INTRODUCTION ............................................................. 198 CELLULAR LIPIDS ......................................................... 198 Extraction .................................................................. 198 Subcellular Distribution .................................................... 198 Cytoplasmic (plasma) membrane .......................................... 198 Intracytoplasmic elements ................................................. 199 Cell wall ................................................................. 199 LIPID COMPONENTS ....................................................... 199 Fatty Acids ................................................................. 199 Chemical nature .......................................................... 199 Mutant strains ............................................................ 201 Biosynthesis ............................................................. 201 Biological significance .................................................... 202 Taxonomy ................................................................ 202 Hydrocarbons and Sterols ................................................... 202 Hydrocarbons ............................................................ 202 Sterols (i) Chemical nature ................................................ 202 (ii) Mutant strains......................................................... 204 (iii) Biosynthesis ......................................................... 204 (iv) Biological significance ................................................ 206 Glycerophospholipids ....................................................... 207 Chemical nature .......................................................... 207 Biosynthesis ............................................................. 208 Cellular distribution ...................................................... 208 Biological significance .................................................... 208 Sphingolipids ............................................................... 209 Chemical nature .......................................................... 209 Biological significance .................................................... 209 Glycolipids ................................................................. 209 FACTORS INFLUENCING CELLULAR LIPID COMPOSITION ..... ......... 209 Growth Cycle ............................................................... 209 Sporulation ................................................................. 210 Carbon Source ............................................................. 210 Glucose .................................................................. 210 Hydrocarbons ............................................................ 211 Nutrients ................................................................... 213 Nitrogen ................................................................. 213 Phosphorus .............................................................. 213 Growth factors ........................................................... 213 (i) Inositol ................................................................ 213 (ii) Pantothenic acid ...................................................... 213 (iii) Vitamin B6 ........................................................... 213 (iv) Biotin ................................................................ 214 Miscellaneous Additives .................................................... 214 Sodium chloride .......................................................... 214 Choline and ethanolamine ................................................ 214 Benzo(a)pyrene and dibenzanthracene ..................................... 214 Propanediol .............................................................. 214 Oxygen ..................................................................... 214 Respiratory-deficient (Crabtree-positive) yeasts (i) Lipid composition ....... 214 (ii) Lipid supplementation of growth medium ....... ....................... 215 Respiratory-sufficient (Crabtree-negative) yeasts ...... .................... 216 Temperature ............................................................... 216 pH ..................................................................... 217 CONCLUDING COMMENTS ................................................. 217 LITERATURE CITED ....................................................... 219

https://mmbr.asm.org/content/mmbr/39/3/197.full.pdf

Lipids of yeasts.

(1978). Metabolism of Sterols in Yeast. CRC Critical Reviews in Microbiology: Vol. 6, No. 4, pp. 301-341.

Metabolism of sterols in yeast.

The addition of unsaturated fatty acids to cultures of Saccharomyces cerevisiae significantly altered the microsomal lipid composition. Supplementation with either of the naturally occurring palmitoleic (16:1) or oleic (18:1) acids caused increased levels in membrane phospholipids and reduced levels of the complementary acid. Growth in the presence of equimolar quantities of 16:1 and 18:1 acids, however, produced a fatty acid composition similar to that found in unsupplemented cell membranes. Linoleic acid (18:2) was not found in S. cerevisiae grown under normal conditions. It was preferentially internalized and incorporated into microsomes, however, at levels exceeding 50% of the total fatty acid species. This resulted in an almost total loss of 16:1 and a reduction of 18:1 to 25% of its normal level. The delta-9 fatty acid desaturase, a microsomal enzyme that forms 16:1 and 18:1 from saturated acyl coenzyme A precursors, was affected by the presence of exogenous fatty acids. Enzyme activity toward the 16:0 coenzyme A substrate was elevated in microsomes from saturated-fatty-acid-supplemented cultures and sharply repressed following the addition of unsaturated fatty acids, including 18:2. Northern (RNA blot) and slot-blot analyses of mRNA encoded by the OLE1 gene, which appears to be the structural gene for the delta-9 desaturase, indicated that it was sharply reduced in unsaturated-fatty-acid-fed cells. These data suggest that a significant part of the regulation involves modulation of available transcripts.

Nutritional regulation of yeast delta-9 fatty acid desaturase activity.

Publisher Summary This chapter discusses various aspects of sterols in relation to polyene activity. It begins with the distribution, nature, and function of sterols, followed by their role in the mode of action of the polyenes. Sterols have only one major function in the majority of fungi, a structural role in maintaining membrane integrity and permeability. Sterols have a marked condensing effect on experimental membranes composed of phospholipid, reducing their permeability to polyo1 compounds, such as glucose, glycerol, and erythritol and to water. The presence of sterols is a prerequisite for sensitivity to polyene antibiotics. At the multicellular level, the action of polyenes has been studied on a wide variety of organisms: mycoplasmas; yeasts and filamentous fungi; chloroplasts; protozoa; flatworms; flies and moths; amphibian tissues; chick intestine; and various mammalian tissues. The chapter also describes the role of sterols in resistance mechanisms in yeasts. Polyene resistance has been demonstrated in Candida spp. and dermatophyte strains and founded in organisms of mycological interest—namely, Sacchromyces cerevisiae and Neurospora crassa.

Fungal Sterols and the Mode of Action of the Polyene Antibiotics

Many pathogenic fungi are dimorphic and switch between yeast and filamentous states. This switch alters host-microbe interactions and is critical for pathogenicity. However, in zygomycetes, whether dimorphism contributes to virulence is a central unanswered question. The pathogenic zygomycete Mucor circinelloides exhibits hyphal growth in aerobic conditions but switches to multi-budded yeast growth under anaerobic/high CO2 conditions. We found that in the presence of the calcineurin inhibitor FK506, Mucor exhibits exclusively multi-budded yeast growth. We also found that M. circinelloides encodes three calcineurin catalytic A subunits (CnaA, CnaB, and CnaC) and one calcineurin regulatory B subunit (CnbR). Mutations in the latch region of CnbR and in the FKBP12-FK506 binding domain of CnaA result in hyphal growth of Mucor in the presence of FK506. Disruption of the cnbR gene encoding the sole calcineurin B subunit necessary for calcineurin activity yielded mutants locked in permanent yeast phase growth. These findings reveal that the calcineurin pathway plays key roles in the dimorphic transition from yeast to hyphae. The cnbR yeast-locked mutants are less virulent than the wild-type strain in a heterologous host system, providing evidence that hyphae or the yeast-hyphal transition are linked to virulence. Protein kinase A activity (PKA) is elevated during yeast growth under anaerobic conditions, in the presence of FK506, or in the yeast-locked cnbR mutants, suggesting a novel connection between PKA and calcineurin. cnaA mutants lacking the CnaA catalytic subunit are hypersensitive to calcineurin inhibitors, display a hyphal polarity defect, and produce a mixture of yeast and hyphae in aerobic culture. The cnaA mutants also produce spores that are larger than wild-type, and spore size is correlated with virulence potential. Our results demonstrate that the calcineurin pathway orchestrates the yeast-hyphal and spore size dimorphic transitions that contribute to virulence of this common zygomycete fungal pathogen.

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Calcineurin plays key roles in the dimorphic transition and virulence of the human pathogenic zygomycete Mucor circinelloides.

Fatty acid and sterol content and composition were determined for the dimorphic mold, Mucor genevensis, grown under a variety of experimental conditions. Fatty acids account for 6 to 9% of the dry weight of aerobically grown mycelium, and 70 to 80% of these are unsaturated. The organism contains gamma-linolenic acid which is characteristic for Phycomycetes, and in sporangiospores this compound represents 40% of the total fatty acids. Of the sterols found in mycelium, 80% is ergosterol, and stigmasterol was positively identified as one of the minor components. In anaerobically grown yeastlike cells, sterol content is less than 10% of the level found in aerobically grown cells, and fatty acids amount to less than 2% of the dry weight. These fatty acids are predominantly short chain and less than 10% are unsaturated. Yeastlike cells obtained under aerobic conditions by growth in the presence of phenethyl alcohol have fatty acid and sterol compositions characteristic of aerobically grown mycelium. It is concluded that the dimorphology of the organism is not directly related to lipid composition.

Fatty Acid and Sterol Composition of Mucor genevensis in Relation to Dimorphism and Anaerobic Growth

The synthesis of mitochondrial enzymes induced by exposure of anaerobically grown, lipid-depleted Saccharomyces cerevisiae to oxygen is inhibited by d(−)-threo-chloramphenicol and erythromycin. The concentration of these antibiotics required to cause 50% inhibition of this synthesis is less than 1 mm; this is also approximately the concentration required to inhibit by the same amount mitochondrial protein synthesis in situ. The synthesis of unsaturated fatty acids, ergosterol, and phospholipid induced by aeration is inhibited by d(−)-threo-chloramphenicol at high concentrations (12 mm) but is unaffected by erythromycin. l(+)-threo-Chloramphenicol affects neither enzyme nor lipid synthesis and is without effect on mitochondrial protein synthesis in situ. All three compounds inhibit the oxidative activity of isolated mitochondria; the chloramphenicol isomers also inhibit phosphorylation. In a euflavine-derived petite mutant, lacking mitochondrial protein synthesis and respiration, aeration results in the normal development of lipid in the cells, but no synthesis of mitochondrial enzymes. d(−)-threo-Chloramphenicol does not inhibit lipid synthesis in these cells. Thus inhibition of mitochondrial protein synthesis with erythromycin or genetic deletion of mitochondrial protein synthesis results in loss of the capacity to synthesize enzymes during aeration. d(−)-threo-Chloramphenicol, as well as inhibiting induced enzyme formation, inhibits lipid synthesis induced by oxygen. It is unlikely that the latter effect of chloramphenicol is due to inhibition of energy production and transformation, to direct effects on lipid synthesis, or to an inhibition of mitochondrial protein synthesis. It is, however, an effect not shared with the l isomer.

Effects of chloramphenicol isomers and erythromycin on enzyme and lipid synthesis induced by oxygen in wild-type and petite yeast.

The extent of development of respiratory function induced by aeration of an anaerobically grown unsaturated fatty acid auxotroph of Saccharomyces cerevisiae is determined by the availability, endogenous or externally supplied, of unsaturated fatty acid. The synthesis of mitochondrial and cytoplasmic enzymes during aeration appears to have a similar basis of regulation by available unsaturated fatty acid. Levels of unsaturated fatty acid that permit the synthesis of mitochondrial enzymes also result in a substantial stimulation of cellular protein synthesis.

Effect of Unsaturated Fatty Acids on the Development of Respiration and on Protein Synthesis in an Unsaturated Fatty Acid Mutant of Saccharomyces cerevisiae

P. A. Gordon

Papers

Fatty Acid and Sterol Composition of Mucor genevensis in Relation to Dimorphism and Anaerobic Growth

Effects of chloramphenicol isomers and erythromycin on enzyme and lipid synthesis induced by oxygen in wild-type and petite yeast.

Effect of Unsaturated Fatty Acids on the Development of Respiration and on Protein Synthesis in an Unsaturated Fatty Acid Mutant of Saccharomyces cerevisiae