Characterization of the biotin transport system in Saccharomyces cerevisiae

Vitamins in wine: Which, what for, and how much?

Vitamins are essential compounds to yeasts, and notably in winemaking contexts. Vitamins are involved in numerous yeast metabolic pathways, including those of amino acids, fatty acids, and alcohols, which suggests their notable implication in fermentation courses, as well as in the development of aromatic compounds in wines. Although they are major components in the course of those microbial processes, their significance and impact have not been extensively studied in the context of winemaking and wine products, as most of the studies focusing on the subject in the past decades have relied on relatively insensitive and imprecise analytical methods. Therefore, this review provides an extensive overview of the current knowledge regarding the impacts of vitamins on grape must fermentations, wine-related yeast metabolisms, and requirements, as well as on the profile of wine sensory characteristics. We also highlight the methodologies and techniques developed over time to perform vitamin analysis in wines, and assess the importance of precisely defining the role played by vitamins in winemaking processes, to ensure finer control of the fermentation courses and product characteristics in a highly complex matrix.

Microbial extraction of biotin from lignocellulose biomass and its application on glutamic acid production

Biotin (vitamin B₇) is an important nutrient for various fermentations. It is abundant in agricultural lignocellulose biomass and maintains stable in biorefinery processing chain including acid pretreatment, biodetoxification and saccharification. Here we show a microbial extraction of biotin from biotin-rich corn leaves hydrolysate. Corynebacterium glutamicum was found to have the highest biotin uptake capacity among different biotin auxotrophic microorganisms, and it was further significantly increased by overexpressing the bioYMN gene cluster encoding biotin transporter. Finally 250 folds greater biotin was extracted by recombinant C. glutamicum (303.8 mg/kg dry cell) from virgin corn leaves (1.2 mg/kg), which was far higher than that in commonly used fermentation additives including yeast extract (∼2 mg/kg), molasses (∼1 mg/kg) and corn steep liquor (∼0.75 mg/kg). The biotin extracted from corn leaves was successfully applied to glutamic acid fermentation. This is the first report on microbial extraction of biotin from lignocellulose biomass and fermentation promotion application.

Development of a Synthetic Malonyl-CoA Sensor in Saccharomyces cerevisiae for Intracellular Metabolite Monitoring and Genetic Screening

Genetic sensors capable of converting key metabolite levels to fluorescence signals enable the monitoring of intracellular compound concentrations in living cells, and emerge as an efficient tool in high-throughput genetic screening. However, the development of genetic sensors in yeasts lags far behind their development in bacteria. Here we report the design of a malonyl-CoA sensor in Saccharomyces cerevisiae using an adapted bacterial transcription factor FapR and its corresponding operator fapO to gauge intracellular malonyl-CoA levels. By combining this sensor with a genome-wide overexpression library, we identified two novel gene targets that improved intracellular malonyl-CoA concentration. We further utilized the resulting recombinant yeast strain to produce a valuable compound, 3-hydroxypropionic acid, from malonyl-CoA and enhanced its titer by 120%. Such a genetic sensor provides a powerful approach for genome-wide screening and could further improve the synthesis of a large range of chemicals derived from malonyl-CoA in yeast.

111In-DTPA-Biotin uptake by Staphylococcus aureus

The potential of indium-111 labelled diethylenetriaminepentaacetic acid α,ω-bis(biocytinamide) (In-DTPA-Biotin) as a specific tracer in nuclear medicine imaging of vertebral osteomyelitis has been shown in a large series of consecutive patients. Biocytin is known to serve as a biotin source for a number of different microorganisms and quantitative studies on staphylococci indicated that on a molar basis biocytin seemed to have an activity equal to that of biotin. In this study, we evaluated the possibility of an illicit transport of In-DTPA-Biotin in cultures of Staphylococcus aureus on continued incubation for 24 h. Radiolabelled biocytin was prepared as described earlier and the stability and radiochemical purity was assessed in vitro for 24 h after labelling. Our data seem to demonstrate a passive transport of In-DTPA-Biotin into the cells of the microorganisms.

Optimisation of culture conditions with respect to biotin requirement for the production of recombinant avidin in Pichia pastoris

Due to its very high affinity to biotin, avidin is one of the most widely exploited proteins in modern biotechnological and biomedical applications. Since biotin is an essential vitamin for the growth of many microorganisms, we examined the effect of biotin deficiency on growth for a recombinant Pichia pastoris strain expressing and secreting a recombinant glycosylated avidin. The results showed that biotin deficiency lowers growth rate and biomass yield for P. pastoris. Substitution of biotin in the medium by the two structurally unrelated compounds, aspartic acid and oleic acid, which do not bind to recombinant avidin was analyzed quantitatively. These two compounds had a growth promoting effect in biotin-deficient medium, but did not replace biotin completely. Indeed, in chemostat culture, wash-out occurred after about six liquid residence times and recombinant avidin productivity was lowered. However, addition of low amounts of biotin (20 microg L(-1) of biotin for a cell density of 8 g L(-1)) resulted in stable chemostat cultures on methanol with the production of recombinant biotin-free avidin. The specific avidin production rate was 22 microg g(-1) h(-1) at a dilution rate of 0.06 h(-1).

Vhr1p, a New Transcription Factor from Budding Yeast, Regulates Biotin-dependent Expression of VHT1 and BIO5

Transcription of the Saccharomyces cerevisiae vitamin H transporter gene VHT1 is enhanced by low extracellular biotin. Here we present the identification and characterization of Vhr1p as a transcriptional regulator of VHT1 (VHR1 (YIL056w); VHT1 regulator 1) and the identification of the cis-regulatory target sequences for Vhr1p in two yeast promoters. VHR1 was identified in a complementation screening of mutagenized yeast cells that had lost the capacity to express the gene of the green fluorescent protein (GFP) from the VHT1 promoter. Deltavhr1 deletion mutants fail to induce VHT1 on low biotin concentrations. In yeast one-hybrid analyses performed with fusions of Vhr1p N-terminal and C-terminal fragments to the Gal4p activation domain or to the Gal4p DNA-binding domain, the Vhr1p N terminus mediated biotin-dependent DNA binding, and the Vhr1p C terminus triggered biotin-dependent transcriptional activation. The analyzed Vhr1p N-terminal fragment has previously been described as a domain of unknown function (DUF352). Deletion and linker scanning analyses of the VHT1 promoter revealed the palindromic 18-nucleotide sequence AATCA-N8-TGAYT as the vitamin H-responsive element. This sequence was identified also in the BIO5 promoter that is also transcriptionally activated on low biotin concentrations. Bio5p mediates the transport of 7-keto-8-aminopelargonic acid across the yeast plasma membrane, a compound that is used as a precursor in biotin biosynthesis. Deltavhr1 deletion mutants fail to induce BIO5 on low biotin concentrations. The presented data characterize Vhr1p as an essential component of the biotin-dependent signal transduction cascade in yeast.

Biotin sensing in Saccharomyces cerevisiae is mediated by a conserved DNA element and requires the activity of biotin-protein ligase

Biotin is a water-soluble vitamin that functions as a prosthetic group in carboxylation reactions. In addition to its role as a cofactor, biotin has multiple roles in gene regulation. We analyzed biotin effects on gene expression in the yeast Saccharomyces cerevisiae and demonstrated by microarray, Northern, and Western analyses that all yeast genes encoding proteins involved in biotin metabolism are up-regulated following biotin depletion. Many of these genes contain a palindromic promoter element that is necessary and sufficient for mediating the biotin response and functions as an upstream-activating sequence. Mutants lacking the plasma membrane biotin transporter Vht1p display constitutively high expression levels of biotin-responsive genes. However, they react normally to biotin precursors that do not require Vht1p for uptake. The biotin-like effect of precursors with regard to gene expression requires their intracellular conversion to biotin. This demonstrates that Vht1p does not act as a sensor for biotin and that intracellular biotin is crucial for gene expression. Mutants with defects in biotin-protein ligase, similar to vht1delta mutants, also display aberrantly high expression of biotin-responsive genes. Like vht1delta cells, they have reduced levels of protein biotinylation, but unlike vht1delta mutants, they possess normal levels of free intracellular biotin. This indicates that free intracellular biotin is irrelevant for gene regulation and identifies biotin-protein ligase as an important element of the biotin-sensing pathway in yeast.

A permease exhibiting a dual role for lysine and biotin uptake in Saccharomyces cerevisiae

Among Saccharomyces cerevisiae strains each defective in one of 11 amino acid permeases, a lysine permease disruptant (DK) exhibited about 2-fold reductions in maximum cell density and fermentation ability compared to the parent in a synthetic medium. These unusual properties of DK were found to result from the requirement of biotin for growth, in contrast to the parent whose growth was not dependent on external biotin. The rate of 14C-labeled biotin uptake and the intracellular free biotin content of DK were 2-2.5 fold lower than in the parent. We suggest that lysine permease in S. cerevisiae has the ability to transport both lysine and biotin.

Control of gene expression with small molecules: biotin-mediated acylation of targeted lysine residues in recombinant yeast

Chemical inducers of dimerization (CIDs) are powerful tools for controlling diverse cellular processes. These small molecules typically form strong noncovalent interactions with proteins. We report a related approach involving covalent acylation of a specific lysine residue of a target protein by the small molecule biotin. To control protein-protein interactions with biotin, the biotin protein ligase BirA from E. coli was coexpressed in yeast with a streptavidin-LexA fusion protein and Avitag or BCCP biotin acceptor peptides fused to the B42 activation domain. The addition of biotin (10 nM) resulted in BirA-mediated biotinylation of the biotin acceptor protein, recruitment to LexA DNA sites, and maximal activation of reporter gene expression in this yeast tribrid system. The high potency, low toxicity, and low molecular weight of biotin as a covalent CID are attractive properties for controlling cellular processes.

Isolation and characterization of the plasma membrane biotin transporter from Schizosaccharomyces pombe

The fission yeast Schizosaccharomyces pombe is auxotrophic for biotin (vitamin H) and growth depends on biotin uptake over the plasma membrane. Here a biotin transport mutant of Saccharomyces cerevisiae is used to identify the vht1(+) gene encoding the Schizosaccharomyces pombe plasma membrane transport protein for biotin. SpVht1p belongs to the family of allantoate transporters and has only little sequence homology to the S. cerevisiae biotin transporter. Although having dissimilar primary structures, the biotin transporters in Sz. pombe and S. cerevisiae share similar biochemical properties and regulation. Like in S. cerevisiae, biotin uptake in Sz. pombe is a high-affinity process, is optimal at acidic pH values and inhibited by protonophores, indicating that SpVht1p acts as a proton-biotin symporter. Desthiobiotin, the metabolic precursor of biotin, is also imported by SpVht1p. Deletion of vht1(+) abolishes growth on low external concentrations of the vitamin, showing that vht1(+) encodes the only protein that mediates biotin uptake in Sz. pombe. Expression of vht1(+) is maximal at low external biotin concentrations, indicating that Sz. pombe can adjust the rate of biotin uptake to meet the requirement for the vitamin.

Biotin protein ligase from Saccharomyces cerevisiae. The N-terminal domain is required for complete activity

Catalytically active biotin protein ligase from Saccharomyces cerevisiae (EC 6.3.4.15) was overexpressed in Escherichia coli and purified to near homogeneity in three steps. Kinetic analysis demonstrated that the substrates ATP, biotin, and the biotin-accepting protein bind in an ordered manner in the reaction mechanism. Treatment with any of three proteases of differing specificity in vitro revealed that the sequence between residues 240 and 260 was extremely sensitive to proteolysis, suggesting that it forms an exposed linker between an N-terminal 27-kDa domain and the C-terminal 50-kDa domain containing the active site. The protease susceptibility of this linker region was considerably reduced in the presence of ATP and biotin. A second protease-sensitive sequence, located in the presumptive catalytic site, was protected against digestion by the substrates. Expression of N-terminally truncated variants of the yeast enzyme failed to complement E. coli strains defective in biotin protein ligase activity. In vitro assays performed with purified N-terminally truncated enzyme revealed that removal of the N-terminal domain reduced BPL activity by greater than 3500-fold. Our data indicate that both the N-terminal domain and the C-terminal domain containing the active site are necessary for complete catalytic function.

Identification of the plasma membrane H+-biotin symporter of Saccharomyces cerevisiae by rescue of a fatty acid-auxotrophic mutant

Bakers' yeast is auxotrophic for biotin (vitamin H) and depends on the efficient uptake of this compound from the environment. A mutant strain with strongly reduced biotin uptake and with reduced levels of protein biotinylation was identified. The strain was auxotrophic for long-chain fatty acids, and this auxotrophy could be suppressed with high levels of biotin in the medium. After transformation of this mutant with a yeast genomic library, the unassigned open reading frame YGR065C was identified to complement this mutation. This gene codes for a protein with 593 amino acids and 12 putative transmembrane helices. Northern blot analysis revealed that, in wild-type cells, the corresponding mRNA levels were increased at low biotin concentrations. Likewise, cellular biotin uptake was increased with decreasing biotin availability. Expression of YGR065C under the control of the constitutive ADH1 promoter resulted in very high biotin transport rates across the plasma membrane that were no longer regulated by the biotin concentration in the growth medium. We conclude that YGR065C encodes the first biotin transporter identified for a non-mammalian organism and designate this gene VHT1 for vitamin H transporter 1.

Rapid purification of a functionally active plant sucrose carrier from transgenic yeast using a bacterial biotin acceptor domain

A rapid and efficient method has been used for the purification of a Plantago major sucrose carrier from Saccharomyces cerevisiae. The C-terminal fusion of a bacterial biotin acceptor domain to the carrier protein did not interfere with the targeting to the yeast plasma membrane nor with the catalytic activity of the sucrose carrier. The chimeric construct is biotinylated by yeast cells in vivo and represents the only biotinylated protein in yeast membranes. Solubilized biotinylated carrier protein binds selectively to immobilized monomeric avidin and can be eluted as pure protein with free biotin. The purified protein is functionally active and catalyzes the energy-dependent transport of sucrose into proteoliposomes.

Biotin staining in the giant fiber systems of the lobster

The avidin-biotin-complex method is a popular immunocytochemical technique. This method labels consistently a group of neurons in the lobster ventral nerve cord in the absence of primary antibodies. The specific staining is due to a relatively high level of endogenous biotin (or biocytin) in these neurons. These biotin-positive neurons are located in the supraesophageal, thoracic, and abdominal ganglia. Intraaxonal injection of Lucifer yellow followed by Texas red-conjugated streptavidin staining reveals that the neurons are members of the medial giant (MG) and lateral giant (LG) systems, which are important in mediating rapid tail flipping during escape maneuvers. In neuronal somata, staining is restricted to the cytoplasm. Within MG axons, staining appears as punctate, subaxolemmal structures. Preincubating nerve cords in biocytin or direct intraaxonal injection of biocytin enhances staining of these punctate organelles. In LG axons, staining is localized to fragments of braided filamentous structures that also appear to be associated with the axolemma. Preincubation of ventral nerve cords in various concentrations of biocytin results in the appearance of additional groups of stained neurons, suggesting that there are subsets of neurons with specific biocytin-uptake or -retention mechanisms. In the crayfish, biotin-positive staining is confined to the MG neurons; the LG neurons are not stained. In the earthworm, no staining is observed in the MG and LG axon escape systems. In the goldfish, no biotin-staining is seen in the Mauthner neurons and their axons. The significance of specific localization of biotin or biocytin to subsets of neurons is unclear. It may reflect the presence of high levels of biocytin moieties on biotin-dependent enzymes. Biotin is an important cofactor in the catalytic functions of several decarboxylases crucial in energy production and lipogenesis. Axons of the giant fiber systems in lobsters and crayfish may have high energy and fatty acid synthesis requirements. Increased levels of biotin accumulation may also be related to other functions of the giant axon systems, such as the formation of electrical synapses among themselves and with phasic motoneurons.

Biotin uptake in cultured hepatocytes from normal and biotin-deficient rats

Biotin uptake was studied in isolated cultured hepatocytes of normal and biotin-deficient rats. Biotin uptake was temperature-dependent with respect to physical, but not to chemical, processes, proportional to the exogenous biotin concentration in the medium, independent of pH and sodium ion concentrations of the medium, and uneffected by the presence of structural analogues of biotin or metabolic inhibitors in both normal and biotin-deficient hepatocytes. These results suggest that biotin uptake occurs by a passive, nonmediated, non-energy-dependent mechanism in rat hepatocytes.

Role of human serum biotinidase as biotin-binding protein

Biotinidase shows two binding sites for biotin, with Kd = 59 and 3 nM respectively, and requires tryptophan and cysteine residues of the biotinidase protein for biotin-binding activity. Analysis of human serum by various column-chromatographic techniques indicates that biotinidase is the only protein which exchanges with labelled (+)-biotin. It was shown previously that epileptic patients receiving a high average dose of anticonvulsants (containing a carbamide group) have lower biotin concentrations than those receiving a low dose. We have shown in human serum and with purified biotinidase that these anticonvulsant drugs compete with biotin for binding to the protein moiety.

The glucose-dependent transport of L-malate in Zygosaccharomyces bailii

Zygosaccharomyces bailii possesses a constitutive malic enzyme, but only small amounts of malate are decomposed when the cells ferment fructose. Cells growing anaerobically on glucose (glucose cells) decompose malate, whereas fructose cells do not. Only glucose cells show an increase in the intracellular concentration of malate when suspended in a malate-containing solution. The transport system for malate is induced by glucose, but it is repressed by fructose. The synthesis of this transport system is inhibited by cycloheximide. Of the two enantiomers L-malate is transported preferentially. The transport of malate by induced cells is not only inhibited by addition of fructose but also inactivated. This inactivation is independent of the presence of cycloheximide. The transport of malate is inhibited by uranyl ions; various other inhibitors of transport and phosphorylation were of little influence. It is assumed that the inducible protein carrier for malate operates by facilitated diffusion. Fructose cells of Z. bailii and cells of Saccharomyces cerevisiae do not contain a transport system for malate.

Biotin transport by a biotin-deficient strain of Escherichia coli

Biotin uptake has been investigated using an Escherichia coli biotin requiring auxotroph grown under biotin-deficient conditions. This strain accumulated biotin in the free and bound form. In agreement with a previous report by O. Prakash and M.A. Eisenberg (J. Bacteriol. 120 (1974) 785-791), the biotin entry proved to be an active process which depended on an energy source and was inhibited in the presence of uncouplers. The kinetic parameters have been determined (KM = 0.05 microM, Vmax = 7 pmol/min per mg dry weight). The pool of free biotin could be readily exchanged with external biotin and decreased to a very low level in the absence of an energy source. The use of several biotin analogues revealed that this transport system was quite specific for biotin: slight modifications, for instance in the valeric chain, lowered drastically the affinity for the carrier.

Properties of the biotin transport system in Bifidobacterium breve N4

Binding of biotin to resting cells of Bifidobacterium breve N4, which grew in a biotin-deficient medium, was independent of pH from 1 to 9 and of temperature below 50 C. It was not inhibited by metabolic inhibitors including sulfhydryl reagents, but it was inhibited by treatment with 80% ethanol or 5% trichloroacetic acid. It was also competitively inhibited by biotin-sulfone, but not by tetrahydrothiophene nor dethiobiotin. The binding constant was calculated to be 3.3 X 10(8) M--1. The amount of biotin unextractable with hot water, representing part of the transported biotin, increased gradually for 20 min, this increase was inhibited by NaF, hydroxylamine and low temperature. 14C-biotin on the cells was displaced by cold biotin and biotin-sulfone; the displacement was not inhibited by metabolic inhibitors, but it was dependent on temperature. A few minutes after binding, the biotin was released to the medium. The release was dependent on pH and temperature, was affected by energy sources and was inhibited by metabolic inhibitors, e.g. NaF, p-chloromercuribenzoic acid and hydroxylamine. It could be stopped at any time by cooling to 0 C or by adding NaF, and the amount of accumulated biotin did not increase under those conditions. These results suggest that the binding sites on the cell surface decreased in number or in their binding affinity for biotin through an energy-dependent process.

Inhibition of thiamine transport in baker's yeast by methylene blue

Methylene blue was found to inhibit thiamine transport competitively (Ki = 0.63 microM) in baker's yeast. The dye was also effective in abolishing the growth inhibition of Saccharomyces cerevisiae by pyrithiamine which is known to be taken up by a common transport system for thiamine in yeast cells. A possible mechanism for the inhibition by methylene blue of the thiamine transport system in baker's yeast is discussed.

Specific localization and quantification of biotin transport components in yeast by use of a biotin-conjugated, impermeant, electron-dense label

Two approaches are described for the localization and quantification of biotin transport components in yeast cells. One approach is based on tracing the fate of a radioactive affinity label for the biotin transport system, [14C]biotinyl-p-nitrophenyl ester (pBNP), through various stages of subcellular fractionations. A complementary method involves the use of a biotin-derivatized, impermeant, electron-dense, affinity-cytochemical label (ferritin-biotin conjugates) for subsequent visualization by electron microscopy. Values of approximately 8,000 and 4,000 sites/cell, respectively, were achieved by the two methods. Complicating factors, future perspectives and the relevance of the two methods to the isolation of transport components are discussed.

Uptake of extracellular biotin by Escherichia coli biotin prototrophs

Uptake of exogenous biotin by two Escherichia coli biotin prototroph strains, K-12 and Crookes, appeared to involve incorporation at a fixed number of binding sites located at the cell membrane. Incorporation was characterized as a binding process specific for biotin, not requiring energy, and stimulated by acidic pH. Constant saturation quantities of exogenous biotin were incorporated by these cells, and the amounts, which were titrated, depended on whether the cells were resting or dividing. Resting cells incorporated exogenous biotin amounting to 2% of their total intracellular biotin content. Fifty percent of the exogenous biotin was incorporated into their free biotin fraction, and 50% was incorporated into their bound biotin fraction. On the other hand, dividing cells incorporated exogenous biotin into all of their intracellular sites, 88% going into the intracellular-bound biotin fraction, and 12% going into the free biotin fraction. Calculations suggested that each cell contained approximately 3,000 binding sites for biotin. It was postulated that biotin incorporation sites might have been components of acetyl coenzyme A carboxylase located at or near the membrane.

[Studies on the thiamine transport system in Bacillus cereus (author's transl)]

The thiamine transport system in Bacillus cereus exhibits rhythmical changes of resorption- and excretion-phases lasting 1-2 h. These main phases are subdivided in shorter ones with an average duration of 45 s. The velocity of the thiamine uptake is influenced by pH, temperature, age of cells, energy and substrate supply and thiamine concentration of the medium. The Michaelis-Menten-Kinetic can be used to describe the uptake: Km = 1.98 x 10(-8) M; Vmax = 1.19 x 10(-6) mol/g dry weight x min. The rate is enhanced by K+, Ca2+ and Mg2+, and inhibited by Pyrithiamin, EDTA, H+-ions, proton donors and proton acceptors; OH(-)-ions cause a change in the direction of transport. A theoretical explanation can be given by assuming a coupling of the thiamine permeation with proton movements in the membrane.

Reversibility of the affinity labelled-biotin transport system in yeast cells

Transport of biotin by Saccharomyces cerevisiae is inhibited by biotynyl p-nitrophenyl ester. Conversion of the inhibited cells to spheroplasts or simple treatment with thiols results in a total restoration of vitamin transport. Biotynyl p-nitrophenyl ester-induced inhibition is not due to an intracellular accumulation of the vitamin and consequent regulation, but appears to be due to specific labelling of the transport system.

Active transport of biotin in Escherichia coli K-12

The transport of [(14)C]biotin into cells of a biotin prototroph, Escherichia coli K-12 strain Y10-1, was investigated. The vitamin taken up by the cells in this strain existed primarily in the free form. Addition of glucose enhanced the rate of uptake six- to eightfold and the steady level was reached in 2 to 3 min resulting in accumulation of biotin against a concentration gradient. The uptake showed marked dependence on temperature (Q(10), 2.3; optimum, 37 C) and pH (optimum 6.6) and was inhibited by iodoacetate. Energy of activation for glucose-dependent uptake was calculated to be 16,200 cal per mol. The rate of biotin uptake with increasing biotin concentrations showed saturation kinetics with an apparent K(m) and V(max) values of 1.4 x 10(-7) M and 6.6 pmol per mg of dry cells per min respectively. The cells also accumulated biotin against a concentration gradient in the absence of added glucose, although at a much lower rate. This accumulation was much more susceptible to inhibition by azide and uncouplers of oxidative phosphorylation suggesting that the energy source was supplied through the electron-transport chain. Inhibition studies with a number of biotin analogues indicated the requirement for an intact ureido ring. The biotin uptake was inhibited in cells grown in biotin-containing medium and was shown to be the result of repression of the transport system, suggesting the control of the biotin transport.

Biotin uptake by cold-shocked cells, spheroplasts, and repressed cells of Saccharomyces cerevisiae: lack of feedback control

Cold-osmotic-shocked cells and spheroplasts of Saccharomyces cerevisiae (ATCC 9896) display a biotin uptake system similar to that observed in intact cells. 2-Mercaptoethanol was found to inhibit biotin transport. Cells repressed for biotin uptake by growth in excess biotin (25 ng/ml) possess an energy-dependent transport system that has a K(m) for biotin of 6.6 x 10(-7) M and a V(max) equal to 39 pmol per mg (dry weight) per min. A similar K(m) (6.4 x 10(-7) M) but a considerably higher V(max) (530 pmol per mg (dry weight) per min) was determined for biotin uptake by cells grown in sufficient biotin (0.25 ng/ml). The V(max) rates of biotin uptake by both repressed and derepressed cells were increased approximately 35-fold in the presence of glucose. These yeast cells appear to regulate their biotin uptake by two mechanisms. An exit system provides for immediate adjustments, whereas turnover of the transport system and repression of new synthesis establishes a slower adaptation to changes in the environment. Feedback inhibition was ruled out as a mechanism of regulation of transport.

Irreversible inhibition of biotin transport in yeast by biotinyl-p-nitrophenyl ester

Biotinyl-p-nitrophenyl ester (BNP), an active-ester derivative of biotin, irreversibly inactivates biotin transport in the yeast Saccharomyces cerevisiae. Transport inactivation is progressive with time and occurs at concentrations of the ester as low as 10(-7) M. In the presence of sodium azide, a reagent known to block biotin accumulation in yeast, the derivative is still effective. The specificity of inactivation by the ester is revealed by the following findings: (a) Biotinyl-p-nitroanilide and acetyl-p-nitrophenyl ester do not affect biotin transport; (b) the nitrophenyl ester does not affect the transport of lysine and aspartic acid, or that of L-sorbose; (c) inactivation of biotin transport by the ester is partially prevented when the cells are incubated with it in the presence of relatively high concentrations of biotin.

Uranyl nitrate inhibition of transport systems in Saccharomyces cerevisiae

Uranyl nitrate inhibited the transport of several amino acids, a vitamin, and a disaccharide into yeast.

Regulation of biotin transport in Saccharomyces cerevisiae

The metabolic control of biotin transport in Saccharomyces cerevisiae was investigated. Nonproliferating cells harvested from cultures grown in excess biotin (25 ng/ml) took up small amounts of biotin, whereas cells grown in biotin-sufficient medium (0.25 ng/ml) accumulated large amounts of the vitamin. Transport was inhibited maximally in cells grown in medium containing 9 ng (or more) of biotin per ml. When avidin was added to biotin-excess cultures, the cells developed the ability to take up large amounts of biotin. Boiled avidin was without effect, as was treatment of cells with avidin in buffer. Avidin did not relieve transport inhibition when added to biotin-excess cultures treated with cycloheximide, suggesting that protein synthesis was required for cells to develop the capacity to take up biotin after removal of extracellular vitamin by avidin. Cycloheximide did not inhibit the activity of the preformed transport system in biotin-sufficient cells. The presence of high intracellular free biotin pools did not inhibit the activity of the transport system. The characteristics of transport in biotin-excess cells (absence of temperature or pH dependence, no stimulation by glucose, absence of iodoacetate inhibition, independence of uptake on cell concentration, and nonsaturation kinetics) indicated that biotin entered these cells by diffusion. The results suggest that the synthesis of the biotin transport system in S. cerevisiae may be repressed during growth in medium containing high concentrations of biotin.