ON THE SYNTHESIS OF ORNITHINE CARBAMOYLTRANSFERASE IN BIOTIN-DEFICIENT SACCHAROMYCES CEREVISIAE

Continuous cultivation of Saccharomyces cerevisiae at different biotin concentrations in nutrient media

An increase of biotin concentration in nutrient media increased the content of protein, phosphorus, total ribonucleic acids, activity of pyruvate carboxylase and isocitrate lyase in cells and decreased the content of trehalose, glycogen and respiratory quotient of yeast cells in the course of continuous cultivation of Saccharomyces cerevisiae.

Changes in the lipid composition and fine structure of Saccharomyces cerevisiae during ascus formation

Eighty to ninety percent of vegetative cells of Saccharomyces cerevisiae DCL 740 incubated in KCl-acetate medium form asci, the majority of which are four-spored. Ascospores are visible in asci after about 24 hr, and spore formation is complete after about 48 hr. The dry weight of the cells increases by about 75% during 48 hr of incubation, while the lipid content of the cells increases by a factor of four. The increase in lipid content is attributed mainly to an increased synthesis of sterol esters and triacylglycerols and to a lesser extent of phospholipids. The phospholipid and sterol compositions do not change appreciably, but there is a marked increase in the proportion of unsaturated fatty acid residues in ascan lipids. Uniformly labeled (14)C-acetate is incorporated mainly into sterol esters and triacylglycerols and phospholipids. Pulse-labeling by adding acetate-U-(14)C to sporulating cultures and harvesting after a further 6 hr of incubation reveal two main periods of acetate incorporation, namely between 0 and 18 hr, and between 24 and 30 hr. Electron micrographs of thin sections through developing asci show that the principal changes in fine structure occur between 18 and 24 hr and include the appearance of numerous electron-transparent vesicles which become aligned around the meiotic nucleus, and the laying down of extensive endoplasmic reticulum membranes. Changes in fine structure are discussed in relation to the alterations in lipid content and composition of asci.

Osmotic properties of spheroplasts from Saccharomyces cerevisiae grown at different temperatures

Spheroplasts were prepared from cells of Saccharomyces cerevisiae NCYC 366, grown at 30 or 15 C, by incubating cells with snail-gut juice after pretreatment with 2-mercaptoethanol. Walls of cells grown batchwise or in continuous culture at 15 C were more resistant to digestion with snail juice than walls on cells grown under the same conditions as 30 C. Spheroplasts lysed when suspended in hypotonic solutions of mannitol. The resistance of spheroplasts to osmotic lysis tended to increase when the test temperature was lowered below 30 C. The increased resistance was greater with spheroplasts from cells grown at 15 C. Cations, especially Ca(2+), protected spheroplasts against osmotic lysis. In general, the protective effects, measured at 30 C, were smaller with spheroplasts from cells grown at 15 C compared with 30 C. Citrate and ethylenediaminetetraacetate (EDTA) decreased the resistance of spheroplasts to osmotic lysis. On the whole, the decrease was greater with spheroplasts from cells grown at 30 C rather than 15 C. In the presence of EDTA, spheroplasts from cells grown at 30 C were less resistant to osmotic lysis at 5 C than at 30 C; when spheroplasts from cells grown at 15 C were similarly examined, they were more resistant to lysis at 5 C than at 30 C. Spheroplast membranes from cells grown at 15 C had slightly but significantly greater contents of Mg(2+), Ca(2+), K(+), and Na(+) compared with spheroplast membranes from cells grown at 15 C. Mg(2+) and Ca(2+) were more easily extracted with EDTA from membranes of 30 C-grown cells than from 15 C-grown cells.

Composition of the protoplast membrane from Saccharomyces cerevisiae

1. Protoplasts of Saccharomyces cerevisiae N.C.Y.C. 366 were prepared by incubating washed exponential-phase cells in buffered mannitol (0.8m) containing 10mm-magnesium chloride and snail gut juice (about 8mg. of protein/ml. of reaction mixture). Protoplast membranes were obtained by bursting protoplasts in ice-cold phosphate buffer (pH7.0) containing 10mm-magnesium chloride. 2. Protoplast membranes accounted for 13-20% of the dry weight of the yeast cell. They contained on a weight basis about 39% of lipid, 49% of protein, 6% of sterol (assayed spectrophotometrically) and traces of RNA and carbohydrate (glucan+mannan). 3. The principal fatty acids in membrane lipids were C(16:0), C(16:1) and C(18:1) acids. Whole cells contained a slightly greater proportion of C(16:0) and a somewhat smaller proportion of C(18:1) acids. Membrane and whole-cell lipids included monoglycerides, diglycerides, triglycerides, sterols, sterol esters, phosphatidylcholine, lysophosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol+phosphatidylserine. Phosphorus analyses on phospholipid fractions from membranes and whole cells showed that membranes contained proportionately more phosphatidylethanolamine and phosphatidylinositol+phosphatidylserine than whole cells, which in turn were richer in phosphatidylcholine. Phospholipid fractions from membranes and whole cells had similar fatty acid compositions. 4. Membranes and whole cells contained two major and three minor sterol components. Gas-liquid chromatography, mass spectrometry and u.v. and i.r. spectra indicated that the major components were probably Delta(5,7,22,24(28))-ergostatetraen-3beta-ol and zymosterol. The minor sterol components in whole cells were probably episterol (or fecosterol), ergosterol and a C(29) di-unsaturated sterol. 5. Defatted whole cells contained slightly more glutamate and ornithine and slightly less leucine and isoleucine than membranes. Otherwise, no major differences were detected in the amino acid compositions of defatted whole cells and membranes.

A specific requirement for biotin in the synthesis of ornithine carbamoyltransferase by yeast

1. Growth of a biotin-requiring strain of Saccharomyces cerevisiae in a medium containing a suboptimum concentration of biotin for growth caused a decreased synthesis of ornithine carbamoyltransferase as compared with yeast grown in a medium containing an optimum concentration of biotin. Inclusion of the biotin homologues norbiotin or homobiotin, but not bishomobiotin, in the biotin-deficient medium caused an appreciable increase in ornithine carbamoyltransferase synthesis without affecting growth or synthesis of total RNA and protein. The addition of norbiotin to biotin-deficient medium had no effect on the respiratory activity of the yeast or on the synthesis of aspartate carbamoyltransferase, acid phosphatase, beta-fructofuranosidase or malate dehydrogenase. 2. Synthesis of acetylornithine deacetylase and acetylornithine acetyltransferase was slightly diminished by the imposition of biotin deficiency, but the effect was not as great as on ornithine carbamoyltransferase synthesis. Incorporation of norbiotin in the biotin-deficient medium had no marked effect on the synthesis of any other arginine-pathway enzyme except ornithine carbamoyltransferase. 3. l-Ornithine induced synthesis of ornithine carbamoyltransferase in yeast grown in biotin-deficient medium, but in yeast grown in this medium supplemented with norbiotin it repressed synthesis of the enzyme. l-Arginine had no detectable effect on ornithine carbamoyltransferase synthesis by the yeast grown in biotin-deficient medium with or without norbiotin. l-Aspartate repressed synthesis of ornithine carbamoyltransferase in biotin-deficient yeast and completely nullified the stimulatory effect of norbiotin on synthesis of the enzyme in this yeast. 4. There was no increase in ornithine carbamoyltransferase synthesis in biotin-deficient yeast incubated in phosphate buffer, pH4.5, containing glucose and biotin or norbiotin. In biotin-deficient yeast suspended in complete medium containing an optimum concentration of biotin, there was an increase in ornithine carbamoyltransferase synthesis only after the onset of growth.