of protein with appropriate post-translational modifications, conformation and function, and secretion to the extracellular medium.

Transfer and expression of heterologous genes in yeasts other than Saccharomyces cerevisiae

In the past few years, yeasts other than those belonging to the genus Saccharomyces have become increasingly important for industrial applications. Species such as Pichia pastoris, Hansenula polymorpha, Schizosaccharomyces pombe, Yarrowia lipolytica and Kluyveromyces lactis have been modified genetically and used for the production of heterologous proteins. For a number of additional yeasts such as Schwanniomyces occidentalis, Zygosaccharomyces rouxii, Trichosporon cutaneum, Pachysolen tannophilus, Pichia guilliermondii and members of the genus Candida genetic transformation systems have been worked out. Transformation was achieved using either dominant selection markers based on antibiotic resistance genes or auxotrophic markers in conjunction with cloned biosynthetic genes involved in amino acid or nucleotide metabolism.

Molecular characterization of the 3-phosphoglycerate kinase gene (PGK1) from the methylotrophic yeast Pichia pastoris

We report the cloning of the 3-phosphoglycerate kinase gene (PGK1) from the methylotrophic yeast Pichia pastoris by a PCR approach. The coding sequence of the PGK1 gene comprises 1251 bp with the potential to encode a polypeptide of 416 amino acid residues, which shows high identity to homologous proteins from other yeasts. The promoter region of this gene (P(PGK1)) contains regulatory cis-elements found in other PGK1 genes, such as TATA box, CT-rich block and a heat shock element. In the 3' downstream region we identified a tripartite element 5'-TAG-TAGT-TTT-3', which is supposed to be important for transcription termination. As in other yeasts, the PGK1 gene from P. pastoris is present as a single-copy gene. Northern blot analysis revealed that the gene is transcribed as a 1.5 kb mRNA; when cells are grown on glucose the levels of this mRNA are increased two-fold in comparison to cells grown on glycerol. The transcriptional regulation of this gene by the carbon source was further confirmed when the alpha-amylase gene from Bacillus subtilis was placed under the control of P(PGK1): higher levels of expression were obtained when cells were grown on glucose as compared to glycerol and methanol. Preliminary results related to the strength of P(PGK1) show that it represents a potential alternative to constitutive heterologous expression in P. pastoris.

Application of a gratuitous induction system in Kluyveromyces lactis for the expression of intracellular and secreted proteins during fed-batch culture

A gratuitous induction system in the yeast Kluyveromyces lactis was evaluated for the expression of intracellular and extracellular products during fed-batch culture. The Escherichia coli lacZ gene (beta-galactosidase; intracellular) and MFalpha1 leader-BPTI cassette (bovine pancreatic trypsin inhibitor; extracellular) were placed under the control of the inducible K. lactis LAC4 promotor, inserted into partial-pKD1 plasmids, and transformed into a ga1-209 K. lactis strain. To obtain a high level of production, culture conditions for growth and expression were initially evaluated in tube cultures. A selective medium containing 5 g/L glucose (as carbon source) and 0.5 g/L galactose (as inducer) demonstrated the maximum activity of both beta-galactosidase and secreted BPTI. This level of expression had no significant effect on the growth of the recombinant cells; growth rate dropped by approximately 11%, whereas final biomass concentrations remained the same. In shake-flask culture, biomass concentration, beta-galactosidase activity, and BPTI secreted activity were 4 g/L, 7664 U/g dry cell, and 0.32 mg/L, respectively. Fed-batch culture (with a high glucose concentration and a low galactose [inducer] concentration feed) resulted in a 6.5-fold increase in biomass, a 23-fold increase in beta-galactosidase activity, and a 3-fold increase in BPTI secreted activity. The results demonstrate the success of gratuitous induction during high-cell-density fed-batch culture of K. lactis. A very low concentration of galactose feed was sufficient for a high production level.

Impaired cutinase secretion in Saccharomyces cerevisiae induces irregular endoplasmic reticulum (ER) membrane proliferation, oxidative stress, and ER-associated degradation

Impaired secretion of the hydrophobic CY028 cutinase invokes an unfolded protein response (UPR) in Saccharomyces cerevisiae cells. Here we show that the UPR in CY028-expressing S. cerevisiae cells is manifested as an aberrant morphology of the endoplasmic reticulum (ER) and as extensive membrane proliferation compared to the ER morphology and membrane proliferation of wild-type CY000-producing S. cerevisiae cells. In addition, we observed oxidative stress, which resulted in a 21-fold increase in carbonylated proteins in the CY028-producing S. cerevisiae cells. Moreover, CY028-producing S. cerevisiae cells use proteasomal degradation to reduce the amount of accumulated CY028 cutinase, thereby attenuating the stress invoked by CY028 cutinase expression. This proteasomal degradation occurs within minutes and is characteristic of ER-associated degradation (ERAD). Our results clearly show that impaired secretion of the heterologous, hydrophobic CY028 cutinase in S. cerevisiae cells leads to protein aggregation in the ER, aberrant ER morphology and proliferation, and oxidative stress, as well as a UPR and ERAD.

Identification and characterisation of two transcriptional repressor elements within the coding sequence of the Saccharomyces cerevisiae HXK2 gene

A well-defined set of isogenic yeast strains has been constructed whereby each strain contains a different HXK2::lacZ gene fusion integrated at the URA3 locus. These HXK2::lacZ fusions differ in the amount of the HXK2 gene (encoding hexokinase 2 isoenzyme) that is fused to the lacZ reporter gene. Comparison of the beta-galactosidase activities of each strain during growth on glucose or ethanol revealed that some part of the coding region between +39 and +404 bp is involved in repressing gene expression in a carbon source dependent manner. A series of deletions of this HXK2 coding region were constructed and fused upstream of a minimal CYC1::lacZ promoter. beta-Galactosidase activities on glucose or ethanol growth yeast calls revealed that two different regulatory elements are present in this DNA region. Gel mobility shift analysis and in vitro DNase I footprinting have shown that proteins bind specifically to two downstream repressor sequences (DRS1 located from +140 to +163 and DRS2 located between +231 and +251) that influence the rate of HXK2 transcription when ethanol is used as carbon source by Saccharomyces cerevisiae. We identified and partially purified a 18 kDa protein that binds specifically to synthetic double-stranded oligonucleotides containing the (A/C)(A/G)GAAAT box sequence. Our data suggest that p18 synthesis is under the control of genes involved in glucose repression (MIG1 = CAT4) and glucose derepression (SNF1 = CAT1).

The 3-phosphoglycerate kinase gene of the yeast Yarrowia lipolytica de-represses on gluconeogenic substrates

We have isolated the 3-phosphoglycerate kinase (PGK) gene of the yeast Yarrowia lipolytica by probing a genomic library with a PCR fragment amplified with primers deduced from two highly conserved regions of various PGKs. It is a unique sequence encoding a polypeptide of 417 residues with extensive homology to other PGKs, especially to that of Aspergillus nidulans (76% identity). The expression of the Y. lipolytica PGK1 gene proved to be higher on gluconeogenic substrates than on glycolytic ones. Haploid strains harboring a disrupted allele were able to grow on mixtures of a gluconeogenic carbon source and of a glycolytic one, but required proline supplementation in the presence of glucose, and were inhibited by glycerol.

The glucose-dependent transactivation activity of ABF1 on the expression of the TDH3 gene in yeast

Autonomously replicating sequence binding factor 1 (ABF1) has been implicated in the control of a variety of gene expressions in Saccharomyces cerevisiae. In this paper evidence is presented that ABF1 is involved in the glucose-dependent expression of the TDH3 gene which encodes glyceraldehyde-3-phosphate dehydrogenase. ABF1 binds to consensus sites located between -420 and -250, and between +77 and +200, and acts as a transactivator in an orientation-independent manner on both upstream and downstream sites. TDH3-lacZ fusions having an ABF1 consensus motif showed glucose-dependent expression of TDH3, whereas in the abf1 mutant strain JCA35 glucose-dependent expression disappeared. These findings suggest that ABF1 functions as a glucose-dependent transactivator for the expression of the TDH3 gene.

Industrial production of heterologous proteins by fed-batch cultures of the yeast Saccharomyces cerevisiae

This review concerns the issues involved in the industrial development of fed-batch culture processes with Saccharomyces cerevisiae strains producing heterologous proteins. Most of process development considerations with fed-batch recombinant cultures are linked to the reliability and reproducibility of the process for manufacturing environments where quality assurance and quality control aspects are paramount. In this respect, the quality, safety and efficacy of complex biologically active molecules produced by recombinant techniques are strongly influenced by the genetic background of the host strain, genetic stability of the transformed strain and production process factors. An overview of the recent literature of these culture-related factors is coupled with our experience in yeast fed-batch process development for producing various therapeutic grade proteins. The discussion is based around three principal topics: genetics, microbial physiology and fed-batch process design. It includes the fundamental aspects of yeast strain physiology, the nature of the recombinant product, quality control aspects of the biological product, features of yeast expression vectors, expression and localization of recombinant products in transformed cells and fed-batch process considerations for the industrial production of Saccharomyces cerevisiae recombinant proteins. It is our purpose that this review will provide a comprehensive understanding of the fed-batch recombinant production processes and challenges commonly encountered during process development.

Human interferon-beta, expressed in Saccharomyces cerevisiae, is predominantly directed to the vacuoles. Influence of modified co-expression of secretion factors and chaperones

Expression of the human interferon-beta (hIFN-beta) gene was found to be very toxic for Saccharomyces cerevisiae. An integrative expression cassette, containing the hIFN-beta gene under control of the inducible galactokinase (GAL1) promoter in combination with the alpha-factor prepro-secretion signal, was used to study the secretion process in more detail. Specific differences were found between a vacuolar proteinase--mutant and a normal laboratory yeast strain. Cell organelle fractionation, carried out with the recombinant C13-ABYS66 strain, revealed that 99% of the hIFN-beta remained intracellular and that the majority was associated with the vacuolar fraction. The secretion efficiency in the latter strain was investigated by overexpressing chaperone molecules (HSP70 and BiP) and homologous secretion factors (SEC1 and SEC18). Only the presence of HSP70 resulted in a 5-fold increase in secreted hIFN-beta.

Yeast intragenic transcriptional control: activation and repression sites within the coding region of the Saccharomyces cerevisiae LPD1 gene

Though widely recognized in higher eukaryotes, the regulation of Saccharomyces cerevisiae genes transcribed by RNA polymerase II by proteins that bind within the coding sequence remains largely speculative. We have shown for the LPD1 gene, encoding lipoamide dehydrogenase, that the coding sequence between +13 and +469 activated gene expression of an LPD1::lacZ fusion by up to sixfold in the presence of the upstream promoter. This downstream region, inserted upstream of a promoterless CYC1::lacZ fusion, activated gene expression in a carbon source-dependent manner by a factor of 15 to 111, independent of orientation. Deletion and mutational analysis identified two downstream activation sites (DAS1 and DAS2) and two downstream repressor sites (DRS1 and DRS2) that influence the rate of LPD1 transcription rather than mRNA degradation or translation. Activation from the DAS1 region (positions +137 to +191), encompassing a CDEI-like element, is twofold under derepressive conditions. Activation from DAS2 (+291 to +296), a CRE-like motif, is 12-fold for both repressed and derepressed states. DRS1, a pair of adjacent and opposing ABF1 sites (+288 to +313), is responsible for a 1.3- to 2-fold repression of transcription, depending on the carbon source. DRS1 requires the concerted action of DRS2 (a RAP1 motif at position +406) for repression of transcription only when the gene is induced. Gel mobility shift analysis and in vitro footprinting have shown that proteins bind in vitro to these downstream elements.

Yeast intragenic transcriptional control: activation and repression sites within the coding region of the Saccharomyces cerevisiae LPD1 gene

Though widely recognized in higher eukaryotes, the regulation of Saccharomyces cerevisiae genes transcribed by RNA polymerase II by proteins that bind within the coding sequence remains largely speculative. We have shown for the LPD1 gene, encoding lipoamide dehydrogenase, that the coding sequence between +13 and +469 activated gene expression of an LPD1::lacZ fusion by up to sixfold in the presence of the upstream promoter. This downstream region, inserted upstream of a promoterless CYC1::lacZ fusion, activated gene expression in a carbon source-dependent manner by a factor of 15 to 111, independent of orientation. Deletion and mutational analysis identified two downstream activation sites (DAS1 and DAS2) and two downstream repressor sites (DRS1 and DRS2) that influence the rate of LPD1 transcription rather than mRNA degradation or translation. Activation from the DAS1 region (positions +137 to +191), encompassing a CDEI-like element, is twofold under derepressive conditions. Activation from DAS2 (+291 to +296), a CRE-like motif, is 12-fold for both repressed and derepressed states. DRS1, a pair of adjacent and opposing ABF1 sites (+288 to +313), is responsible for a 1.3- to 2-fold repression of transcription, depending on the carbon source. DRS1 requires the concerted action of DRS2 (a RAP1 motif at position +406) for repression of transcription only when the gene is induced. Gel mobility shift analysis and in vitro footprinting have shown that proteins bind in vitro to these downstream elements.

Cloning and nucleotide sequence analysis of the Candida albicans enolase gene

The complete nucleotide sequence of the coding region as well as the flanking non-coding region of Candida albicans enolase gene was determined. A continuous open reading frame of 1323 nucleotides with no introns was identified. The deduced amino acid sequence showed 87% similarity to the enolases from the yeast Saccharomyces cerevisiae. The two isoforms of enolase are encoded by two non-tandemly arrayed genes in S. cerevisiae. However, DNA hybridisation analysis indicates that in C. albicans enolase is encoded by a single gene. The position of the transcription start site, putative TATA box and polyadenylation signal of the C. albicans enolase gene have been identified. The location of these sequences are similar to those of the S. cerevisiae enolase genes.

Authentic temperature-regulation of a heat shock gene inserted into yeast on a high copy number vector. Influences of overexpression of HSP90 protein on high temperature growth and thermotolerance

Heat shock protein HSP90 is relatively abundant in eukaryotic cells even in the absence of heat shock. Its precise function is still unclear, although it is apparently required in higher levels for growth at high temperatures. In this study Saccharomyces cerevisiae transformants were constructed with 50-150 copies of the homologous heat-inducible gene for HSP90 (HSP82) present on a high copy number episomal vector. These transformants were then used to demonstrate: (i) that this heat shock gene displays essentially normal regulation when present in yeast at high copy numbers; (ii) that yeast is an expression host suitable for the high level synthesis of HSP90; and (iii) that increasing normal cellular levels of HSP90 affects a number of physiological properties. The HSP82 gene is normally single-copy in the haploid yeast genome, yet even at 50 to 150 copies per cell it displayed almost normal basal and heat shock-induced levels of expression. Proper regulation of the heat shock element sequence controlling HSP82 is therefore not lost at high gene copy levels. In unstressed cultures in exponential growth at 25 degrees C the low basal expression of the multiple HSP82 gene copies caused a 3 to 7-fold HSP90 overproduction, but HSP90 levels increased 10-fold to 30-40% of total cell protein following temperature upshift to 39 degrees C for 75 min. Heat induction of the chromosomal genes for other heat shock proteins in the same cells was not suppressed relative to cells which were isogenic but for the possession of just a single HSP82 gene, this constituting further evidence that yeast can authentically regulate a large number of heat shock genes. HSP90 overproduction was not protective against heat killing, causing strain-dependent reductions in growth at 37.5 degrees C and in thermotolerance.

Solution structure of the fibrin binding finger domain of tissue-type plasminogen activator determined by 1H nuclear magnetic resonance

The amino acid sequence of the first domain of tissue-type plasminogen activator (t-PA) includes eight residues that are highly conserved in the type 1 finger domains found in human fibronectin. A construct comprising 50 residues from this finger domain of t-PA has been expressed and its solution structure has been determined by two-dimensional nuclear magnetic resonance spectroscopy. A total of 782 experimental restraints consisting of 723 interproton distances derived from nuclear Overhauser effect measurements, 43 torsion angles, and 16 hydrogen bond restraints were used as the input for dynamical simulated annealing structure calculations. Twenty-eight structures were obtained that satisfied the experimental data with no single distance violation greater than 0.3 A. The average atomic root-mean-square distribution for the backbone atoms of the final structures was 0.41 (+/- 0.13) A for the well defined part of the structure (residues 4 to 47). The overall fold of the t-PA finger domain shows a striking similarity to that of the seventh type 1 repeat of human fibronectin with the side-chains of conserved residues lying in similar conformations. One significant difference between the two molecules is that hydrophobic residues cover the exposed surface of the principal beta-sheet region in the t-PA finger domain. It is suggested that one face of this region may interact with parts of the complete t-PA protein.

Starvation for His-tRNAHis in yeast causes translational arrest without a high level of misincorporation of glutamine at histidine codons

The hts1.1 temperature-sensitive histidinyl-tRNA synthetase mutation enables Saccharomyces cerevisiae to be starved for His-tRNAHis by upshift to the non-permissive temperature of 38 degrees C. If yeast behaves similarly to bacterial and mammalian cells, this lack of His-tRNAHis should greatly enhance misreading at histidine codons (CAU/CAC) by Gln-tRNAGln, resulting in substitution of the neutral amino acid glutamine in place of histidine, a basic amino acid. Such misreading causes the isoelectric point (pI) of proteins to shift to lower values, and is readily detectable as "stuttering" on two-dimensional (2D) protein gels. By gel analysis of pulse-labelled proteins of hts1.1 yeast cells that were overexpressing phosphoglycerate kinase (PGK), our study sought to detect this specific translational error in PGK protein. It was not detected by this relatively sensitive technique, indicating that missense errors due to glutamine insertion at histidine codons do not occur in yeast at the readily-detectable level found in bacterial and mammalian cells.

A 3' transcriptional enhancer within the coding sequence of a yeast gene encoding the common subunit of two multi-enzyme complexes

A well-defined set of isogenic yeast strains has been constructed whereby each strain contains a different LPD::lacZ gene fusion integrated at the ura3 locus. These LPD::lacZ fusions differ in the amount of the LPD1 gene (encoding lipoamide dehydrogenase) that is fused to the lacZ reporter. Comparison of the beta-galactosidase activities of each strain during growth on glucose or ethanol revealed that some part of the LPD1 coding region between +13 and +700 is involved in activating gene expression in a carbon source-dependent manner. This activation occurs at the mRNA level, and is not mediated by changes in mRNA stability. Therefore, the LPD1 gene appears to contain a transcriptional enhancer that lies 3' to the transcriptional start site, and which responds to carbon source.

Production of hepatitis B surface antigen (PreS2 + S) by high-cell density cultivations of a recombinant yeast

A high-cell density bioprocess has been developed for the production of hepatitis B surface protein (preS2 + S) by recombinant yeast. This fed-batch process utilizes a growth medium containing yeast extract, soy peptone and glucose which was fed at a constant rate to maintain cells in a respiratory state. Cell densities of up to 60 g l-1 dry weight were achieved, which represented a 6-fold increase over those from batch bioprocesses. This increase in cell mass was attained without compromising specific activity; therefore, volumetric productivities of six times those of batch bioprocesses were achieved.

Structure of the fibronectin type 1 module

The rapid accumulation of sequence data has provided insight into the evolution of proteins and led to the identification of 'mosaic proteins'. These proteins have evolved by duplication, insertion and deletion of a common pool of structural units or modules, yet their biological functions are diverse. They are involved in cell adhesion and migration, embryogenesis and the pathways of blood clotting, fibrinolysis and complement. The modular units are defined by 'consensus sequences' which often include conserved disulphide bonds. Despite the available sequence information, little is known of the tertiary structure of mosaic proteins. If, however, the 'consensus structure' of the modules were known, valuable structural information could be inferred about a wide variety of proteins and biological systems. An important mosaic protein is fibronectin, an extracellular matrix protein that consists of three types of module (see refs 3, 7 for reviews). Here we describe the structure of the fibronectin type 1 module which appears twelve times in fibronectin and is also found in factor XII and tissue plasminogen activator. The module was produced using a yeast expression system and the structure was determined in solution using 1H NMR. This methodology promises to be extremely powerful in the investigation of modules from a wide range of mosaic proteins.

Host-vector systems

In 1980 it was only possible to express foreign genes in bacteria and a few easily cultured animal cells. During the subsequent eight years specialized vectors have been developed to allow the genetic manipulation of a wide range of both prokaryotes and eukaryotes. One of the major goals of biotechnology in 1980 was to use host cells as 'factories' for the production of proteins that were only available in minute quantities from natural sources. This has already lead to a new generation of pharmaceutical products. Advances in our understanding of host-vector systems have defined new goals. The basic concepts of expression vector design will be illustrated. Some of the new goals are discussed with particular reference to the exploitation of novel host-vector systems to develop vaccines and anti-viral agents against AIDS.

Simultaneous synthesis and assembly of various hepatitis B surface proteins in Saccharomyces cerevisiae

Yeast transposon of class-1-based vectors, allowing integration at a series of chromosomal loci by homologous recombination with resident transposons, were constructed. Using such vectors, we have introduced several copies of an expression cassette encoding the major hepatitis B surface protein as well as expression cassettes encoding the middle (M) or/and the large (L) surface protein into Saccharomyces cerevisiae. In extracts of such strains, coassembly of the different proteins into a single lipoprotein structure is observed. This was demonstrated by immunoprecipitation of the major protein using monoclonal antibodies directed specifically against epitopes that are present only on the M or the L protein. These results show that hepatitis B surface antigen envelope proteins synthesized in yeast are able to assemble into structures composed of different polypeptides. This opens the possibility of producing in yeast a variety of particles carrying well-defined amounts of preS epitopes on their surface. Also, one can envisage the production of mixed particles containing different foreign epitopes on their surface, in defined relative abundance, which could be useful for vaccine applications.

5'-secondary structure formation, in contrast to a short string of non-preferred codons, inhibits the translation of the pyruvate kinase mRNA in yeast

The effects of poor codon bias and secondary structure formation upon the translation of the pyruvate kinase (PYK1) mRNA have been investigated in Saccharomyces cerevisiae. Following insertion mutagenesis at the 5'-end of the PYK1 coding region, the gene was transformed into yeast, and translation assessed directly in vivo by determining the distribution of the modified PYK1 mRNAs across polysomes fractionated by sucrose density gradient centrifugation. The chromosomally-encoded (wild-type) PYK1 mRNA, and the actin, ribosomal protein L3 and glyceraldehyde-3-phosphate dehydrogenase mRNAs were used to control for minor differences between polysome preparations. An insertion containing 13 non-preferred codons at the 5'-end of the coding region was found to have no significant effect upon PYK1 mRNA translation. In contrast, translation was inhibited by an insertion which increased the formation of secondary structures at the 5'-end of the mRNA (overall delta G = -36.6 kcal/mol). Control insertions were also analysed to exclude the possibility that alterations to the amino acid sequence of pyruvate kinase affect the translation of its mRNA. These insertions, which introduced preferred codons or restored wild-type levels of secondary structure formation, did not significantly influence PYK1 mRNA translation.

Transcript characterisation, gene disruption and nucleotide sequence of the Saccharomyces cerevisiae WH12 gene

WH12 is a gene which plays a prominent role in regulating growth and proliferation in Saccharomyces cerevisiae. It is expressed as a 2.0-kb mRNA transcript. Disruption of this transcript in a WH12+ cell results in the mutant phenotype being displayed. The nucleotide sequence of the cloned gene has been determined and found to include a 487-codon long open reading frame coding for a 55.3-kDa protein. The protein showed no extensive homologies to any previously identified protein. The 5' and 3' noncoding regions contained many of the features found in other yeast genes.

Cloning of human lysozyme gene and expression in the yeast Saccharomyces cerevisiae

cDNA clones encoding human lysozyme were isolated from a human histiocytic cell line (U-937) and a human placenta cDNA library. The clones, ranging in size from 0.5 to 0.75 kb, were identified by direct hybridization with synthetic oligodeoxynucleotides. The nucleotide sequence coding for the entire protein was determined. The derived amino acid sequence has 100% homology with the published amino acid (aa) sequence; the leader sequence codes for 18 aa. Expression and secretion of human lysozyme in Saccharomyces cerevisiae was achieved by placing the cloned cDNA under the control of a yeast gene promoter (ADH1) and the alpha-factor peptide leader sequence.

A heat shock element in the phosphoglycerate kinase gene promoter of yeast

The phosphoglycerate kinase (PGK) promoter is often employed in yeast expression vectors due to its very high efficiency. Its activity in unstressed cells has been shown to be due to an upstream activator site (UASPGK) at -402 to -479. Since levels of PGK mRNA can sometimes be elevated by heat shock of yeast cultures this investigation determined how specific deletions of PGK promoter sequences effect levels of PGK mRNA both before and after heat shock. A series of PGK promoter deletions was inserted on a high copy plasmid into cells having a TRP1 gene disruption of the solitary chromosomal PGK locus. This enabled PGK transcripts of plasmid and chromosomal origin to be distinguished by virtue of their different sizes. Certain deletions lacking UASPGK displayed activities that were very low in unstressed cells, but which increased fifty to one-hundred fold after heat shock. With UASPGK present heat shock had only a relatively small or negligible effect on PGK mRNA levels. Heat shock activation was abolished when the -256 to -377 region with homology to the heat shock element consensus of eukaryotes was deleted in addition to UASPGK, but was unaffected by the deletion of regions further downstream containing TATA- and CAAT- sequence motifs. This is the first demonstration of a heat shock element, an activator site normally found upstream of eukaryotic heat shock protein genes, as a natural constituent of a high efficiency glycolytic promoter. It is proposed that PGK may be one member of a small subset of yeast genes that are highly expressed in unstressed cells yet possess a heat shock element to ensure their continued transcription after heat shock.

Amplification of plasmid copy number by thymidine kinase expression in Saccharomyces cerevisiae

A 2 micron circle-based chimaeric plasmid containing the yeast LEU2 and the Herpes Simplex Virus type 1 thymidine kinase (HSV-1 TK) genes was constructed. Transformants grown under selective conditions for the LEU2 gene harboured the plasmid at about 15 copies per cell whilst selection for the HSV-1 TK gene led to an increase to about 100 copies per cell. Furthermore, the plasmid copy number could be controlled by the stringency of selection for the TK gene, and the increase in TK gene dosage was reflected in an increase in intracellular thymidine kinase activity. The mitotic stability of the plasmid in "high-copy" and "low-copy" number cells was determined. "High-copy" number cells showed a greater mitotic stability. The relationship of TK expression to plasmid copy number may be useful for the isolation of plasmid copy number mutants in yeast and the control of heterologous gene expression.

Translation and stability of an Escherichia coli beta-galactosidase mRNA expressed under the control of pyruvate kinase sequences in Saccharomyces cerevisiae

Plasmids were assembled in which the coding region of the pyruvate kinase (PYK) gene of Saccharomyces cerevisiae was replaced by that of the B-galactosidase (LacZ) gene from Escherichia coli. Analysis of the resultant, chimaeric transcripts from low copy number, centromeric plasmids indicated that this substitution caused a dramatic reduction in the steady-state level of the messenger RNA (mRNA). This fluctuation cannot be wholly accounted for by the 2-fold decrease in mRNA stability observed. This is consistent with the existence of a transcriptional Downstream Activation Site (DAS) within the PYK coding region, analogous to the DAS reported within the yeast phosphoglycerate kinase gene (PGK; Kingsman, S M et al. (1985) Biotech. Gen. Eng. Rev. 3, 377). At these low levels of heterologous gene expression, comparison of the distribution of PYK and PYK/LacZ transcripts across polysome gradients revealed no significant effect mediated by their striking disparity in codon usage. Nevertheless, upon increasing B-galactosidase mRNA levels, via manipulation of plasmid copy number, a distinct decline in ribosome loading was observed for the heterologous PYK/LacZ transcript which was not mirrored by either endogenous PYK transcripts or other yeast mRNAs of high (Ribosomal protein 1) or moderate (Actin) codon bias. However, high levels of the PYK/LacZ mRNA did affect the translation of an endogenous mRNA with poor codon bias (TRP2). The possible basis for this phenomenon is discussed.

The UAS of the yeast PGK gene contains functionally distinct domains

The upstream activation site (UAS) of the yeast phosphoglycerate kinase gene (PGK) has been localised by deletion analysis (1). Here we show that the UASPGK contains two functionally distinct domains. These two domains, designated activator (A) and modulator (M), appear to be located within bases -460 to -402 and -531 to -461, respectively, relative to the initiating ATG; although it is possible that part of the M domain resides within the A domain. They have been shown, using a heterologous assay promoter, to have distinct transcriptional functions. Domain A is responsible for activation of transcription whilst domain M is required for carbon source dependent regulation of transcription. Protein-DNA binding studies have demonstrated that the DNA fragment containing domain M has high affinity for at least one specific DNA-binding protein, whilst domain A does not appear to interact strongly in protein-binding assays under the same conditions. The domain M binding activity is dependent on the carbon source in the growth medium and may be functional in the carbon source control of PGK expression.

A transcriptional activator is located in the coding region of the yeast PGK gene

Expression of heterologous genes from the PGK promoter on high copy number plasmids in yeast is relatively poor compared to the intact PGK gene because of low steady-state RNA levels. In this paper we show that low levels of heterologous RNA are not due to instability of mRNA but result from inefficient transcription due to a defect in RNA synthesis. A comparison of RNA levels from homologous and heterologous transcription units allowed the identification of a positive activator for transcription within the PGK coding region which is required for efficient expression of the PGK gene. Deletion of this region, the "downstream activator sequence", causes a six to ten fold reduction in transcriptional efficiency from the PGK 5' noncoding region.

Construction of expression plasmids for Saccharomyces cerevisiae: application for synthesis of poliovirus protein VP2

A series of expression plasmids was constructed to compare the usefulness of various promoters for the synthesis of a given protein in the Saccharomyces cerevisiae. The plasmids pMBL212, -213, -214, -215 and -216 can be used to synthesize the protein of interest directly as a non-fused protein or, if the protein is difficult to detect, indirectly as an enzymatically active beta-galactosidase fusion protein. The plasmids were employed to identify which yeast promoter and strain are suitable for the synthesis of poliovirus protein VP2. It was concluded that the GAL7 and PGK promoters in combination with strain X904 can be used for efficient synthesis of a VP2 in the form of a N-terminally fused VP2-beta-galactosidase protein.

Transcription of the phosphoglycerate kinase gene of Saccharomyces cerevisiae increases when fermentative cultures are stressed by heat-shock

The single gene for phosphoglycerate kinase (PGK) in the haploid genome of Saccharomyces cerevisiae is expressed to a very high level in cultures fermenting glucose. Despite this it responds to heat-shock. When S. cerevisiae growing exponentially on glucose media was shifted from 25 degrees C to 38 degrees C transient increases of 6-7-fold in cellular PGK mRNA were observed. This elevation in PGK mRNA still occurred in the presence of the protein-synthesis inhibitor cycloheximide, but was not observed in cells bearing the rna1.1 mutation. From the kinetics of continuous labelling of PGK mRNA, relative to the labelling of other RNAs in the same cultures whose levels do not alter with heat-shock, it was shown that the elevation in PGK mRNA in response to temperature upshift reflects primarily an increased synthesis of this mRNA and not an alteration of its half-life. PGK mRNA synthesis is therefore one target of a response mechanism to thermal stress. Synthesis of PGK enzyme in glucose-grown cultures is efficient after mild (25 degrees C to 38 degrees C) or severe (25 degrees C to 42 degrees C) heat-shocks. Following the severe shock, the synthesis of most proteins is abruptly terminated, but synthesis of PGK and a few other glycolytic enzymes continues at levels comparable to the levels of synthesis of most of those proteins dramatically induced by heat (heat-shock proteins). Cells that overproduce PGK due to the presence of multiple copies of the PGK gene on a high-copy-number plasmid continue their overproduction of this enzyme during severe thermal stress. Therefore PGK mRNA is both elevated in level in response to heat-shock and translated efficiently at supra-optimal temperatures.

Construction of stable laboratory and industrial yeast strains expressing a foreign gene by integrative transformation using a dominant selection system

An expression cassette of mouse dihydrofolate reductase (Mdhfr) cDNA under control of the yeast cytochrome c promoter was inserted in a yeast plasmid containing the ARS1 sequence. The ARS replicating function was destroyed by BglII treatment prior to yeast transformation. Using this linearized plasmid, genomic transformants could be obtained from either laboratory or industrial strains of bakers' yeast based on direct methotrexate (MTX)-resistance selection. The entire sequence of the linearized plasmid was integrated by homologous recombination at the ARS region of the host chromosome. The results indicate that repetitive and homologous recombination occurs readily in such transformations. The stability of the constructed integrants was more than 99.95% per generation in non-selective medium, and tandem repeats of up to six copies (i.e., about 44 kb) were not changed even after 30 generations in rich medium. Expression in rich medium of cointegrated, human interleukin 2 cDNA under control of the triose phosphate isomerase promoter was shown by Western blot experiments in both laboratory and industrial yeast strains. Furthermore, a comparison of the transcription efficiency of the Mdhfr gene in the chromosome with that in the plasmid revealed that the efficiency was almost proportional to the number of gene copies, irrespective of the location of the transcription unit. These results show that by using the MTX/Mdhfr dominant selection-amplification system one can construct stable recombinant yeast strains suitable for heterologous gene expression in laboratory as well as in industrial fermentation conditions.