Export of an invertase by yeast Candida utilis cells

Export and accumulation of various forms of invertase (EC 3.2.1.26) in the cell wall and culture medium of the yeast Candida utilis was investigated. It was found that there is the high-molecular-weight invertase in the cell wall (CW-form). This form is not exported into the culture medium, and it is by a third more glycosylated than the previously described exported S-form. It was shown that one of the two forms of invertase exported into the culture medium—the glycosylated S-form—is retained in the cell wall, while the other one-the nonglycosylated F-form—was not detected in the cell wall. Based on these results, as well as data on the distribution dynamics of the enzyme in the culture medium and in the cell wall during different growth stages of a yeast culture, we suggested that the nonglycosylated form was exported into the culture medium via the zone of abnormal cell wall permeability and the glycosylated forms of this enzyme (both exported and nonexported) did not use this pathway and the degree of N-glycosylation is an important factor determining the final localization of the enzyme.     

Cobalamins and cobalamin-dependent enzymes in Candida utilis.

Candida utilis has been shown to contain 4.7 pmol of cobalamin per g of wet cell paste. Purification of the cobalamin showed it to be a mixture of methylcobalamin and adenosylcobalamin. Two cobalamin-dependent enzyme systems have been found in the yeast: methylcobalamin-dependent methionine biosynthesis and leucine 2,3-aminomutase. The cobalamin extracted from the yeast is as effective as authentic adenosylcobalamin in stimulating leucine 2,3-aminomutase.     

Pentose Metabolism in Candida utilis

In attempts to obtain GMP producing strains, Brevibacterium ammoniagenes was treated with UV, N.T.G. or D.E.S. as a mutagen. Adenine-guanine requiring mutants were obtained from an adenine-requiring mutant of Brev. ammoniagenes, KY 3482–9 and two of them, presumably adenine-xanthine requiring mutants, were then reverted to mutants which required only adenine for their growth.

Although these revertants were not able to accumulate a copious amount of GMP, most of them and of adenine-guanine requiring mutants produced larger amounts of IMP than the parent adenine-requiring strain.

Effects of Mn²⁺ and purine bases in the medium on IMP production by these mutants were examined and IMP productivities of these mutants were compared with the parent strain under optimal conditions.

These mutagenic treatments were thus proved to be effective for the increase of de novo IMP production by Brev. ammoniagenes mutants.

Brevibacterium ammoniagenes ATCC 6872 accumulates 5′-GDP and -GTP, or 5′-ADP and -ATP together with GMP or AMP in nucleotide fermentation by salvage synthesis.

With cell free extract of this strain, transphosphorylating reactions of AMP or GMP were investigated.

ATP-AMP transphosphorylating enzyme(s) was partially purified to 21.7 fold with acid treatment, salting-out and column chromatography.

In ATP-AMP and ATP-GMP transphosphorylating reactins, optimal conditions were decided such as for concentrations of enzyme, of MgCl₂ and of phosphate donor, pH and cell age as the enzyme sources.

Specificities of phosphate donors and acceptors were examined with both the partially purified enzymes or the sonicate. AMP and GMP were phosphorylated by ATP rapidly, but IMP and XMP were not, therefore supporting our previous finding that Brev. ammoniagenes could not accumulated IDP, ITP, XDP and XTP in IMP and XMP fermentation, respectively.

Although ATP was the best donor for both AMP and GMP phosphorylations, other nucleoside triphosphates and PRPP were used as phosphate donors.

Furthermore, phosphorylation of ADP to ATP was investigated and possible mechanisms of nucleoside di- or triphosphates synthesis in the nucleotide fermentation were discussed.

From these results, it is suggested as a possible mechanism for nucleoside di- and triphosphate accumulation by Brev. Ammoniagenes, that a nucleoside monophosphate formed is phosphorylated to a nucleoside di-phosphate with ATP or other phosphate donors and then the nucleoside diphosphate is converted to a triphosphate with these phosphate donors.

Both AMP and GMP were transphosphorylated rapidly to the corresponding nucleoside-diphosphates and triphosphates by ATP and by other high energy phosphate compounds with cell free extracts of Brevibacterium ammoniagenes.

Some enzyme inhibitors, such as metals and PCMB were shown to inhibit the phosphorylations of AMP and GMP. Higher levels of ATP, ADP, GTP and GDP also inhibited the activity of the partially purified ATP-AMP transphosphorylating enzyme(s).

In guanine nucleotides fermentation by salvage synthesis with this strain, addition of these inhibitors to the medium increased the amounts of GMP and total guanine nucleotides accumulated.

On the contrary, supplement of xylene or of other organic solvents to the medium stimulated the accumulation of both GTP and total guanine compouuds in this fermentation. From enzymatic studies, these solvents are presumed to have the ability to change cell permeability.

Such findings give an effective method for controlling the amounts of nucleotides accumulated in these fermentations.     

Disruption of Candida utilis cells in high pressure flow devices*

The disruption of Candida utilis cells in suspensions subjected to different types of stress was investigated. Stresses caused by impingement of a high velocity jet of suspended cells against a stationary surface were found to be significantly more effective for disruption than either shear or normal stresses. The fraction of cells disrupted by impingement is a first order function of the number of passes through the disruptor and, over a prescribed range of operating pressures, is a power function of pressure. These results indicate that impingement is the predominant mechanism causing cells disruption in high pressure flow devices such as Manton–Gaulin homogenizers. The impingement results suggest that cells grown in cyclic batch culture are easier to disrupt than cells grown at a lower specific growth rate in continuous culture. In addition to determining the fraction of cells disrupted, the release of invertase activity was determined for the impingement experiments. The fraction of total invertase activity released was found to be somewhat greater than the fraction of cells disrupted.     

Differential growth rates of Candida utilis mother and daughter cells under phased cultivation.

The yeast Candida utilis was continuously synchronized by the phased method of cultivation with the nitrogen source as the growth-limiting nutrient. The doubling time (phasing period) of cells was 6 h. Both cell number and deoxyribonucleic acid synthesis showed a characteristic stepwise increase during the phased growth. The time of bud emergence coincided with the time of initiation of deoxyribonucleic acid synthesis. Size distribution studies combined with microscopic analysis showed that the cells expanded only during the unbudded phase of growth. Usually the cells stopped increasing in size about 30 min before bud emergence, and the arrest of the increase in cell volume coincided with the exhaustion of nitron from the medium. There was no net change in the volume of cells during the bud expansion phase of growth, suggesting that as the bud expanded, the volume of the mother portion of the cell decreased. After division the cells expanded slightly. The postdivision expansion of cells, unlike the growth before bud initiation, occurred in the absence of the growth-limiting nutrient. The newly formed daughter cells were smaller than the mother cells and expanded at a faster rate, so that both types of cells reached maximum size at the same time. Possible reasons for the different rates of expansion of mother and daughter cells are discussed.     

Morphological changes in a chemostat culture of Candida utilis undergoing cyclic changes of pH and pO2

The effect of cyclic pH and pO2 changes on the morphology of Candida utilis was studied with time intervals measured in minutes. The pH was varied from 4.5 to 2.6, the pO2 from 70% of the saturation to 0%. The morphology was studied both visually and using the technique of optical-structural computer analysis. The regime with pH changes increased the biomass yield. However, the averaged morphological characteristics showed that the growth was slightly inhibited. Therefore, the yeast population was very heterogeneous, some cells were inhibited while other cells were stimulated, which made the economic coefficient rise. The cells were also inhibited when they were exposed in the conditions of oxygen deficiency for a short period of time. However, the cells could not be inhibited if, at the same time, the pH was extremely low. Changes in the morphology were detected earlier than either the inhibition or stimulation of growth was recorded by measuring the weight of biomass.     

Regulation of glucose transport in Candida utilis

The transport systems for glucose present in Candida utilis cells, growing in batch and continuous cultures on several carbon sources, have been studied. Two different systems were found: a proton symport and a facilitated diffusion system. The high-affinity symport (Km for glucose about 15 microM) transported one proton per mole of glucose and was partially constitutive, appearing in cells grown on gluconeogenic substrates such as lactate, ethanol and glycerol. It was also induced by glucose concentrations up to 0.7 mM and repressed by higher ones. The level of repression depended on the external glucose concentration at which cells had grown in a way similar to that shown by the maltose-uptake system, so both systems seem to be under a common glucose control. Initial uptake by facilitated diffusion, the only transport system present in cells growing at glucose concentrations higher than 10 mM, showed a complex kinetic dependence on the extracellular glucose concentration. This could be explained either by the presence of at least two different systems simultaneously active, one with a Km around 2 mM and the other with a Km of about 1 M, or by the allosteric or hysteretic behaviour of a single carrier whose apparent Km would oscillate between 2 and 70 mM.     

Extraction of uricase from Candida utilis

Summary Six methods of uricase extraction from Candida utilis were tested and compared. X-press disintegration and sonic oscillation gave the best results.     

Purification of an exo-1,3-beta-glucanase from Candida utilis.

An exo-1,3-beta-glucanase (EC 3.2.1.-) has been purified from the culture fluid of the yeast Candida utilis, and its biochemical properties have been studied. The amino acid analysis revealed a high content of acidic amino acids. The purified enzyme had 20% carbohydrate and a net negative charge showing higher affinity for laminarin than for p-nitrophenyl-beta-D-glucopyranoside and yeast cell-wall 1,3-beta-glucans. In addition, the enzyme hydrolyzed the substrates starting from the nonreducing ends, releasing glucose as the exclusive hydrolysis product. The enzyme activity was strongly inhibited by lactones and also by some heavy-metal ions.     

产朊假丝酵母(Candida utilis)基因工程研究进展

产朊假丝酵母是生物安全(Generally Recognized as Safe,GRAS)的微生物,也是一种重要的工业微生物。近20年来,随着分子生物学技术的发展,产朊假丝酵母的基因表达系统和基因工程研究及开发应用取得了显著的进展,使得利用该菌表达多种物质成为可能。本文概述了产朊假丝酵母的生物学特点、外源基因表达系统、基因敲除、遗传转化等方面的研究和应用进展。     

Peptidoglycan biosynthesis in stationary-phase cells of Escherichia coli.

The ability of stationary-phase cells of Escherichia coli W7 to incorporate radioactive precursors into macromolecular murein has been studied. During the initial 6 h of the stationary phase, resting cells incorporated meso-[3H]diaminopimelic acid at a rate corresponding to the insertion of 1.3 X 10(4) disaccharide units min-1 cell-1. Afterwards, the rate of incorporation dropped drastically (90%) to a low but still detectable level. Incorporation during stationary phase did not result in an increased amount of total murein in the culture, suggesting that it was related to a turnover process. Analysis of the effects of a number of beta-lactam antibiotics indicated that incorporation of murein precursors in stationary-phase cells was mediated by penicillin-binding proteins, suggesting that the activity of penicillin-binding protein 2 was particularly relevant to this process.