Structure and expression of fatty acid desaturases

A Temperature-sensitive Mechanism That Regulates Post-translational Stability of a Plastidial ω-3 Fatty Acid Desaturase (FAD8) in Arabidopsis Leaf Tissues

Trienoic fatty acids (TAs) are the major constituents in plant membrane lipids. In Arabidopsis, two plastidial isozymes of omega-3 fatty acid desaturase, FAD7 and FAD8, are the major contributors for TA production in leaf tissues. Despite a high degree of structural relatedness, activities of these two isozymes are regulated differentially in response to temperature. Elevated temperatures lead to decreases in leaf TA level due to temperature sensitivity of FAD8 activity. A series of FAD7-FAD8 chimeric genes, each encoding a functional plastidial omega-3 desaturase, were introduced into the Arabidopsis fad7fad8 double mutant. Constructs with or without a c-Myc epitope tag were tested. Functionality of each chimeric gene in response to temperature was assayed by Northern and Western analyses and by examining the fatty acid composition. All transformants harboring a chimeric gene containing the FAD8-derived C-terminal coding region (44 amino acids) showed a marked decrease in TA level when exposed to high temperature, similarly as transgenic lines complemented with the native form of FAD8. The reduction of TA level was accompanied by a decrease in the amount of omega-3 desaturase protein but not necessarily by a decrease in its transcript level. Analysis of the decay of c-Myc-tagged products after inhibiting protein synthesis revealed that the FAD8-derived C-terminal region acts in an autoregulatory fashion to destabilize the protein at high temperature. This suggests that the regulation of post-translational stability of FAD8 provides an important regulatory mechanism for modifying its activity in response to temperature, mediating a decrease in TA level at elevated temperatures.

Temperature adaptation in Dictyostelium: role of Delta5 fatty acid desaturase

Membrane fluidity is critical for proper membrane function and is regulated in part by the proportion of unsaturated fatty acids present in membrane lipids. The proportion of these lipids in turn varies with temperature and may contribute to temperature adaptation in poikilothermic organisms. The fundamental question posed in this study was whether the unsaturation of fatty acids contributes to the ability to adapt to temperature stress in Dictyostelium. First, fatty acid composition was analysed and it was observed that the relative proportions of dienoic acids changed with temperature. To investigate the role of dienoic fatty acids in temperature adaptation, null mutants were created in the two known Delta5 fatty acid desaturases (FadA and FadB) that are responsible for the production of dienoic fatty acids. The fadB null mutant showed no significant alteration in fatty acid composition or in phenotype. However, the disruption of fadA resulted in a large drop in dienoic fatty acid content from 51.2 to 4.1 % and a possibly compensatory increase in monoenoic fatty acids (40.9-92.4 %). No difference was detected in temperature adaptation with that of wild-type cells during the growth phase. However, surprisingly, mutant cells developed more efficiently than the wild-type at elevated temperatures. These results show that the fatty acid composition of Dictyostelium changes with temperature and suggest that the regulation of dienoic fatty acid synthesis is involved in the development of Dictyostelium at elevated temperatures, but not during the growth phase.

Membrane fluidity and its roles in the perception of environmental signals

Poikilothermic organisms are exposed to frequent changes in environmental conditions and their survival depends on their ability to acclimate to such changes. Changes in ambient temperature and osmolarity cause fluctuations in the fluidity of cell membranes. Such fluctuations are considered to be critical to the initiation of the regulatory reactions that ultimately lead to acclimation. The mechanisms responsible for the perception of changes in membrane fluidity have not been fully characterized. However, the analysis of genome-wide gene expression using DNA microarrays has provided a powerful new approach to studies of the contribution of membrane fluidity to gene expression and to the identification of environmental sensors. In this review, we focus on the mechanisms that regulate membrane fluidity, on putative sensors that perceive changes in membrane fluidity, and on the subsequent expression of genes that ensures acclimation to a new set of environmental conditions.

Fatty acid synthesis in Xylella fastidiosa: correlations between genome studies, 13C NMR data, and molecular models

Xylella fastidiosa was the first plant pathogen to have its complete genome sequence elucidated. Routine database analyses suggested that two enzymes essential for fatty acid synthesis were missing, one of these is the holo-acyl-carrier-protein synthase. However, here we demonstrate, using (13)C NMR spectroscopy, that X. fastidiosa is indeed able to synthesize fatty acids from acetate via an apparently conventional metabolic pathway. We further identify a gene product HetI, an alternative phosphopantetheinyl transferase, which we propose to fill the missing link. Homology modeling of HetI shows conservation of the Coenzyme A binding site suggesting it to be an active enzyme and reveals several interesting structural features when compared with the surfactin synthase-activating enzyme, on which the model was built. These include a simplified topology due to N- and C-terminal deletions and the observation of a novel serine ladder.

Two Delta9-stearic acid desaturases are required for Aspergillus nidulans growth and development

Unsaturated fatty acids are important constituents of all cell membranes and are required for normal growth. In the filamentous fungus Aspergillus nidulans, unsaturated fatty acids and their derivatives also influence asexual (conidial) and sexual (ascospore) sporulation processes. To investigate the relationship between fatty acid metabolism and fungal development, we disrupted the A. nidulans sdeA and sdeB genes, both encoding Delta9-stearic acid desaturases responsible for the conversion of palmitic acid (16:0) and stearic acid (18:0) to palmitoleic acid (16:1) and oleic acid (18:1). The effects of sdeA deletion on development were profound, such that growth, conidial and ascospore production were all reduced at 22 and 37 degrees C. Total fatty acid content was increased over 3-fold in the DeltasdeA strain, reflected in up-regulation of the expression of the fasA gene encoding the alpha chain of the fatty acid synthase, compared to wild type. Stearic acid accumulated approximately 3-fold compared to wild type in the DeltasdeA strain, while unsaturated fatty acid production was decreased. In contrast, disruption of sdeB reduced fungal growth and conidiation at 22 degrees C, but did not affect these processes at 37 degrees C compared to wild type. Interestingly, ascospore production was increased at 37 degrees C for DeltasdeB compared to wild type. Total fatty acid content was not increased in this strain, although stearic acid accumulated 2-fold compared to wild type, and unsaturated fatty acid production was decreased. Combining the DeltasdeA and DeltasdeB alleles created a synthetic lethal strain requiring the addition of oleic acid to the medium for a modicum of growth. Taken together, our results suggest a role for sdeA in growth and development at all temperatures, while sdeB is involved in growth and development at lower temperatures.

Domain swapping localizes the structural determinants of regioselectivity in membrane-bound fatty acid desaturases of Caenorhabditis elegans

Most fatty acid desaturases are members of a large superfamily of integral membrane, O(2)-dependent, iron-containing enzymes that catalyze a variety of oxidative modifications to lipids. Sharing a similar primary structure and membrane topology, these enzymes are broadly categorized according to their positional specificity or regioselectivity, which designates the preferred position for substrate modification. To investigate the structural basis of regioselectivity in membrane-bound desaturases, the Caenorhabditis elegans omega-3 (FAT-1) and "Delta12" (FAT-2) desaturases were used as a model system. With the use of unnatural substrates, the regioselectivity of C. elegans FAT-2 was clearly defined as nu+3, i.e. it "measures" three carbons from an existing double bond. The structural basis for nu+3 and omega-3 regioselectivities was examined through construction and expression of chimeric DNA sequences based on FAT-1 and FAT-2. Each sequence was divided into seven domains, and chimeras were constructed in which specific domains were replaced with sequence from the other desaturase. When tested by expression in yeast using exogenously supplied substrates, chimeric sequences were found in which domain swapping resulted in a change of regioselectivity from nu+3 to omega-3 and vice versa. In this way, the structural determinants of regioselectivity in FAT-1 and FAT-2 have been localized to two interdependent regions: a relatively hydrophobic region between the first two histidine boxes and the carboxyl-terminal region.

Saccharomyces kluyveri FAD3 encodes an ω3 fatty acid desaturase

Fungi, like plants, are capable of producing the 18-carbon polyunsaturated fatty acids linoleic acid andα-linolenic acid. These fatty acids are synthesized by catalytic reactions of Δ12 andω3 fatty acid desaturases. This paper describes the first cloning and functional characterization of a yeastω3 fatty acid desaturase gene. The deduced protein encoded by theSaccharomyces kluyveri FAD3gene (Sk-FAD3) consists of 419 amino acids, and shows 30–60 % identity with Δ12 fatty acid desaturases of several eukaryotic organisms and 29–31 % identity withω3 fatty acid desaturases of animals and plants. DuringSk-FAD3expression inSaccharomyces cerevisiae,α-linolenic acid accumulated only when linoleic acid was added to the culture medium. The disruption ofSk-FAD3led to the disappearance ofα-linolenic acid inS. kluyveri. These findings suggest thatSk-FAD3is the onlyω3 fatty acid desaturase gene in this yeast. Furthermore, transcriptional expression ofSk-FAD3appears to be regulated by low-temperature stress in a manner different from the other fatty acid desaturase genes inS. kluyveri.

Long chain polyunsaturated fatty acids are required for efficient neurotransmission in C. elegans

The complex lipid constituents of the eukaryotic plasma membrane are precisely controlled in a cell-type-specific manner, suggesting an important, but as yet, unknown cellular function. Neuronal membranes are enriched in long-chain polyunsaturated fatty acids (LC-PUFAs) and alterations in LC-PUFA metabolism cause debilitating neuronal pathologies. However, the physiological role of LC-PUFAs in neurons is unknown. We have characterized the neuronal phenotype of C. elegans mutants depleted of LC-PUFAs. The C. elegans genome encodes a single Delta6-desaturase gene (fat-3), an essential enzyme for LC-PUFA biosynthesis. Animals lacking fat-3 function do not synthesize LC-PUFAs and show movement and egg-laying abnormalities associated with neuronal impairment. Expression of functional fat-3 in neurons, or application of exogenous LC-PUFAs to adult animals rescues these defects. Pharmacological, ultrastructural and electrophysiological analyses demonstrate that fat-3 mutant animals are depleted of synaptic vesicles and release abnormally low levels of neurotransmitter at cholinergic and serotonergic neuromuscular junctions. These data indicate that LC-PUFAs are essential for efficient neurotransmission in C. elegans and may account for the clinical conditions associated with mis-regulation of LC-PUFAs in humans.

Functional expression of a Delta12 fatty acid desaturase gene from spinach in transgenic pigs

Linoleic acid (18:2n-6) and alpha-linolenic acid (18:3n-3) are polyunsaturated fatty acids that are essential for mammalian nutrition, because mammals lack the desaturases required for synthesis of Delta12 (n-6) and n-3 fatty acids. Many plants can synthesize these fatty acids and, therefore, to examine the effects of a plant desaturase in mammals, we generated transgenic pigs that carried the fatty acid desaturation 2 gene for a Delta12 fatty acid desaturase from spinach. Levels of linoleic acid (18:2n-6) in adipocytes that had differentiated in vitro from cells derived from the transgenic pigs were approximately 10 times higher than those from wild-type pigs. In addition, the white adipose tissue of transgenic pigs contained approximately 20% more linoleic acid (18:2n-6) than that of wild-type pigs. These results demonstrate the functional expression of a plant gene for a fatty acid desaturase in mammals, opening up the possibility of modifying the fatty acid composition of products from domestic animals by transgenic technology, using plant genes for fatty acid desaturases.

Trypanosoma brucei oleate desaturase may use a cytochrome b5 -like domain in another desaturase as an electron donor

An open reading frame with fatty acid desaturase similarity was identified in the genome of Trypanosoma brucei. The 1224 bp sequence specifies a protein of 408 amino acids with 59% and 58% similarity to Mortierella alpina and Arabidopsis thaliana Delta12 desaturase, respectively, and 51% with A. thaliana omega3 desaturases. The histidine tracks that compose the iron-binding active centers of the enzyme were more similar to those of the omega3 desaturases. Expression of the trypanosome gene in Saccharomyces cerevisiae resulted in the production of fatty acids that are normally not synthesized in yeast, namely linoleic acid (18:2Delta9,12) and hexadecadienoic acid (16:2Delta9,12), the levels of which were dependent on the culture temperature. At low temperature, the production of bi-unsaturated fatty acids and the 16:2/18:2 ratio were higher. Transformed yeast cultures supplemented with 19:1Delta10 fatty acid yielded 19:2Delta10,13, indicating that the enzyme is able to introduce a double bond at three carbon atoms from a pre-existent olefinic bond. The expression of the gene in a S. cerevisiae mutant defective in cytochrome b5 showed a significant reduction in bi-unsaturated fatty acid production, although it was not totally abolished. Based on the regioselectivity and substrate preferences, we characterized the trypanosome enzyme as a cytochrome b5-dependent oleate desaturase. Expression of the ORF in a double mutant (ole1Delta,cytb5Delta) abolished all oleate desaturase activity completely. OLE1 codes for the endogenous stearoyl-CoA desaturase. Thus, Ole1p has, like Cytb5p, an additional cytochrome b5 function (actually an electron donor function), which is responsible for the activity detected when using the cytb5Delta single mutant.

Identification and characterization of a novel Δ6 -fatty acid desaturase gene from Rhizopus arrhizus

A cDNA sequence putatively encoding a Delta(6)-fatty acid desaturase was isolated from Rhizopus arrhizus using reverse transcription polymerase chain reaction and rapid amplification of cDNA ends methods. Sequence analysis indicated that this cDNA sequence had an open reading frame of 1377 bp encoding 458 amino acids of 52 kDa. The deduced amino acid sequence showed high similarity to those of fungal Delta(6)-fatty acid desaturases which comprised the characteristics of membrane-bound desaturases, including three conserved histidine-rich motifs and hydropathy profile. A cytochrome b(5)-like domain was observed at the N-terminus. To elucidate the function of this novel putative desaturase, the coding sequence was expressed heterologously in Saccharomyces cerevisiae strain INVScl. The result demonstrated that the coding product of the sequence exhibited Delta(6)-fatty acid desaturase activity by the accumulation of gamma-linolenic acid.

Unique mechanism of action of the thiourea drug isoxyl on Mycobacterium tuberculosis

The thiourea isoxyl (thiocarlide; 4,4'-diisoamyloxydiphenylthiourea) is known to be an effective anti-tuberculosis drug, active against a range of multidrug-resistant strains of Mycobacterium tuberculosis and has been used clinically. Little was known of its mode of action. We now demonstrate that isoxyl results in a dose-dependent decrease in the synthesis of oleic and, consequently, tuberculostearic acid in M. tuberculosis with complete inhibition at 3 microg/ml. Synthesis of mycolic acid was also affected. The anti-bacterial effect of isoxyl was partially reversed by supplementing growth medium with oleic acid. The specificity of this inhibition pointed to a Delta9-stearoyl desaturase as the drug target. Development of a cell-free assay for Delta9-desaturase activity allowed direct demonstration of the inhibition of oleic acid synthesis by isoxyl. Interestingly, sterculic acid, a known inhibitor of Delta9-desaturases, emulated the effect of isoxyl on oleic acid synthesis but did not affect mycolic acid synthesis, demonstrating the lack of a relationship between the two effects of the drug. The three putative fatty acid desaturases in the M. tuberculosis genome, desA1, desA2, and desA3, were cloned and expressed in Mycobacterium bovis BCG. Cell-free assays and whole cell labeling demonstrated increased Delta9-desaturase activity and oleic acid synthesis only in the desA3-overexpressing strain and an increase in the minimal inhibitory concentration for isoxyl, indicating that DesA3 is the target of the drug. These results validate membrane-bound Delta9-desaturase, DesA3, as a new therapeutic target, and the thioureas as anti-tuberculosis drugs worthy of further development.

Application of microarray technology in environmental and comparative physiology

DNA microarray technology is revolutionizing many aspects of biological research, allowing the expression of many thousands of gene transcripts to be monitored simultaneously. This provides powerful tools for the genome-wide correlation of gene transcript levels with physiological responses and alterations in physiological states. To date, microarray analyses have been applied almost exclusively to a few model species for which the abundant gene sequence data permit the fabrication of whole-genome microarrays. However, many interesting physiological traits and responses are poorly expressed or absent in model species and may be better illustrated in nonmodel organisms. Comparative approaches to understanding function traditionally focus on species that by virtue of their unusual adaptations, lifestyles, and phylogeny are particularly suited to address a specific biological process or problem. In this review, we show that microarray technology can be successfully applied to these nonmodel species and used to generate new insights of comparative and evolutionary significance into animal function.

MFE1, a member of the peroxisomal hydroxyacyl coenzyme A dehydrogenase family, affects fatty acid metabolism necessary for morphogenesis in Dictyostelium spp

Beta-oxidation of long-chain fatty acids and branched-chain fatty acids is carried out in mammalian peroxisomes by a multifunctional enzyme (MFE) or D-bifunctional protein, with separate domains for hydroxyacyl coenzyme A (CoA) dehydrogenase, enoyl-CoA hydratase, and steroid carrier protein SCP2. We have found that Dictyostelium has a gene, mfeA, encoding MFE1 with homology to the hydroxyacyl-CoA dehydrogenase and SCP2 domains. A separate gene, mfeB, encodes MFE2 with homology to the enoyl-CoA hydratase domain. When grown on a diet of bacteria, Dictyostelium cells in which mfeA is disrupted accumulate excess cyclopropane fatty acids and are unable to develop beyond early aggregation. Axenically grown mutant cells, however, developed into normal fruiting bodies composed of spores and stalk cells. Comparative analysis of whole-cell lipid compositions revealed that bacterially grown mutant cells accumulated cyclopropane fatty acids that remained throughout the developmental stages. Such a persistent accumulation was not detected in wild-type cells or axenically grown mutant cells. Bacterial phosphatidylethanolamine that contains abundant cyclopropane fatty acids inhibited the development of even axenically grown mutant cells, while dipalmitoyl phosphatidylethanolamine did not. These results suggest that MFE1 protects the cells from the increase of the harmful xenobiotic fatty acids incorporated from their diets and optimizes cellular lipid composition for proper development. Hence, we propose that this enzyme plays an irreplaceable role in the survival strategy of Dictyostelium cells to form spores for their efficient dispersal in nature.

Transformation of tobacco with a gene for the thermophilic acyl-lipid desaturase enhances the chilling tolerance of plants

The desC gene for the acyl-lipid Delta9-desaturase from the thermophilic cyanobacterium Synechococcus vulcanus was introduced into Nicotiana tabacum under control of the 35S promoter. Expression of the desaturase was confirmed by Western blotting. Lipid analysis revealed that lipid content and the extent of fatty acid unsaturation significantly increased in leaves of transgenic plants. Chilling tolerance of those plants also increased, as estimated by the electrolyte leakage from the tissues damaged by cold treatments. Seeds of plants that expressed the desC gene imbibed at low temperatures demonstrated higher chilling tolerance than those of the control plants. The results demonstrate that the cyanobacterial thermophilic acyl-lipid desaturase was efficiently expressed in tobacco at ambient temperatures, and its expression resulted in the enhanced chilling tolerance of the transgenic plants.

Seasonal changes in oleosomic lipids and fatty acids of perennial root nodules of beach pea

Seasonal changes in the fatty acid composition of phospholipids (PL), monoglycerides (MG), diglycerides (DG), free fatty acids (FA) and triglycerides (TG) separated from oleosomes (lipid bodies) of perennial root nodules of beach pea (Lathyrus maritimus) were analysed. Thin layer chromatography (TLC) revealed that PL and MG are the major lipids in nodule oleosomes. The fatty acid profile and overall double bond index (DBI) varied among lipid classes depending upon the season. High DBI in PL and MG found during late winter and early spring indicated that they may play a major role in winter survival and regeneration of perennial nodules. The DBI of DG was high at the end of the fall season and the DBI of FA and TG was high in summer months. The dominant fatty acids are C16:0 followed by C18:0 and C18:1. The levels of many unsaturated fatty acids such as C18:1, C18:2 and C18:3 increased while saturated fatty acid C18:0 decreased during winter. These unsaturated fatty acids possibly play an important role in the protection of nodule cells from cold stress. Nodules seem to retain some fatty acids and selectively utilize specific fatty acids to survive the winter and regenerate in spring.

New insight into Phaeodactylum tricornutum fatty acid metabolism. Cloning and functional characterization of plastidial and microsomal delta12-fatty acid desaturases

In contrast to 16:3 plants like rapeseed (Brassica napus), which contain alpha-linolenic acid (18:3(Delta9,12,15)) and hexadecatrienoic acid (16:3(Delta7,10,13)) as major polyunsaturated fatty acids in leaves, the silica-less diatom Phaeodactylum tricornutum contains eicosapentaenoic acid (EPA; 20:5(Delta5,8,11,14,17)) and a different isomer of hexadecatrienoic acid (16:3(Delta6,9,12)). In this report, we describe the characterization of two cDNAs having sequence homology to Delta12-fatty acid desaturases from higher plants. These cDNAs were shown to code for a microsomal and a plastidial Delta12-desaturase (PtFAD2 and PtFAD6, respectively) by heterologous expression in yeast (Saccharomyces cerevisiae) and Synechococcus, respectively. Using these systems in the presence of exogenously supplied fatty acids, the substrate specificities of the two desaturases were determined and compared with those of the corresponding rapeseed enzymes (BnFAD2 and BnFAD6). The microsomal desaturases were similarly specific for oleic acid (18:1(Delta9)), suggesting that PtFAD2 is involved in the biosynthesis of EPA. In contrast, the plastidial desaturase from the higher plant and the diatom clearly differed. Although the rapeseed plastidial desaturase showed high activity toward the omega9-fatty acids 18:1(Delta9) and 16:1(Delta7), in line with the fatty acid composition of rapeseed leaves, the enzyme of P. tricornutum was highly specific for 16:1(Delta9). Our results indicate that in contrast to EPA, which is synthesized in the microsomes, the hexadecatrienoic acid isomer found in P. tricornutum (16:3(Delta6,9,12)) is of plastidial origin.

Recent advances in the study of fatty acid desaturases from animals and lower eukaryotes

The biosynthesis of polyunsaturated fatty acids (PUFAs) in different organisms can involve a variety of pathways, catalyzed by a complex series of desaturation and elongation steps. A range of different desaturases have been identified to date, capable of introducing double bonds at various locations on the fatty acyl chain. Some recently identified novel desaturases include a delta4 desaturase from marine fungi, and a bi-functional delta5/delta6 desaturase from zebrafish. Using molecular genetics approaches, these desaturase genes have been isolated, identified, and expressed in variety of heterologous hosts. Results from these studies will help increase our understanding of the biochemistry of desaturases and the regulation of PUFA biosynthesis. This is of significance because PUFAs play critical roles in multiple aspects of membrane physiology and signaling mechanisms which impact human health and development.

Glucose oversupply increases Delta9-desaturase expression and its metabolites in rat skeletal muscle

AIM/HYPOTHESIS

Previous studies have shown that prolonged glucose infusion causes insulin resistance and triglyceride accumulation in rat skeletal muscle. In this study, we investigated a possible relationship between insulin resistance and the composition of different accumulated lipid fractions in rat skeletal muscle.

METHODS

Continuous glucose infusion was carried out in rats for 7 days. Lipids were extracted from skeletal muscle, separated by thin layer chromatography and fatty acid composition of phospholipids, triglycerides, diglycerides, free fatty acids and cholesterol esters fractions was analysed by gas chromatography. Delta9-Desaturase mRNA was measured by real time polymerase chain reaction. The enzyme activity was measured in the microsomal fractions.

RESULTS

Prolonged glucose infusion (5 days) increased the relative content of palmitoleic acid (16:1 N7) several-fold (2.3- to 5.8-fold) in four out of five lipid fractions and enhanced oleic acid (18:1 N9) two-fold in three lipid fractions suggesting increased Delta9-desaturase activity while the content of several polyunsaturated fatty acids was reduced. In parallel, Delta9-Desaturase mRNA contents and enzyme activities in skeletal muscle were increased 10-fold, 75-fold, 2.6-fold and 7.7-fold after 2 and 5 days of glucose infusion, respectively.

CONCLUSION/INTERPRETATION

Our results suggest that long-term glucose oversupply induces a rapid increase in Delta9-desaturase expression and enzyme activity in skeletal muscle which leads to fast and specific changes in fatty acid metabolism possibly contributing to the insulin resistance in this animal model.

Stearoyl-CoA desaturase, a short-lived protein of endoplasmic reticulum with multiple control mechanisms

Stearoyl-CoA desaturase (SCD) is a short-lived, polytopic membrane-bound non-heme iron enzyme localized primarily in the endoplasmic reticulum. SCD is required for the biosynthesis of monounsaturated fatty acids, and plays a key role in hepatic synthesis of triglycerides and very-low-density lipoproteins. The intracellular concentration of SCD fluctuates in a wide range in response to complex and often competing hormonal and dietary factors. A combination of transcriptional regulation and rapid protein degradation produces transient elevations of SCD enzyme activity in response to physiologic demands. Dysregulation of SCD has been implicated in non-alcoholic fatty liver disease, hyperlipidemia, and obesity.

A novel regulatory gene for light-induced carotenoid synthesis in the bacterium Myxococcus xanthus

Myxococcus xanthus cells respond to blue light by producing carotenoids. Light triggers a network of regulatory actions that lead to the transcriptional activation of the carotenoid genes. By screening the colour phenotype of a collection of Tn5-lac insertion mutants, we have isolated a new mutant devoid of carotenoid synthesis. We map the transposon insertion, which co-segregates with the mutant phenotype, to a previously unknown gene designated here as carF. An in frame deletion within carF causes the same phenotype as the Tn5-lac insertion. The carF deletion prevents the activation of the normally light-inducible genes, without affecting the expression of any of the regulatory genes known to be expressed in a light-independent manner. Until now, the switch that sets off the regulatory cascade had been identified with light-driven inactivation of protein CarR, an antisigma factor. The exact mechanism of this inactivation has remained elusive. We show by epistatic analysis that the carF gene product participates in the light-dependent inactivation of CarR. The predicted CarF amino acid sequence reveals no known prokaryotic homologues. On the other hand, CarF is remarkably similar to Kua, a family of proteins of unknown function that is widely distributed among eukaryotes.

Bacterial cold shock responses

As a measure for molecular motion, temperature is one of the most important environmental factors for life as it directly influences structural and hence functional properties of cellular components. After a sudden increase in ambient temperature, which is termed heat shock, bacteria respond by expressing a specific set of genes whose protein products are designed to mainly cope with heat-induced alterations of protein conformation. This heat shock response comprises the expression of protein chaperones and proteases, and is under central control of an alternative sigma factor (sigma 32) which acts as a master regulator that specifically directs RNA polymerase to transcribe from the heat shock promotors. In a similar manner, bacteria express a well-defined set of proteins after a rapid decrease in temperature, which is termed cold shock. This protein set, however, is different from that expressed under heat shock conditions and predominantly comprises proteins such as helicases, nucleases, and ribosome-associated components that directly or indirectly interact with the biological information molecules DNA and RNA. Interestingly, in contrast to the heat shock response, to date no cold-specific sigma factor has been identified. Rather, it appears that the cold shock response is organized as a complex stimulon in which post-transcriptional events play an important role. In this review, we present a summary of research results that have been acquired in recent years by examinations of bacterial cold shock responses. Important processes such as cold signal perception, membrane adaptation, and the modification of the translation apparatus are discussed together with many other cold-relevant aspects of bacterial physiology and first attempts are made to dissect the cold shock stimulon into less complex regulatory subunits. Special emphasis is placed on findings concerning the nucleic acid-binding cold shock proteins which play a fundamental role not only during cold shock adaptation but also under optimal growth conditions.

Effect of glutamate on arachidonic acid production from Mortierella alpina

AIMS

To evaluate the effect of glutamate on arachidonic acid production from Mortierella alpina.

METHODS AND RESULTS

Cell growth, arachidonic acid production, proportions of poly-unsaturated fatty acids (PuFAs) in fatty acids and glucose-6-phosphate dehydrogenase (G6PDH) activity were analysed when glutamate concentration was 0.8 g l(-1). Biomass and arachidonic acid production were higher in the culture containing glutamate than those in the control culture, and both reached their maximum of 25 g l(-1) and 1.4 g l(-1) after 7 d, respectively. The proportions of some PuFAs, oleic acid, gamma-linolenic acid and dihomo-gamma-linolenic acid were decreased while linoleic acid and arachidonic acid were enhanced by glutamate addition. Glutamate addition enhanced G6PDH activity compared with the control during the whole culture process.

CONCLUSIONS

Addition of 0.8 g l(-1) glutamate was beneficial to enhance arachidonic acid production from Mortierella alpina, which was a result of activating the pentose phosphate pathway (PPP).

SIGNIFICANCE AND IMPACT OF THE STUDY

This study shows that the addition of glutamate and regulation of PPP had a positive influence on arachidonic acid synthesis in Mortierella species.

Lipid-dependent subcellular relocalization of the acyl chain desaturase in yeast

The degree of acyl chain desaturation of membrane lipids is a critical determinant of membrane fluidity. Temperature-sensitive mutants of the single essential acyl chain desaturase, Ole1p, of yeast have previously been isolated in screens for mitochondrial inheritance mutants (Stewart, L.C. and Yaffe, M.P. (1991). J. Cell Biol. 115, 1249-1257). We now report that the mutant desaturase relocalizes from its uniform ER distribution to a more punctuate localization at the cell periphery upon inactivation of the enzyme. This relocalization takes place within minutes at nonpermissive conditions, a time scale at which mitochondrial morphology and inheritance is not yet affected. Relocalization of the desaturase is fully reversible and does not affect the steady state localization of other ER resident proteins or the kinetic and fidelity of the secretory pathway, indicating a high degree of selectivity for the desaturase. Relocalization of the desaturase is energy independent but is lipid dependent because it is rescued by supplementation with unsaturated fatty acids. Relocalization of the desaturase is also observed in cells treated with inhibitors of the enzyme, indicating that it is independent of temperature-induced alterations of the enzyme. In the absence of desaturase function, lipid synthesis continues, resulting in the generation of lipids with saturated acyl chains. A model is discussed in which the accumulation of saturated lipids in a microdomain around the desaturase could induce the observed segregation and relocalization of the enzyme.

Fatty acid desaturation: variations on an oxidative theme

Significant progress in our understanding of the mechanism of fatty acid desaturation has been achieved. The site of initial oxidation has been determined for several membrane-bound desaturases and a common cryptoregiochemical theme has been revealed. The results of several studies, including a detailed analysis of a soluble plant desaturase system, point to a close mechanistic relationship between dehydrogenation and hydroxylation pathways.

Coping with the cold: the cold shock response in the Gram-positive soil bacterium Bacillus subtilis

All organisms examined to date, respond to a sudden change in environmental temperature with a specific cascade of adaptation reactions that, in some cases, have been identified and monitored at the molecular level. According to the type of temperature change, this response has been termed heat shock response (HSR) or cold shock response (CSR). During the HSR, a specialized sigma factor has been shown to play a central regulatory role in controlling expression of genes predominantly required to cope with heat-induced alteration of protein conformation. In contrast, after cold shock, nucleic acid structure and proteins interacting with the biological information molecules DNA and RNA appear to play a major cellular role. Currently, no cold-specific sigma factor has been identified. Therefore, unlike the HSR, the CSR appears to be organized as a complex stimulon rather than resembling a regulon. This review has been designed to draw a refined picture of our current understanding of the CSR in Bacillus subtilis. Important processes such as temperature sensing, membrane adaptation, modification of the translation apparatus, as well as nucleoid reorganization and some metabolic aspects, are discussed in brief. Special emphasis is placed on recent findings concerning the nucleic acid binding cold shock proteins, which play a fundamental role, not only during cold shock adaptation but also under optimal growth conditions.

The same rat Delta6-desaturase not only acts on 18- but also on 24-carbon fatty acids in very-long-chain polyunsaturated fatty acid biosynthesis

The recently cloned Delta6-desaturase is known to catalyse the first step in very-long-chain polyunsaturated fatty acid biosynthesis, i.e. the desaturation of linoleic and alpha-linolenic acids. The hypothesis that this enzyme could also catalyse the terminal desaturation step, i.e. the desaturation of 24-carbon highly unsaturated fatty acids, has never been elucidated. To test this hypothesis, the activity of rat Delta6-desaturase expressed in COS-7 cells was investigated. Recombinant Delta6-desaturase expression was analysed by Western blot, revealing a single band at 45 kDa. The putative involvement of this enzyme in the Delta6-desaturation of C(24:5) n-3 to C(24:6) n-3 was measured by incubating transfected cells with C(22:5) n-3. Whereas both transfected and non-transfected COS-7 cells were able to synthesize C(24:5) n-3 by elongation of C(22:5) n-3, only cells expressing Delta6-desaturase were also able to produce C(24:6) n-3. In addition, Delta6-desaturation of [1-(14)C]C(24:5) n-3 was assayed in vitro in homogenates from COS-7 cells expressing Delta6-desaturase or not, showing that Delta6-desaturase catalyses the conversion of C(24:5) n-3 to C(24:6) n-3. Evidence is therefore presented that the same rat Delta6-desaturase catalyses not only the conversion of C(18:3) n-3 to C(18:4) n-3, but also the conversion of C(24:5) n-3 to C(24:6) n-3. A similar mechanism in the n-6 series is strongly suggested.

Regulation of the desaturation of fatty acids and its role in tolerance to cold and salt stress

The expression of cold-inducible genes is regulated by a two-component system in Synechocystis and Bacillus subtilis. The cold sensors are membrane-bound histidine kinases and it seems likely that they sense and transduce changes in the fluidity of membranes. Desaturation of fatty acids in membrane lipids has been implicated in tolerance to cold and salt stress.

Molecular cloning and functional expression of the gene for a cotton Delta-12 fatty acid desaturase (FAD2)

Two overlapping genomic clones spanning 16.5 kb of cotton DNA were found to encompass a Delta-12 fatty acid desaturase (FAD2-3) gene. A partial FAD2-3 cDNA clone was also analyzed. The FAD2-3 gene has one large intron of 2967 bp entirely within its 5'-untranslated region, only 12 bp upstream from the ATG initiation codon. Several potential promoter elements, including several light-responsive motifs, occur in the 5'-flanking region. The continuous FAD2-3 coding region is 1155 bp and would encode a protein of 384 amino acids. The polypeptide has four putative membrane-spanning helices, indicative of an integral membrane protein, and is most likely localized in the endoplasmic reticulum. Yeast cells transformed with a plasmid construct containing the cotton FAD2-3 coding region accumulate an appreciable amount of linoleic acid (18:2), not normally present in wild-type yeast cells, indicating that the gene encodes a functional FAD2 enzyme.

Molecular cloning and sequence analysis of stearoyl-CoA desaturase in milkfish, Chanos chanos

Stearoyl-CoA desaturase (EC 1.14.99.5) is a key enzyme in the biosynthesis of polyunsaturated fatty acids and the maintenance of the homeoviscous fluidity of biological membranes. The stearoyl-CoA desaturase cDNA in milkfish (Chanos chanos) was cloned by RT-PCR and RACE, and it was compared with the stearoyl-CoA desaturase in cold-tolerant teleosts, common carp and grass carp. Nucleotide sequence analysis revealed that the cDNA clone has a 972-bp open reading frame encoding 323 amino acid residues. Alignments of the deduced amino acid sequence showed that the milkfish stearoyl-CoA desaturase shares 79% and 75% identity with common carp and grass carp, and 63%-64% with other vertebrates such as sheep, hamsters, rats, mice, and humans. Like common carp and grass carp, the deduced amino acid sequence in milkfish well conserves three histidine cluster motifs (one HXXXXH and two HXXHH) that are essential for catalysis of stearoyl-CoA desaturase activity. However, RT-PCR analysis showed that stearoyl-CoA desaturase expression in milkfish is detected in the tissues of liver, muscle, kidney, brain, and gill, and more expression sites were found in milkfish than in common carp and grass carp. Phylogenic relationships among the deduced stearoyl-CoA desaturase amino acid sequence in milkfish and those in other vertebrates showed that the milkfish stearoyl-CoA desaturase amino acid sequence is phylogenetically closer to those of common carp and grass carp than to other higher vertebrates.

Probing the mechanism of a cyanobacterial Delta9 fatty acid desaturase from Spirulina platensis C1 (Arthrospira sp. PCC 9438)

The initial and rate determining step in the mechanism of fatty acid desaturases has been proposed to be breakage of one of the C&z.sbnd;H bonds at the site of the incipient double bond. This has been investigated and supported for a number of eukaryotic fatty acid desaturases through the use of kinetic isotope effect experiments with deuterated substrates. In order to probe the reaction catalyzed by the cyanobacterial Delta9 desaturase and compare it to the eukaryotic desaturases, the desC gene of Spirulina platensis, strain C1 (Arthrospira sp. PCC 9438) was expressed in a desaturase mutant of baker's yeast. Kinetic isotope effects were performed by culturing yeast transformants with deuterated thia-substituted stearic acids. A large kinetic isotope effect was found for the 9 position, in qualitative agreement with results from eukaryotic desaturases.

Genetic Connection between Fatty Acid Metabolism and Sporulation in Aspergillus nidulans

In the Ascomycete fungus Aspergillus nidulans, the ratio of conidia (asexual spores) to ascospores (sexual spores) is affected by linoleic acid moieties including endogenous sporogenic factors called psi factors. Deletion of odeA (Delta odeA), encoding a Delta-12 desaturase that converts oleic acid to linoleic acid, resulted in a strain depleted of polyunsaturated fatty acids (18:2 and 18:3) but increased in oleic acid (18:1) and total percent fatty acid content. Linoleic acid-derived psi factors were absent in this strain but oleic acid-derived psi factors were increased relative to wild type. The Delta odeA strain was reduced in conidial production and mycelial growth; these effects were most noticeable when cultures were grown at 26 degrees C in the dark. Under these environmental conditions, the Delta odeA strain was delayed in ascospore production but produced more ascospores than wild type over time. This suggests a role for oleic acid-derived psi factors in affecting the asexual to sexual spore ratio in A. nidulans. Fatty acid composition and spore development were also affected by veA, a gene previously shown to control light driven conidial and ascospore development. Taken together our results indicate an interaction between veA and odeA alleles for fatty acid metabolism and spore development in A. nidulans.

Molecular basis of thermosensing: a two-component signal transduction thermometer in Bacillus subtilis

Both prokaryotes and eukaryotes respond to a decrease in temperature with the expression of a specific subset of proteins. Although a large body of information concerning cold shock-induced genes has been gathered, studies on temperature regulation have not clearly identified the key regulatory factor(s) responsible for thermosensing and signal transduction at low temperatures. Here we identified a two-component signal transduction system composed of a sensor kinase, DesK, and a response regulator, DesR, responsible for cold induction of the des gene coding for the Delta5-lipid desaturase from Bacillus subtilis. We found that DesR binds to a DNA sequence extending from position -28 to -77 relative to the start site of the temperature-regulated des gene. We show further that unsaturated fatty acids (UFAs), the products of the Delta5-desaturase, act as negative signalling molecules of des transcription. Thus, a regulatory loop composed of the DesK-DesR two-component signal transduction system and UFAs provides a novel mechanism for the control of gene expression at low temperatures.

Molecular cloning of full-length cDNA encoding delta-9 desaturase through PCR strategies and its genomic organization and expression in grass carp (Ctenopharyngodon idella)

Desaturases are enzymes that catalyze double bond formation in fatty acids, which is a critical step in the synthesis of unsaturated fatty acids in organisms. Desaturase cDNA has been cloned from various species. Here we report the cloning of a full-length cDNA of Delta(9)-desaturase from grass carp (Ctenopharyngodon idella), using a combination of PCR techniques: reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE). The resolved cDNA encompasses 2420 bp, containing an open reading frame corresponding to 324 amino acids. The deduced amino acid sequence shares high homology with those of mammalian desaturases. Northern blot and RT-PCR analyses demonstrated a high abundance of the transcript in liver tissue but low abundance in brain tissue. Furthermore, the structure of the gene has been resolved by screening its cognate genomic DNA library. The analysis shows that this gene is composed of six exons and five introns, encompassing a region of 8.5 kb. In particular, the last exon contains a length of the 3' untranslated region as long as 1382 bp. Although the primary sequence and the genomic organization are phylogenetically conserved between fish and mammals, the regulation of the gene expression appears to be divergent among species.

Role of the Bacillus subtilis fatty acid desaturase in membrane adaptation during cold shock

In our attempt to understand the cold shock response of Bacillus subtilis, we report on the role of the B. subtilis fatty acid desaturase (FA-D) Des during membrane adaptation to low temperatures and demonstrate its importance during cold shock. A des null mutant was constructed and analysed in comparison with its parental strain. Growth studies and large-scale comparative fatty acid (FA) analysis revealed a severe cold-sensitive phenotype of the des deletion mutant during the absence of isoleucine and showed that four unsaturated fatty acid (UFA) species differing in length, branching pattern and position of the double bond are synthesized in B. subtilis JH642 but not in the des null mutant. Apart from the lack of UFA synthesis, the FA-D deletion strain showed a dramatically altered saturated fatty acid (SFA) profile at the onset of the stationary growth phase in the presence of exogenous isoleucine sources. Expression of des integrated in trans at the amyE locus of the des deletion strain not only cured the cold-sensitive phenotype observed for the des mutant but allowed much better growth than in strain JH642 after a shift from 37 degrees C to 15 degrees C. These results show that, during cold shock adaptation, des expression can completely replace the isoleucine-dependent, long-term, FA branching adaptation mechanism. We conclude that the crucial aspect in cold adaptation of the cytoplasmic membrane is not its specific molecular composition but rather its physical status in terms of its fluidity.

Expression of fungal desaturase genes in cultured mammalian cells

Long-chain polyunsaturated fatty acids (LC-PUFA) are important components of cellular structure and function. Most of LC-PUFA are derived from linoleic acid and alpha-linolenic acid. In plants and fungi, these two acids can be synthesized from oleic acid via the action of two enzymes, delta12 and delta15-desaturases. Due to lack of these enzymatic activities and the ability to synthesize these two essential fatty acids, animals must obtain them from the diet. In this report, we demonstrated the expression of a fungal delta12-desaturase gene in mouse L cells incubated in serum-free medium. The results showed a significant increase in the amount of linoleic acid with a concomitant decrease of oleic acid in cellular lipids. Most of the newly formed linoleic acid was incorporated into cellular phospholipids, particularly phosphatidylcholine. The increase of linoleic acid provided the substrate for the endogenous synthesis of (n-6) LC-PUFA, such as eicosadienoic acid (EDA), dihomo-gamma-linoleic acid (DGLA) and arachidonic acid (AA). Prolonged incubation further increased the levels of linoleic acid derived from oleic acid by the action of delta12-desaturase, and the levels of 20:2n-6 produced from linoleic acid by the action of the endogenous elongase. However, prolonged incubation suppressed significantly the formation of DGLA and AA. In a separate study, a fungal delta6-desaturase gene has also been expressed in the mouse L cells incubated in serum-containing medium. The result shows a significant increase in levels of 20:3n-6 and 20:4n-6. These findings demonstrate that through genetic modification, it is possible to (1) generate cell lines which no longer require dietary 'essential' fatty acids and (2) alter the endogenous fatty acid metabolism to enhance the production of LC-PUFA and their derivatives.

Chimeras of Delta6-fatty acid and Delta8-sphingolipid desaturases

The Borago officinalis Delta6 fatty acid desaturase (Boofd6) shares 58% identity in its amino acid sequence with Boofd8, a Delta8 sphingolipid desaturase from the same plant species. In order to localise the distinct catalytic properties of Boofd6 and Boofd8 to individual regions within them, a set of chimeras of these two enzymes were constructed and expressed in yeast. Chimera 2 is different from the other chimeras and Boofd6 in that it did not have any detectable desaturase activity on 18 carbon fatty acids. However, it desaturated C16 palmitoleic and C14 myristoleic acid, and the conversion rate for the later one was more than three times higher than that of Boofd6. These results suggest that the predicted membrane helices 1 and 2 of Boofd6 are involved in forming the substrate-binding site. This site appears to place constraints on the chain length of fatty acid substrates, which is similar to hydrophobic substrate binding pockets.

Dietary cholesterol induces changes in molecular species of hepatic microsomal phosphatidylcholine

After 21 days on a diet containing 1 g% cholesterol and 0.5 g% cholic acid, rats had an increased content of cholesterol in liver microsomal lipids. In liver, both cholesterol content and delta9 desaturase activity increased, whereas delta6 and delta5 desaturase activities decreased. These changes correlated with increases in oleic, palmitoleic, and linoleic acids and decreases in arachidonic and docosahexenoic acids in total microsomal lipids. Similar fatty acid changes were found in phosphatidylcholine (PC), the principal lipid of the microsomal membrane. In PC the predominant molecular fatty acid species (67% of the total) in the control rats were 18:0/20:4, 16:0/20:4, and 16:0/18:2; and they mainly determined the contribution of PC to the biophysical and biochemical properties of the phospholipid bilayer. The cholesterol diet decreased specifically the 18:0/20:4 species, and to a lesser extent, 16:0/20:4 and 18:0/22:6. The 18:1-containing species, especially 18:1/18:2 and less so 16:0/18:1 and 18:1/20:4, were increased. A new 18:1/18:1 species appeared. The independent effects of the presence of cholesterol and change of the fatty acid composition of the phospholipid bilayer of liver microsomes on the packing were studied by fluorescence methods using 6-lauroyl-2,4-dimethylaminonaphthalene, 1,6-diphenyl-1,3,5-hexatriene and 1-(4-trimethylammonium phenyl)-6-phenyl-1,3,5-hexatriene, which test different parameters and depths of the bilayer. Data showed that the increase of cholesterol in the membrane, and not the change of the fatty acid composition of phospholipids, was the main determinant of the increased bulk packing of the bilayer. The increase of fluid oleic- and linoleic-containing species almost compensated for the drop in 20:4- and 22:6-containing molecules. But the most important effect was that the general drop in essential n-6 and n-3 polyunsaturated fatty acids meant that this endogenous source for the needs of the animal decreased.

The transcriptional signature of dioxin in human hepatoma HepG2 cells

We have used a high density microarray hybridization approach to characterize the transcriptional response of human hepatoma HepG2 cells to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). We find that exposure to 10 nM TCDD for 8 hr alters by at least a factor of 2.1 the expression of 310 known genes and of an equivalent number of expressed sequence tags. Treatment with TCDD in the presence of 20 microg/mL of cycloheximide blocked the effect on 202 of these genes, allowing us to distinguish between primary effects of TCDD exposure, which take place whether cycloheximide is present or not, and secondary effects, which are blocked by inhibition of protein synthesis. Of the 310 known genes affected by TCDD, 30 are up-regulated and 78 are down-regulated regardless of cycloheximide treatment, and 84 are up-regulated and 118 are down-regulated only when protein synthesis is not inhibited. Functional clustering of genes regulated by TCDD reveals many potential physiological interactions that might shed light on the multiple biological effects of this compound. Our results, however, suggest that arriving at a sound understanding of the molecular mechanisms governing the biological outcome of TCDD exposure promises to be orders of magnitude more complicated than might have been previously imagined.

cDNA cloning, genomic structure, and chromosomal localization of three members of the human fatty acid desaturase family

The insertion of double bonds into specific positions of fatty acids is achieved by the action of distinct desaturase enzymes. Here we report the cloning and characterization of three members of the fatty acid desaturase (FADS) gene family in humans. Initially identified as cDNA fragments by direct cDNA selection within a defined 1.4-Mb region in 11q12-q13.1, full-length fatty acid desaturase-1 (FADS1) and fatty acid desaturase-2 (FADS2) transcripts were obtained by EST sequence assembly. A third member, fatty acid desaturase-3 (FADS3), was identified in silico revealing 62 and 70% nucleotide sequence identity with FADS1 and FADS2, respectively. The three genes are clustered within 92 kb of genomic DNA located 2 kb telomeric to FEN1 and 50 kb centromeric to VMD2 and are likely to have arisen evolutionarily from gene duplication as they share a remarkably similar exon/intron organization. Protein database searches identified FADS1, FADS2, and FADS3 as fusion products composed of an N-terminal cytochrome b5-like domain and a C-terminal multiple membrane-spanning desaturase portion.

The Crd1 gene encodes a putative di-iron enzyme required for photosystem I accumulation in copper deficiency and hypoxia in Chlamydomonas reinhardtii

Chlamydomonas reinhardtii adapts to copper deficiency by degrading apoplastocyanin and inducing Cyc6 and Cpx1 encoding cytochrome c(6) and coproporphyrinogen oxidase, respectively. To identify other components in this pathway, colonies resulting from insertional mutagenesis were screened for copper- conditional phenotypes. Twelve crd (copper response defect) strains were identified. In copper-deficient conditions, the crd strains fail to accumulate photosystem I and light-harvesting complex I, and they contain reduced amounts of light-harvesting complex II. Cyc6, Cpx1 expression and plastocyanin accumulation remain copper responsive. The crd phenotype is rescued by a similar amount of copper as is required for repression of Cyc6 and Cpx1 and for maintenance of plastocyanin at its usual stoichiometry, suggesting that the affected gene is a target of the same signal transduction pathway. The crd strains represent alleles at a single locus, CRD1, which encodes a 47 kDa, hydrophilic protein with a consensus carboxylate-bridged di-iron binding site. Crd1 homologs are present in the genomes of photosynthetic organisms. In Chlamydomonas, Crd1 expression is activated in copper- or oxygen-deficient cells, and Crd1 function is required for adaptation to these conditions.

cDNA cloning of acyl-CoA desaturase homologs in the silkworm, Bombyx mori

We have isolated two acyl-CoA desaturase clones from a pheromone gland cDNA library by using the EST (expressed sequence tag) database of Bombyx mori. The putative acyl-CoA desaturases encoded by the clones desat 1 (2029bp) and desat 2 (2341bp) have 98% identity, and both proteins show 61% identities to Trichoplusia ni acyl-CoA Delta(11) desaturase. The deduced amino acid sequences conserve well the histidine clusters that are catalytically essential for acyl-CoA desaturase activity. Northern blot and RT-PCR analyses revealed that both transcripts of desat 1 and desat 2 were expressed predominantly in the pheromone gland. Both transcripts detected 3days before adult eclosion dramatically increased a day before adult eclosion, keeping the mRNA levels high even after eclosion. These results, combined with the fact that Delta(11) and Delta(10, 12) desaturation of palmitate is a key step to synthesize pheromone in B. mori, suggest that the desaturases encoded by desat 1 and desat 2 are involved in either or both of the desaturation steps in the pheromone biosynthetic pathway of B. mori. The mRNA levels of desat 1 and desat 2 were not affected by decapitation or injection of the pheromone biosynthesis activating neuropeptide (PBAN) into the adult female moth, suggesting that the transcription of desat 1 and desat 2 is not regulated by PBAN. In addition to the clones in the pheromone gland, eight other clones encoding the same Delta(9) desaturase homolog were found in an embryonic cDNA library by searching from the EST database of B. mori. The deduced amino acid sequence from one of the clones (desat 3) shows 79% identity to T. ni Delta(9) desaturase but only 52% identity to the desaturases in the pheromone gland of B. mori. Northern blot analysis showed that the mRNA corresponding to the desat 3 was detected in the ovary and fat body, but not in the pheromone gland. Abundance of the Delta(9) desaturase clones (eight out of the 762 randomly sequenced clones) in the library prepared from diapause-destined embryos (40h after oviposition) suggests that the Delta(9) desaturase encoded by desat 3 plays an important role in embryonic development in B. mori.

The pathway for perception and transduction of low-temperature signals in Synechocystis

Low temperature is an important environmental factor that has effects on all living organisms. Various low-temperature-inducible genes encode products that are essential for acclimation to low temperature, but low-temperature sensors and signal transducers have not been identified. However, systematic disruption of putative genes for histidine kinases and random mutagenesis of almost all the genes in the genome of the cyanobacterium Synechocystis sp. PCC 6803 have allowed us to identify two histidine kinases and a response regulator as components of the pathway for perception and transduction of low-temperature signals. Inactivation, by targeted mutagenesis, of the gene for each of the two histidine kinases and inactivation of the gene for the response regulator depressed the transcription of several lowtemperature-inducible genes.