IDENTIFICATION OF THE STRUCTURAL GENE FOR β-GLUCOSIDASE IN SACCHAROMYCES LACTIS

Identification of strong promoters based on the transcriptome of Bacillus licheniformis

OBJECTIVES

To expand the repertoire of strong promoters for high level expression of proteins based on the transcriptome of Bacillus licheniformis.

RESULTS

The transcriptome of B. licheniformis ATCC14580 grown to the early stationary phase was analyzed and the top 10 highly expressed genes/operons out of the 3959 genes and 1249 operons identified were chosen for study promoter activity. Using beta-galactosidase gene as a reporter, the candidate promoter pBL9 exhibited the strongest activity which was comparable to that of the widely used strong promoter p43. Furthermore, the pro-transglutaminase from Streptomyces mobaraensis (pro-MTG) was expressed under the control of promoter pBL9 and the activity of pro-MTG reached 82 U/ml after 36 h, which is 23% higher than that of promoter p43 (66.8 U/ml).

CONCLUSION

In our analyses of the transcriptome of B. licheniformis, we have identified a strong promoter pBL9, which could be adapted for high level expression of proteins in the host Bacillus subtilis.

Protoplast fusion in a petite-negative yeast, Kluyveromyces lactis

The technique of protoplast fusion has been applied to the problem of unstable diploidy in the yeast Kluyveromyces lactis. By protoplast fusion between heterothallic strains of like mating-type, sporulation-deficient hybrids can be obtained. Biochemical, cytological, and genetical characterisation of these hybrids suggests that the majority of fusion products are diploid. Sporulating hybrids can be constructed by protoplast fusion between homothallic strains. Tetrad analysis of these hybrids demonstrates conclusively the diploid nature of fusion products.

L-rhamnose induction of Aspergillus nidulans α-L-rhamnosidase genes is glucose repressed via a CreA-independent mechanism acting at the level of inducer uptake

BACKGROUND

Little is known about the structure and regulation of fungal α-L-rhamnosidase genes despite increasing interest in the biotechnological potential of the enzymes that they encode. Whilst the paradigmatic filamentous fungus Aspergillus nidulans growing on L-rhamnose produces an α-L-rhamnosidase suitable for oenological applications, at least eight genes encoding putative α-L-rhamnosidases have been found in its genome. In the current work we have identified the gene (rhaE) encoding the former activity, and characterization of its expression has revealed a novel regulatory mechanism. A shared pattern of expression has also been observed for a second α-L-rhamnosidase gene, (AN10277/rhaA).

RESULTS

Amino acid sequence data for the oenological α-L-rhamnosidase were determined using MALDI-TOF mass spectrometry and correspond to the amino acid sequence deduced from AN7151 (rhaE). The cDNA of rhaE was expressed in Saccharomyces cerevisiae and yielded pNP-rhamnohydrolase activity. Phylogenetic analysis has revealed this eukaryotic α-L-rhamnosidase to be the first such enzyme found to be more closely related to bacterial rhamnosidases than other α-L-rhamnosidases of fungal origin. Northern analyses of diverse A. nidulans strains cultivated under different growth conditions indicate that rhaA and rhaE are induced by L-rhamnose and repressed by D-glucose as well as other carbon sources, some of which are considered to be non-repressive growth substrates. Interestingly, the transcriptional repression is independent of the wide domain carbon catabolite repressor CreA. Gene induction and glucose repression of these rha genes correlate with the uptake, or lack of it, of the inducing carbon source L-rhamnose, suggesting a prominent role for inducer exclusion in repression.

CONCLUSIONS

The A. nidulans rhaE gene encodes an α-L-rhamnosidase phylogenetically distant to those described in filamentous fungi, and its expression is regulated by a novel CreA-independent mechanism. The identification of rhaE and the characterization of its regulation will facilitate the design of strategies to overproduce the encoded enzyme - or homologs from other fungi - for industrial applications. Moreover, A. nidulans α-L-rhamnosidase encoding genes could serve as prototypes for fungal genes coding for plant cell wall degrading enzymes regulated by a novel mechanism of CCR.

Genomic exploration of the hemiascomycetous yeasts: 11. Kluyveromyces lactis

Random sequencing of the Kluyveromyces lactis genome allowed the identification of 2235-2601 open reading frames (ORFs) homologous to S. cerevisiae ORFs, 51 ORFs which were homologous to genes from other species, 64 tRNAs, the complete rDNA repeat, and a few Ty1- and Ty2-like sequences. In addition, the complete sequence of plasmid pKD1 and a large coverage of the mitochondrial genome were obtained. The global distribution into general functional categories found in Saccharomyces cerevisiae and as defined by MIPS is well conserved in K. lactis. However, detailed examination of certain subcategories revealed a small excess of genes involved in amino acid metabolism in K. lactis. The sequences are deposited at EMBL under the accession numbers AL424881-AL430960.

Genetics and molecular physiology of the yeast Kluyveromyces lactis

With the recent development of powerful molecular genetic tools, Kluyveromyces lactis has become an excellent alternative yeast model organism for studying the relationships between genetics and physiology. In particular, comparative yeast research has been providing insights into the strikingly different physiological strategies that are reflected by dominance of respiration over fermentation in K. lactis versus Saccharomyces cerevisiae. Other than S. cerevisiae, whose physiology is exceptionally affected by the so-called glucose effect, K. lactis is adapted to aerobiosis and its respiratory system does not underlie glucose repression. As a consequence, K. lactis has been successfully established in biomass-directed industrial applications and large-scale expression of biotechnically relevant gene products. In addition, K. lactis maintains species-specific phenomena such as the "DNA-killer system, " analyses of which are promising to extend our knowledge about microbial competition and the fundamentals of plasmid biology.

The genes for the Golgi apparatus N-acetylglucosaminyltransferase and the UDP-N-acetylglucosamine transporter are contiguous in Kluyveromyces lactis

The mannan chains of Kluyveromyces lactis mannoproteins are similar to those of Saccharomyces cerevisiae except that they lack mannose phosphate and have terminal alpha(1-->2)-linked N-acetylglucosamine. Previously, Smith et al. (Smith, W. L. Nakajima, T., and Ballou, C. E. (1975) J. Biol. Chem. 250, 3426-3435) characterized two mutants, mnn2-1 and mnn2-2, which lacked terminal N-acetylglucosamine in their mannoproteins. The former mutant lacks the Golgi N-acetylglucosaminyltransferase activity, whereas the latter one was recently found to be deficient in the Golgi UDP-GlcNAc transporter activity. Analysis of extensive crossings between the two mutants led Ballou and co-workers (reference cited above) to conclude that these genes were allelic or tightly linked. We have now cloned the gene encoding the K. lactis Golgi membrane N-acetylglucosaminyltransferase by complementation of the mnn2-1 mutation and named it GNT1. The mnn2-1 mutant was transformed with a 9.5-kilobase (kb) genomic fragment previously shown to contain the gene encoding the UDP-GlcNAc transporter; transformants were isolated, and phenotypic correction was monitored after cell surface labeling with fluorescein isothiocyanate-conjugated Griffonia simplicifolia II lectin, which binds terminal N-acetylglucosamine, and a fluorescence-activated cell sorter. The above 9.5-kb DNA fragment restored the wild-type lectin binding phenotype of the transferase mutant; further subcloning of this fragment yielded a smaller one containing an opening reading frame of 1,383 bases encoding a protein of 460 amino acids with an estimated molecular mass of 53 kDa, which also restored the wild-type phenotype. Transformants had also regained the ability to transfer N-acetylglucosamine to 3-0-alpha-D-mannopyranosyl-D-mannopyranoside. The gene encoding the above transferase was found to be approximately 1 kb upstream from the previously characterized MNN2 gene encoding the UDP-GlcNAc Golgi transporter. Each gene can be transcribed independently by their own promoter. To our knowledge this is the first demonstration of two Golgi apparatus functionally related genes being contiguous in a genome.

AN INDUCIBLE SYSTEM FOR THE HYDROLYSIS AND TRANSPORT OF BETA-GLUCOSIDES IN YEAST. I. CHARACTERISTICS OF THE BETA-GLUCOSIDASE ACTIVITY OF INTACT AND OF LYSED CELLS

A strain of bakers' yeast was isolated which could utilize cellobiose and other β-D-glucosides quantitatively as carbon and energy sources for growth. Cellobiose-grown cells contained a largely cryptic enzyme active against the chromogenic substrate p-nitrophenyl-β-D-glucoside. The patent (intact cell) activity of such cells was inhibited by azide and, competitively, by cellobiose; neither agent inhibited the β-glucosidase activity of lysed cells or of extracts. The enzyme induced by growth in cellobiose medium had no affinity for cellobiose as either substrate or inhibitor; its substrate specificity classifies it as an aryl-β-glucosidase. It was concluded that growth in cellobiose also induced the formation of a stereospecific and energy-dependent system whose function determined the rate at which intact cells could hydrolyze substrates of the intracellular β-glucosidase.

BETA-GLUCOSIDASE SYSTEM OF NEUROSPORA CRASSA. I. BETA-GLUCOSIDASE AND CELLULASE ACTIVITIES OF MUTANT AND WILD-TYPE STRAINS

Eberhart, Bruce (University of North Carolina, Greensboro), David F. Cross, and Lewis R. Chase. beta-Glucosidase system of Neuspora crassa. I. beta-Glucosidase and cellulose activities of mutant and wild-type strains. J. Bacteriol. 87:761-770. 1964.-A mutant strain, gluc-1, of Neurospora crassa was isolated and characterized by its low level of beta-glucosidase activity. The mutant was selected by testing irradiated colonies for extracellular beta-glucosidase activity. Strains containing the gluc-1 gene were also visibly detected by their reduced ability to destroy esculin in their growth media. The mutant strain grew at wild-type rates with cellobiose or carboxymethylcellulose as carbon sources. This auxotrophic similarity with wild type is explained by the presence of at least two beta-glucosidases (and possibly two cellulases) in Neurospora that act complementarily. The thermolabile beta-glucosidase was destroyed after 1 min of incubation at 60 C. This enzyme was present in mycelia but absent in conidial extracts. A second beta-glucosidase that is comparatively stable at 60 C was present in both mycelia and conidia. A partial separation of these enzymes was achieved with ammonium fractionation of mycelial extracts of gluc-1 and wild-type strains. Thermolabile beta-glucosidase and cellulase activity appear not to be affected by the gluc-1 mutation, whereas the thermostable glucosidase is greatly reduced in gluc-1 strains.

Molecular cloning of the Golgi apparatus uridine diphosphate-N-acetylglucosamine transporter from Kluyveromyces lactis

The mannan chains of Kluyveromyces lactis mannoproteins are similar to those of Saccharomyces cerevisiae except that they lack mannose phosphate and have terminal alpha1-->2-linked N-acetylglucosamine. The biosynthesis of these chains probably occurs in the lumen of the Golgi apparatus, by analogy to S. cerevisiae. The sugar donors, GDP-mannose and UDP-GlcNAc, must first be transported from the cytosol, their site of synthesis, via specific Golgi membrane transporters into the lumen where they are substrates in the biosynthesis of these mannoproteins. A mutant of K. lactis, mnn2-2, that lacks terminal N-acetylglucosamine in its mannan chains in vivo, has recently been characterized and shown to have a specific defect in transport of UDP-GlcNAc into the lumen of Golgi vesicles in vitro. We have now cloned the gene encoding the K. lactis Golgi membrane UDP-GlcNAc transporter by complementation of the mnn2-2 mutation. The mnn2-2 mutant was transformed with a genomic library from wild-type K. lactis in a pKD1-derived vector; transformants were isolated and phenotypic correction was monitored following cell surface labeling with fluorescein isothiocyanate conjugated to Griffonia simplicifolia II lectin, which binds terminal N-acetylglucosamine, and a fluorescent activated cell sorter. A 2.4-kb DNA fragment was found to restore the wild-type lectin binding phenotype. Upon loss of the plasmid containing this fragment, reversion to the mutant phenotype occurred. The above fragment contained an open reading frame for a multitransmembrane spanning protein of 328 amino acids. The protein contains a leucine zipper motif and has high homology to predicted proteins from S. cerevisiae and C. elegans. In an assay in vitro, Golgi vesicles isolated from the transformant had regained their ability to transport UDP-GlcNAc. Taken together, the above results strongly suggest that the cloned gene encodes the Golgi UDP-GlcNAc transporter of K. lactis.

Production of beta-glucosidase and diauxic usage of sugar mixtures by Candida molischiana

The fermentation of cellobiose is a rare trait among yeasts. Of the 308 yeast species that utilize cellobiose aerobically, only 12 species ferment it, and only 2 species, Candida molischiana and Candida wickerhamii, also ferment cellodextrins. Candida molischiana produced beta-glucosidase activity on all carbon sources tested, except glucose, mannose, and fructose. When these sugars were added to cultures growing on cellobiose, the synthesis of beta-glucosidase ceased. However, the total amount of enzyme activity remained constant, indicating that the C. molischiana beta-glucosidase is catabolite repressed and not catabolite inactivated. When grown in medium initially containing glucose plus xylose, cellobiose, maltose, mannitol, or glucitol, C. molischiana preferentially utilized glucose and produced little beta-glucosidase activity until glucose was nearly depleted from the medium. When grown in medium containing cellobiose plus either fructose or mannose, the yeast preferentially utilized the monosaccharides and produced little beta-glucosidase activity. Candida molischiana produced beta-glucosidase and co-utilized cellobiose and xylose, maltose, or trehalose. Glucose and fructose, mannose, or trehalose were co-utilized; however, no beta-glucosidase activity was detected. Thus, the order of substrate preference groups appeared to be (glucose, trehalose, fructose, mannose) > (cellobiose, maltose, xylose) > (mannitol, glucitol).

Utilization of glucose and cellobiose byCandida molischiana

Some of the factors that influence the biosynthesis of the Candida molischiana β-glucosidase were investigated. The yeast produced maximal enzyme activity when grown at 28 °C in a carbohydrate-containing complex medium (YM) in which the initial pH was adjusted to 6.0. The enzyme appeared to be produced constitutively, as activity was detected when either ethanol, glycerol, xylose, glucitol, mannitol, maltose, trehalose, cellobiose, cellodextrins, or soluble starch was used as the carbohydrate source. The presence of either glucose, mannose, or fructose (> 25 g/L) repressed β-glucosidase expression; however, C. molischiana did produce β-glucosidase when the initial glucose concentration was <25 g/L. When the yeast was grown in YM medium containing glucose plus cellobiose, diauxic utilization of the carbon sources was observed, and β-glucosidase activity was not detected until the glucose was depleted from the medium.Key words: β-glucosidase, glucose repression, fermentation, yeast.

The promoter of the beta-glucosidase gene from Kluyveromyces fragilis contains sequences that act as upstream repressing sequences in Saccharomyces cerevisiae

The relationship between the promoter length of the Kluyveromyces fragilis beta-glucosidase gene and the level of its expression in Saccharomyces cerevisiae was studied by gene fusion between deleted promoter fragments of various lengths and the promoterless beta-galactosidase gene of Escherichia coli. The removal of a region from position -425 to -232 led to a tenfold increase in the expression of the gene. The same results were obtained for the reconstructed beta-glucosidase gene with the same promoter length. It is likely that the deletion of this part of the promoter removes negative regulatory elements which are functional in Saccharomyces cerevisiae. This increase in activity is the main event which may explain the high increase in gene expression (60-fold) previously observed for an upstream deletion obtained during subcloning experiments of the beta-glucosidase gene. It is also shown that the expression of the gene greatly depends upon the nature of the recipient strain, the growth phase of the cell and that of the vector carrying it.

Characterization of a positive regulatory gene, LAC9, that controls induction of the lactose-galactose regulon of Kluyveromyces lactis: structural and functional relationships to GAL4 of Saccharomyces cerevisiae

Lactose or galactose induces the expression of the lactose-galactose regulon in Kluyveromyces lactis. We show here that the regulon is not induced in strains defective in LAC9. We demonstrate that this gene codes for a regulatory protein that acts in a positive manner to induce transcription. The LAC9 gene was isolated by complementation of a lac9 defective strain. DNA sequence analysis of the gene gave a deduced protein of 865 amino acids. Comparison of this sequence with that of the GAL4 protein of Saccharomyces cerevisiae revealed three regions of homology. One region of about 90 amino acid occurs at the amino terminus, which is known to mediate binding of GAL4 protein to upstream activator sequences. We speculate that a portion of this region, adjacent to the "metal-binding finger," specifies DNA binding. We discuss possible functions of the two other regions of homology. The functional implications of these structural similarities were examined. When LAC9 was introduced into a gal4 defective strain of S. cerevisiae it complemented the mutation and activated the galactose-melibiose regulon. However, LAC9 did not simply mimic GAL4. Unlike normal S. cerevisiae carrying GAL4, the strain carrying LAC9 gave constitutive expression of GAL1 and MEL1, two genes in the regulon. The strain did show glucose repression of the regulon, but repression was less severe with LAC9 than with GAL4. We discuss the implications of these results and how they may facilitate our understanding of the LAC9 and GAL4 regulatory proteins.

Characterization of a positive regulatory gene, LAC9, that controls induction of the lactose-galactose regulon of Kluyveromyces lactis: structural and functional relationships to GAL4 of Saccharomyces cerevisiae

Lactose or galactose induces the expression of the lactose-galactose regulon in Kluyveromyces lactis. We show here that the regulon is not induced in strains defective in LAC9. We demonstrate that this gene codes for a regulatory protein that acts in a positive manner to induce transcription. The LAC9 gene was isolated by complementation of a lac9 defective strain. DNA sequence analysis of the gene gave a deduced protein of 865 amino acids. Comparison of this sequence with that of the GAL4 protein of Saccharomyces cerevisiae revealed three regions of homology. One region of about 90 amino acid occurs at the amino terminus, which is known to mediate binding of GAL4 protein to upstream activator sequences. We speculate that a portion of this region, adjacent to the "metal-binding finger," specifies DNA binding. We discuss possible functions of the two other regions of homology. The functional implications of these structural similarities were examined. When LAC9 was introduced into a gal4 defective strain of S. cerevisiae it complemented the mutation and activated the galactose-melibiose regulon. However, LAC9 did not simply mimic GAL4. Unlike normal S. cerevisiae carrying GAL4, the strain carrying LAC9 gave constitutive expression of GAL1 and MEL1, two genes in the regulon. The strain did show glucose repression of the regulon, but repression was less severe with LAC9 than with GAL4. We discuss the implications of these results and how they may facilitate our understanding of the LAC9 and GAL4 regulatory proteins.

Regulation of β-1, 4-Glucosidase Expression by Candida wickerhamii

Candida wickerhamii NRRL Y-2563 expressed β-glucosidase activity (3 to 8 U/ml) constitutively when grown aerobically in complex medium containing either glycerol, succinate, xylose, galactose, or cellobiose as the carbon source. The addition of a high concentration of glucose (>75 g/liter) repressed β-glucosidase expression (<0.3 U/ml); however, this yeast did produce β-glucosidase when the initial glucose concentration was ≤50 g/liter. When grown aerobically in medium containing glucose plus the above-listed carbon sources, diauxic utilization of the carbon source was observed and the expression of β-glucosidase was glucose repressed. Surprisingly, glucose repression did not occur when the cells were grown anaerobically. When grown anaerobically in medium containing 100 g of glucose per liter, C. wickerhamii produced 6 to 9 U of enzyme per ml and did not demonstrate diauxic utilization of glucose-cellobiose mixtures. To our knowledge, this is the first report of apparent derepression of a glucose-repressed enzyme by anaerobiosis.

Cloning and expression of the structural gene for beta-glucosidase of Kluyveromyces fragilis in Escherichia coli and Saccharomyces cerevisiae

Cellobiose, the last product in cellulose degradation, is converted into two molecules of glucose by a beta-glucosidase. S. cerevisiae does posses the structural gene for a beta-glucosidase, but it is very poorly expressed; we thus decided to isolate and characterize that of Kluyveromyces fragilis. We constructed in E. coli HB101 strain a genomic library of the Kluyveromyces fragilis Y610 strain (ATCC 12424), a yeast able to grow on cellobiose and which constitutively produces the beta-glucosidase. The structural gene for beta-glucosidase was identified by its expression in E. coli. The initial isolated cosmid KF1 contained an insert of 35 Kb and by successive subcloning the insert size was reduced to 3.5 Kb (KF4). This cloned beta-glucosidase gene introduced in S. cerevisiae by transformation is expressed at a level of about 500 times that of K. fragilis. We checked by Southern hybridization that the high expression level was not due to a rearrangement of K. fragilis DNA during the cloning experiments. Nevertheless to obtain yeast transformants able to grow on cellobiose a yeast strain whose permeability to sugar is increased must be used and this last point is discussed.

Physiological studies of beta-galactosidase induction in Kluyveromyces lactis

We examined the kinetics of beta-galactosidase (EC 3.2.1.23) induction in the yeast Kluyveromyces lactis. Enzyme activity began to increase 10 to 15 min, about 1/10 of a cell generation, after the addition of inducer and continued to increase linearly for from 7 to 9 cell generations before reaching a maximum, some 125- to 150-fold above the basal level of uninduced cells. Thereafter, as long as logarithmic growth was maintained, enzyme levels remained high, but enzyme levels dropped to a value only 5- to 10-fold above the basal level if cells entered stationary phase. Enzyme induction required the constant presence of inducer, since removal of inducer caused a reduction in enzyme level. Three nongratuitous inducers of beta-galactosidase activity, lactose, galactose, and lactobionic acid, were identified. Several inducers of the lac operon of Escherichia coli, including methyl-, isopropyl- and phenyl-1-thio-beta-d-galactoside, and thioallolactose did not induce beta-galactosidase in K. lactis even though they entered the cell. The maximum rate of enzyme induction was only achieved with lactose concentrations of greater than 1 to 2 mM. The initial differential rate of beta-galactosidase appearance after induction was reduced in medium containing glucose, indicating transient carbon catabolite repression. However, glucose did not exclude lactose from K. lactis, it did not cause permanent carbon catabolite repression of beta-galactosidase synthesis, and it did not prevent lactose utilization. These three results are in direct contrast to those observed for lactose utilization in E. coli. Furthermore, these results, along with our observation that K. lactis grew slightly faster on lactose than on glucose, indicate that this organism has evolved an efficient system for utilizing lactose.

Purification and properties of an inducible beta-galactosidase isolated from the yeast Kluyveromyces lactis

beta-Galactosidase (EC 3.2.1.32) was purified 80-fold from the yeast Kluyveromyces lactis induced for this enzyme by growth on lactose. When the purified enzyme was subjected to electrophoresis on an acrylamide gel in the presence of sodium dodecyl sulfate, one protein with an apparent molecular weight of 135,000 was observed. The enzyme has a sedimentation coefficient of 9.6S. This beta-galactosidase and the one from Escherichia coli are not antigenically related. Maximal enzyme activity requires Na+ and Mn2+ and a reducing agent. beta-Galactosidase has Km values of 12 to 17 and 1.6 mM for lactose and o-nitrophenyl-beta-D-galactoside, respectively. The hydrolase and transgalactosylase activities of the enzyme are similar to those of E. coli beta-galactosidase.

beta-Glucosidase activity in mycobacteria

A procedure to test beta-glucosidase was developed for identification of the mycobacteria. One hundred and thirty-three strains representing 17 species were assayed in a buffered system containing whole cells and the substrate p-nitrophenyl-beta-D-glucoside. Qualitative differences between species indicated that the test was useful in discriminating among the mycobacteria.

Biochemical and genetic characterization of -glucosidase mutants in Saccharomyces lactis

Mutants with reduced activity for beta-glucosidase (beta-d-glucoside glucohydrolase EC 3.2.1.21) were isolated from the haploid yeast Saccharomyces lactis. Tetrad analysis indicated that in each mutant a single genetic factor, closely linked or allelic to the structural gene for beta-glucosidase (B locus), is responsible for the decreased activity. beta-Glucosidases produced by wild-type and mutant strains are similar in molecular size and charge but differ in catalytic properties, thermal stability, and serological specificity, indicating that mutants are in the structural gene. All mutants retained their capacity to be induced by either methyl-beta-d-glucoside or glucose. In all cases, the mutant phenotype was dominant in heterozygous diploids.

Mutants in Saccharomyces lactis controlling both -glucosidase and -galactosidase activities

InSaccharomyces lactis, a class of mutants isolated for low β-glucosidase activity are reduced in activity for β-galactosidase as well. Genetic studies indicate that their properties are the result of a single mutation in a nuclear gene. In diploide containing a wild-type and mutant β-galactosidase allele, the mutant phenotype is partially dominant. The two enzymes can be separated physically and under appropriate conditions are induced independently in wild-type strains.

Comparison of the catalytic and immunological properties of beta-glucosidases from three strains of Saccharomyces lactis

beta-Glucosidase from Saccharomyces lactis strains Y-123 (B(h)), Y-14 (B(m)), and Y-1057A (B(1)) was partially purified. The pH optima, Michaelis constants, and activation energies were determined for the hydrolysis of p-nitro-phenyl-beta-d-glucoside by each of the enzymes. Differences among these constants were not enough to account for the low specific activity of beta-glucosidase in strains Y-14 and Y-1057A. Enzyme-inhibitor constants were measured for a series of alkyl and aryl glucosides. In general, the three enzymes are arylglucosidases. Tris(hydroxymethyl)-aminomethane inhibited all three enzymes in an uncompetitive fashion. The inhibition was antagonized by Mg(++). An antiserum was prepared to the highly purified (200-fold) beta-glucosidase from strain Y-123. The nature and degree of cross-reaction between the three beta-glucosidases was investigated by double diffusion in agar and neutralization tests. Spur formation in the immunodiffusion tests and similar equivalence points in the neutralization tests indicated a strong degree of cross-reaction between the three enzymes. The ratio of enzyme activity to antigenicity was used to compare the relative molecular activity of beta-glucosidase in the three strains. Each strain produced the same amount of beta-glucosidase per milligram of cell protein. The results are consistent either with a lower turnover number for the beta-glucosidase in strains Y-14 and Y-1057A or with the production of beta-glucosidase with a "normal" turnover number and enough cross-reacting material to effectively reduce the specific activity to the observed levels.

Purification of beta-glucosidase from Saccharomyces lactis strain Y-123

Saccharomyces lactis strain Y-123, a constitutive high producer of beta-glucosidase (B(h)), was grown in an enriched medium. beta-Glucosidase, extracted most easily by cell autolysis, was purified by successive ammonium sulfate precipitation, ethyl alcohol precipitation, gel filtration, calcium phosphate gel adsorption-elution, and hydroxyapatite column chromatography. The specific activity of the enzyme increased 200-fold during the purification. The electrophoretic and catalytic properties of the enzyme did not change during the procedure. Polyacrylamide gel disc electrophoresis of the partially purified enzyme revealed one major and several minor protein-staining bands. beta-Glucosidase activity in the polyacrylamide gel columns was measured directly by assaying sections of columns frozen and sliced immediately after electrophoresis. Most of the activity coincided with the major protein-staining band. Prolonged assay produced minor activity coinciding with the less intense protein bands. Properties of the enzyme suggest that it is a single, unconjugated, intracellular, high molecular weight protein. The purification procedure is applicable to strains of S. lactis which possess alleles of the B locus for beta-glucosidase synthesis.

Purification of beta-glucosidase from Saccharomyces lactis strains Y-14 and Y-1057A

Two strains of Saccharomyces lactis (Y-14 and Y-1057A), medium (B(m)) and low (B(1)) constitutive producers of beta-glucosidase, were grown in enriched medium. beta-Glucosidase was extracted by autolysis and purified by ammonium sulfate precipitation, gel filtration, and calcium phosphate gel adsorption-elution. The kinetics of release, purification, and stability of beta-glucosidase from strains Y-14 and Y-1057A were compared with the enzyme from strain Y-123. The ability of glycerol, sorbitol, and mannitol to stabilize the beta-glucosidases is presented. A lower molecular weight, labile form of the Y-14 enzyme is demonstrated. Differences in the initial specific activites of beta-glucosidase among the three strains are discussed.

Respiratory-deficient mutants in Saccharomyces lactis

Six independent ultraviolet-induced respiratory-deficient mutants (petites) of Saccharomyces lactis were isolated and characterized. Two possessed a normal cytochrome spectrum, another displayed an increased level of all the cytochromes, and three suffered from a partial or complete loss of one or more of the cytochromes a, b, c, and c(1). All of the mutants were segregational petites; none was vegetative. Determination of linkage relationships between mutants was restricted because matings between mutants, homozygous or heterozygous, for loci affecting cytochrome content were blocked at various stages in the mating-sporulating sequence. At least three of the petites were genetically nonidentical. Three of the mutations appeared to occupy loci within the same linkage group; two of the three mutations that mapped within this region were cytochrome-deficient. Growth at high or low temperatures, under increased osmotic pressure or in media supplemented with various fatty acids or sterols, did not relieve the physiological defects in these mutants. Reasons for the differences in survival of segregational and vegetative petites within this species are examined.

Evidence for multiple molecular forms of yeast beta-glucosidase in a hybrid yeast

A mixture of beta-glucosidases from Saccharomyces fragilis (Y-18) and S. dobzhanskii (Y-19) eluted from diethylaminoethyl cellulose in two peaks, whereas the enzyme from a hybrid, S. fragilis x S. dobzhanskii (Y-42), eluted in a single broad peak. The highest Y-42 activity fractions eluted at a sodium chloride molarity which was intermediate to the molarities at which most of the Y-18 and Y-19 activity was eluted. In cellulose polyacetate strips, Y-42 enzyme migrated as a diffuse band which spanned the distances migrated by the enzymes from the parent yeast strains. Antisera against either Y-18 or Y-19 enzyme precipitated 80 to 90% of Y-42 enzyme activity. When Y-42 enzyme was dissociated by heat or urea and reacted with parental antiserum, a concomitant increase in the opposite parental activity was demonstrable in both precipitation and complement-fixation (CF) tests. Urea-dissociated beta-glucosidases were resolvable by sucrose-gradient centrifugation into multiple bands displaying specific CF activity. When the enzymes were exposed to 4 m urea for 12 min, particles of approximately 110,000 molecular weight were obtained. By extending the exposure time to 40 min, and incorporating 0.5 m urea in the gradients, smaller particles were detected with molecular weights ranging from 18,000 to 23,000. Attempts to regenerate enzyme activity after dissociation with urea were only moderately successful. Results suggested that a slightly acidic environment favored reassociation, as did the presence of 2-mercaptoethanol. Residual urea also seemed important. It is proposed that the structural genes for both Y-18 and Y-19 enzyme are present in Y-42 cells with either independent or closely interacting regulatory mechanisms. Since synthesis of the two parental-type polypeptides may be unequal, the availability of enzyme subunits for subsequent polymerization in the cell cytoplasm might be equalized at the polysome level. Random association of subunits would produce a binomial distribution of true hybrid enzyme molecules.

GENETIC CONTROL OF β-GLUCOSIDASE SYNTHESIS IN SACCHAROMYCES LACTIS

Herman, Alberta (University of Wisconsin, Madison) and Harlyn Halvorson . Genetic control of β-glucosidase synthesis in Saccharomyces lactis . J. Bacteriol. 85: 901–910. 1963.—Both methyl-β- d -glucoside (2 × 10 −2 m ) and glucose (10 −3 m ) induced β-glucosidase synthesis in selected strains of Saccharomyces lactis . Genetic studies indicated the existence of a single locus specifically affecting β-methyl glucoside inducibility. Glucose-induced β-glucosidase synthesis, on the other hand, was nonspecific (other carbohydrases were simultaneously induced) and appeared to be controlled by more than one genetic factor. In both cases, noninducibility was dominant in diploids. The independent expression of these two modes of induction implied that these loci regulated β-glucosidase induction in a nonsequential manner.