Charles E. Martin

ELO2 and ELO3, homologues of the Saccharomyces cerevisiae ELO1 gene, function in fatty acid elongation and are required for sphingolipid formation.

ELO2 and ELO3 were identified from the Saccharomyces cerevisiae genome data base as homologues of ELO1, a gene involved in the elongation of the fatty acid 14:0 to 16:0. Mutations in these genes have previously been shown to produce pleiotropic effects involving a number of membrane functions. The simultaneous disruption ofELO2 and ELO3 has also been shown to produce synthetic lethality, indicating that they have related and/or overlapping functions. Gas chromatography and gas chromatography/mass spectroscopy analyses reveal that null mutations of ELO2and ELO3 produce defects in the formation of very long chain fatty acids. Analysis of the null mutants indicates that these genes encode components of the membrane-bound fatty acid elongation systems that produce the 26-carbon very long chain fatty acids that are precursors for ceramide and sphingolipids. Elo2p appears to be involved in the elongation of fatty acids up to 24 carbons. It appears to have the highest affinity for substrates with chain lengths less than 22 carbons. Elo3p apparently has a broader substrate specificity and is essential for the conversion of 24-carbon acids to 26-carbon species. Disruption of either gene reduces cellular sphingolipid levels and results in the accumulation of the long chain base, phytosphingosine. Null mutations in ELO3 result in accumulation of labeled precursors into inositol phosphoceramide, with little labeling in the more complex mannosylated sphingolipids, whereas disruption of ELO2 results in reduced levels of all sphingolipids.

Tsc13p Is Required for Fatty Acid Elongation and Localizes to a Novel Structure at the Nuclear-Vacuolar Interface in Saccharomyces cerevisiae

ABSTRACT The TSC13/YDL015c gene was identified in a screen for suppressors of the calcium sensitivity of csg2Δ mutants that are defective in sphingolipid synthesis. The fatty acid moiety of sphingolipids in Saccharomyces cerevisiae is a very long chain fatty acid (VLCFA) that is synthesized by a microsomal enzyme system that lengthens the palmitate produced by cytosolic fatty acid synthase by two carbon units in each cycle of elongation. TheTSC13 gene encodes a protein required for elongation, possibly the enoyl reductase that catalyzes the last step in each cycle of elongation. The tsc13 mutant accumulates high levels of long-chain bases as well as ceramides that harbor fatty acids with chain lengths shorter than 26 carbons. These phenotypes are exacerbated by the deletion of either the ELO2 or ELO3gene, both of which have previously been shown to be required for VLCFA synthesis. Compromising the synthesis of malonyl coenzyme A (malonyl-CoA) by inactivating acetyl-CoA carboxylase in atsc13 mutant is lethal, further supporting a role of Tsc13p in VLCFA synthesis. Tsc13p coimmunoprecipitates with Elo2p and Elo3p, suggesting that the elongating proteins are organized in a complex. Tsc13p localizes to the endoplasmic reticulum and is highly enriched in a novel structure marking nuclear-vacuolar junctions.

ISOLATION AND CHARACTERIZATION OF A GENE AFFECTING FATTY ACID ELONGATION IN SACCHAROMYCES CEREVISIAE

Fatty acid elongation defective mutants were isolated from Saccharomyces cerevisiae by mutagenizing strains that were defective in fatty acid synthase (FAS) activity. Cells of the fatty acid synthase-defective strains can grow when supplemented with tetradecanoic acid (14:0) due to the presence of membrane bound elongation systems that can extend the 14 carbon fatty acid to longer chain species. After mutagenesis and rescue on medium containing a mixture of 14:0, 16:0 and 18:0, cells were screened for their inability to grow on medium containing only 14:0. From 150,000 colonies, four stable isolates were identified, all of which appear to represent the same complementation group. Gas chromatography of lipid extracts from mutant elo1-1 (designated as elongation defective) cells grown with long or medium chain fatty acids indicates that it fails to efficiently elongate (12, 13, or 14) carbon fatty acids. A gene disrupted fas2Δ::LEU2;elo1Δ::HIS3 mutant incorporates 14-18-carbon fatty acids into membrane lipids, indicating that fatty acid transport is not affected by the mutation. Molecular cloning and sequence analysis of the ELO1 gene suggests that the encoded protein is a membrane bound polypeptide that contains at least five potential membrane spanning regions and a presumptive NADPH binding site. Analysis of the ELO1 mRNA levels indicates that the gene is expressed in cells grown on fatty acid deficient medium. It is rapidly induced in wild type cells that are supplemented with 14:0 and is repressed when cells are supplied with 16- and 18-carbon fatty acids.

Identification of a quality-control mechanism for mRNA 5′-end capping

The 7-methylguanosine cap structure at the 5′ end of eukaryotic messenger RNAs is a critical determinant of their stability and translational efficiency. It is generally believed that 5′-end capping is a constitutive process that occurs during mRNA maturation and lacks the need for a quality-control mechanism to ensure its fidelity. We recently reported that the yeast Rai1 protein has pyrophosphohydrolase activity towards mRNAs lacking a 5′-end cap. Here we show that, in vitro as well as in yeast cells, Rai1 possesses a novel decapping endonuclease activity that can also remove the entire cap structure dinucleotide from an mRNA. This activity is targeted preferentially towards mRNAs with unmethylated caps in contrast to the canonical decapping enzyme, Dcp2, which targets mRNAs with a methylated cap. Capped but unmethylated mRNAs generated in yeast cells with a defect in the methyltransferase gene are more stable in a rai1-gene-disrupted background. Moreover, rai1Δ yeast cells with wild-type capping enzymes show significant accumulation of mRNAs with 5′-end capping defects under nutritional stress conditions of glucose starvation or amino acid starvation. These findings provide evidence that 5′-end capping is not a constitutive process that necessarily always proceeds to completion and demonstrates that Rai1 has an essential role in clearing mRNAs with aberrant 5′-end caps. We propose that Rai1 is involved in an as yet uncharacterized quality control process that ensures mRNA 5′-end integrity by an aberrant-cap-mediated mRNA decay mechanism.

Identification and Characterization of a Low Oxygen Response Element Involved in the Hypoxic Induction of a Family ofSaccharomyces cerevisiaeGenes: IMPLICATIONS FOR THE CONSERVATION OF OXYGEN SENSING IN EUKARYOTES

An organisms ability to respond to changes in oxygen tension depends in large part on alterations in gene expression. The oxygen sensing and signaling mechanisms in eukaryotic cells are not fully understood. To further define these processes, we have studied the Δ9 fatty acid desaturase gene OLE1 inSaccharomyces cerevisiae. We have confirmed previous data showing that the expression of OLE1 mRNA is increased in hypoxia and in the presence of certain transition metals.OLE1 expression was also increased in the presence of the iron chelator 1,10-phenanthroline. A 142-base pair (bp) region 3′ to the previously identified fatty acid response element was identified as critical for the induction of OLE1 in response to these stimuli using OLE1 promoter-lacZ reporter constructs. Electromobility shift assays confirmed the presence of an inducible band shift in response to hypoxia and cobalt. Mutational analysis defined the nonameric sequence ACTCAACAA as necessary for transactivation. A 20-base pair oligonucleotide containing this nonamer confers up-regulation by hypoxia and inhibition by unsaturated fatty acids when placed upstream of a heterologous promoter in alacZ reporter construct. Additional yeast genes were identified which respond to hypoxia and cobalt in a manner similar toOLE1. A number of mammalian genes are also up-regulated by hypoxia, cobalt, nickel, and iron chelators. Hence, the identification of a family of yeast genes regulated in a similar manner has implications for understanding oxygen sensing and signaling in eukaryotes.

Fah1p, a Saccharomyces cerevisiae Cytochromeb 5 Fusion Protein, and ItsArabidopsis thaliana Homolog That Lacks the Cytochromeb 5 Domain Both Function in the α-Hydroxylation of Sphingolipid-associated Very Long Chain Fatty Acids

A search of the Saccharomyces cerevisiae genome data base for cytochromeb 5-like sequences identified a 1.152-kilobase pair open reading frame, located on chromosome XIII at locus YMR272C (FAH1). That gene encodes a putative 384-amino acid protein with an amino-terminal cytochrome b 5 domain. The b 5 core domain shows a 52% identity and 70% similarity to that of the yeast microsomal cytochromeb 5 and a 35% identity and 54% similarity to the b 5 core domain of OLE1, theS. cerevisiae Δ-9 fatty acid desaturase. Expression of the S. cerevisiae FAH1 cytochromeb 5 domain in Escherichia coliproduces a soluble protein that exhibits the typical oxidizedversus reduced differential absorbance spectra of cytochrome b 5. Sequence analysis of Fah1p reveals other similarities to Ole1p. Both proteins are predicted to have two hydrophobic domains, each capable of spanning the membrane twice, and both have the HX (2–3)(XH)H motifs that are characteristic of membrane-bound fatty acid desaturases. These similarities to Ole1p suggested that Fah1p played a role in the biosynthesis or modification of fatty acids. Disruption of the FAH1 gene in S. cerevisiaedid not give any visible phenotype, and there was no observable difference in content or distribution of the most abundant long chain saturated and unsaturated 14–18-carbon fatty acid species. Northern blot analysis, however, showed that this gene is expressed at much lower levels (∼150-fold) than the OLE1 gene, suggesting that it might act on a smaller subset of fatty acids. Analysis of sphingolipid-derived very long chain fatty acids revealed an approximately 40-fold reduction of α-HO 26:0 and a complementary increase in 26:0 in the gene-disrupted fah1Δ strain.GAL1 expression of the S. cerevisiae FAH1 genes in the fah1Δ strain restores α-HO 26:0 fatty acids to wild type levels. Also identified are a number of homologs to this gene in other species. Expression of an Arabidopsis thaliana FAH1 gene, which does not contain the cytochromeb 5 domain, in the fah1Δ strain produced an approximately 25-fold increase in α-HO 26:0 and reduced the levels of its 26-carbon precursor, suggesting that it functions in very long chain fatty acid hydroxylation using an alternate electron transfer mechanism.

The Saccharomyces cerevisiae FAT1 Gene Encodes an Acyl-CoA Synthetase That Is Required for Maintenance of Very Long Chain Fatty Acid Levels

The Saccharomyces cerevisiae FAT1 gene appears to encode an acyl-CoA synthetase that is involved in the regulation of very long chain (C20–C26) fatty acids. Fat1p, has homology to a rat peroxisomal very long chain fatty acyl-CoA synthetase. Very long chain acyl-CoA synthetase activity is reduced in strains containing a disrupted FAT1 gene and is increased when FAT1is expressed in insect cells under control of a baculovirus promoter. Fat1p accounts for approximately 90% of the C24-specific acyl-CoA synthetase activity in glucose-grown cells and approximately 66% of the total activity in cells grown under peroxisomal induction conditions. Localization of functional Fat1p:green fluorescent protein gene fusions and subcellular fractionation of C24 acyl-CoA synthetase activities indicate that the majority of Fat1p is located in internal cellular locations. Disruption of the FAT1 gene results in the accumulation of very long chain fatty acids in the sphingolipid and phospholipid fractions. This includes a 10-fold increase in C24 acids and a 6-fold increase in C22 acids. These abnormal accumulations are further increased by perturbation of very long chain fatty acid synthesis. Overexpression of Elo2p, a component of the fatty acid elongation system, in fat1Δ cells causes C20–C26 levels to rise to approximately 20% of the total fatty acids. These data suggest that Fat1p is involved in the maintenance of cellular very long chain fatty acid levels, apparently by facilitating β-oxidation of excess intermediate length (C20–C24) species. Althoughfat1Δ cells were reported to grow poorly in oleic acid-supplemented medium when fatty acid synthase activity is inactivated by cerulenin, fatty acid import is not significantly affected in cells containing disrupted alleles of FAT1 andFAS2 (a subunit of fatty acid synthase). These results suggest that the primary cause of the growth-defective phenotype is a failure to metabolize the incorporated fatty acid rather than a defect in fatty acid transport. Certain fatty acyl-CoA synthetase activities, however, do appear to be essential for bulk fatty acid transport inSaccharomyces. Simultaneous disruption of FAA1and FAA4, which encode long chain (C14–C18) fatty acyl-CoA synthetases, effectively blocks the import of long chain saturated and unsaturated fatty acids.

MGA2 Is Involved in the Low-Oxygen Response Element-Dependent Hypoxic Induction of Genes in Saccharomyces cerevisiae

ABSTRACT Eukaryotes have the ability to respond to changes in oxygen tension by alterations in gene expression. For example,OLE1 expression in Saccharomyces cerevisiae is upregulated under hypoxic conditions. Previous studies have suggested that the pathway regulating OLE1expression by unsaturated fatty acids may involve Mga2p and Spt23p, two structurally and functionally related proteins. To define the possible roles of each of these genes on hypoxia-inducedOLE1 expression, we examined OLE1expression under normoxia, hypoxia, and cobalt treatment conditions in Δmga2 or Δspt23 deletion strains. The results of OLE1promoter-lacZ reporter gene and Northern blot analyses showed that hypoxia- and cobalt-induced OLE1 expression was dramatically decreased in a Δmga2 strain but not in a Δspt23 strain. Further analyses using low-oxygen response element (LORE)-CYC1-lacZ fusion reporter assays and electrophoretic mobility shift assays (EMSAs) demonstrated that MGA2 significantly affects the LORE-dependent hypoxic induction pathway of gene expression. When MGA2 was supplied by a plasmid, the LORE-dependent hypoxia-inducible reporter expression was recovered, as was the hypoxia-inducible complex in EMSAs in the S. cerevisiae Δmga2 strain. Supershift analysis of EMSAs using crude extracts containing mycMga2p indicated that Mga2p is a component of the LORE-binding complex. Another LORE-dependent, hypoxia-inducible gene, ATF1, was similarly affected in the Δmga2 strain. These results indicate thatMGA2 is required for the LORE-dependent hypoxic gene induction in S. cerevisiae.

Chemical genetic profiling and characterization of small-molecule compounds that affect the biosynthesis of unsaturated fatty acids in Candida albicans.

The balance between saturated and unsaturated fatty acids plays a crucial role in determining the membrane fluidity. In the diploid fungal pathogen Candida albicans, the gene for fatty acid Δ9 desaturase, OLE1, is essential for viability. Using a reverse genetic approach, termed the fitness test, we identified a group of structurally related synthetic compounds that induce specific hypersensitivity of the OLE1+/− strain. Genetic repression of OLE1 and chemical inhibition by two selected compounds, ECC145 and ECC188, resulted in a marked decrease in the total unsaturated fatty acids and impaired hyphal development. The resulting auxotroph of both was suppressed by the exogenous monounsaturated fatty acids (16:1Δ9 and 18:1Δ9). These correlations suggest that both compounds affect the level of unsaturated fatty acids, likely by impairing Ole1p directly or indirectly. However, the residual levels of monounsaturated fatty acids (MUFAs) resulted from chemical inhibition were significantly higher than OLE1 repression, indicating even partial inhibition of MUFAs is sufficient to stop cellular proliferation. Although the essentiality of OLE1 was suppressed by MUFAs in vitro, we demonstrated that it was required for virulence in a murine model of systemic candidiasis even when the animals were supplemented with a high fat diet. Thus, the fungal fatty acid desaturase is an attractive antifungal drug target. Taking advantage of the inhibitors and the relevant conditional shut-off strains, we validated several chemical genetic interactions observed in the fitness test profiles that reveal novel genetic interactions between OLE1/unsaturated fatty acids and other cellular processes.

Mga2p Processing by Hypoxia and Unsaturated Fatty Acids in Saccharomyces cerevisiae: Impact on LORE-Dependent Gene Expression

ABSTRACT In Saccharomyces cerevisiae, OLE1 encodes a Δ9 fatty acid desaturase, an enzyme that plays a critical role in maintaining the correct ratio of saturated to monounsaturated fatty acids in the cell membrane. Previous studies have demonstrated that (i) OLE1 expression is repressed by unsaturated fatty acids (UFAs) and induced by low oxygen tension, (ii) a component of this regulation is mediated through the same low oxygen response element (LORE) in the OLE1 promoter, and (iii) Mga2p is involved in LORE-dependent hypoxic induction of OLE1. We now report that LORE-CYC1 basal promoter-lacZ fusion reporter assays demonstrate that UFAs repress the reporter expression under hypoxic conditions in a dose-dependent manner via LORE. Electrophoretic mobility shift assays show that UFAs repress the hypoxia-induced complex formation with LORE. Studies with a construct encoding a truncated form of Mga2p support the hypothesis that both hypoxia and UFA signals affect the processing of Mga2p and the UFA repression of OLE1 hypoxic induction is mediated through Mga2p. Data from Western blot assays provide evidence that under normoxic conditions, Mga2p processing produces approximately equimolar levels of the membrane-bound and processed forms and is unaffected by UFAs. Hypoxic induction of OLE1, however, is associated with increased processing of the protein, resulting in an approximately fivefold increase in the soluble active form that is counteracted by exposure of the cells to unsaturated fatty acids. Data from this study suggest that the Mga2p-LORE interaction plays an important role in OLE1 expression under both normoxic and hypoxic conditions.