Gel to Liquid-Crystalline Phase Transitions of Lipids and Membranes Isolated from Escherichia coli Cells

Differential scanning calorimetry (DSC) was used to examine the relationship of the gel to liquid-crystalline phase transition of lipids to fatty acid composition with membrane lipids and spheroplast membranes isolated from cells of a wild strain and an unsaturated fatty acid auxotroph of Escherichia coli grown under various conditions. These lipids and membranes underwent thermotropic phase transitions at different temperatures depending on the thermal properties of their constituent fatty acids. The lipid phase transition occurred at higher temperatures in biomembranes than in extracted lipids. DSC thermograms of lipids synthesized by bacterial cells which were observed at a temperature scanning rate as slow as 0.3 K min^-1 were characterized by a distinctly plain peak summit. Endothermic peaks given by samples derived from elaidic acid-enriched cells were relatively narrow and asymmetric. The discrepancy between the transition temperatures measured with extracted lipids and with membraneous fractions, and the shape of the endothermic peaks, are discussed.     

The relationships between growth temperature,fatty acid composition and the physical state and fluidity of membrane lipids in Yersinia enterocolitica

The relationship between membrane lipid composition and membrane lipid phase transitions was investigated in Yersinia enterocolitica cells grown at 5, 22 and 37°C. The total phospholipid concentrations were 9.4, 7.3 and 6.3% of the cell dry weight for cells grown at 5, 22 and 37°C, respectively. The relative concentrations of the three major phospholipids, phosphatidylethanolamine (73–76%), phosphatidylglycerol (9–11%) and cardiolipin (11–13%) were essentially the same at all three growth temperatures. The ratios of unsaturated to saturated fatty acids were 2.2, 1.1 and 0.4 for cells grown at 5, 22 and 37°C, respectively. This change in the fatty acid composition in response to temperature changes is similar to the patterns reported for other organisms. Reversible thermotropic phase transitions were detected by calorimetric analysis in both pure lipid preparations and membrane preparations. The mid-points of the thermotropic phase transitions were at ?13, ?9 and 1°C for membranes from cells grown at 5, 22 and 37°C, respectively. The phase transitions of the membranes from cells grown at the three different temperatures occurred below the lowest growth temperature (5°C). The alternations in the fatty acid composition in Y. enterocolitica did not, therefore, appear to be required to adjust membrane fluidity but might rather be required for some other membrane function.     

The influence of branched-chain and omega-alicyclic fatty acids on the transition temperature of bacillus subtilis lipids.

The influence of branched-chain and omega-alicyclic fatty acids on the transition temperature of Bacillus subtilis lipids was studied by measuring the fluorescence depolarisation of the probe 1,6-diphenyl-1,3,5-hexatriene incorporated into lipid bilayers. Only anteiso-C15 and C17 fatty acid-enriched lipids showed no transition in the observed temperature range. Compared to the transition of normal lipids iso-fatty acid-enriched lipids have a slightly higher transition temperature. The incorporation of omega-alicyclic fatty acids with increasing size of the alicycle leads to a decrease in the transition temperature. A possible role of omega-cyclohexane fatty acids in Bacillus acidocaldarius is proposed.     

The effect of alterations in the physical state of the membrane lipids on the ability of Acholeplasma laidlawii B to grow at various temperatures

The physical state of the membrane lipids, as determined by fatty acid composition and environmental temperature, has a marked effect on both the temperature range within which Acholeplasma laidlawii B cells can grow and on growth rates within the permissible temperature ranges. The minimum growth temperature of 8 °C is not defined by the fatty acid composition of the membrane lipids when cells are enriched in fatty acids giving rise to gel to liquid-crystalline membrane lipid phase transitions occurring below this temperature. The elevated minimum growth temperatures of cells enriched in fatty acids giving rise to lipid phase transitions occurring at higher temperatures, however, are clearly defined by the fatty acid composition of the membrane lipids. The optimum and maximum growth temperatures are also influenced indirectly by the physical state of the membrane lipids, being significantly reduced for cells supplemented with lower melting, unsaturated fatty acids. The temperature coefficient of growth at temperatures near or above the midpoint of the lipid phase transition is 16 to 18 $\text{kcal}\text{mol}$, but this value increases abruptly to 40 to 45 $\text{kcal}\text{mol}$ at temperatures below the phase transition midpoint. Both the absolute rates and temperature coefficients of cell growth are similar for cells whose membrane lipids exist entirely or predominantly in the liquid-crystalline state, but absolute growth rates decline rapidly and temperature coefficients increase at temperatures where more than half of the membrane lipids become solidified. Cell growth ceases when the conversion of the membrane lipid to the gel state approaches completion, but growth and replication can continue at temperatures where less than one tenth of the total lipid remains in the fluid state. An appreciable heterogeneity in the physical state of the membrane lipids can apparently be tolerated by this organism without a detectable loss of membrane function.     

The influence of branched-chain and ω-alicyclic fatty acids on the transition temperature of bacillus subtilis lipids

The influence of branched-chain and ω-alicyclic fatty acids on the transition temperature of Bacillus subtilis lipids was studied by measuring the fluorescence depolarisation of the probe 1,6-diphenyl-1,3,5-hexatriene incorporated into lipid bilayers. Only anteiso-C₁₅ and C₁₇ fatty acid-enriched lipids showed no transition in the observed temperature range. Compared to the transition of normal lipids iso-fatty acid-enriched lipids have a slightly higher transition temperature. The incorporation of ω-alicyclic fatty acids with increasing size of the alicycle leads to a decrease in the transition temperature. A possible role of ω-cyclohexane fatty acids in Bacillus acidocaldarius is proposed.     

Characterization of the fatty acid synthetase system of Curtobacterium pusillum

Curtobacterium pusillum contains 11-cyclohexylundecanoic acid as a major component of cellular fatty acids. A trace amount of 13-cyclohexyltridecanoic acid is also present. Fatty acids other than omega-cyclohexyl fatty acids present are 13-methyltetradecanoic, 12-methyltetradecanoic, n-pentadecanoic, 14-methylpentadecanoic, 13-methylpentadecanoic, n-hexadecanoic, 15-methylhexadecanoic, 14-methylhexadecanoic, and n-heptadecanoic acids. The fatty acid synthetase system of this bacterium was studied. Various 14C-labeled precursors were added to the growth medium and the incorporation of radioactivity into cellular fatty acids was analyzed. Sodium [14C]acetate and [14C]glucose were incorporated into almost all species of cellular fatty acids, the incorporation into 11-cyclohexylundecanoic acid being predominant. [14C]Isoleucine was incorporated into 12-methyltetradecanoic and 14-methylhexadecanoic acids: [14C]leucine into 13-methyltetradecanoic and 15-methylhexadecanoic acids; and [14C]valine into 14-methylpentadecanoic acid. [14C]-Shikimic acid was incorporated almost exclusively into omega-cyclohexyl fatty acids. The fatty acid synthetase activity of the crude enzyme preparation of C. pusillum was reconstituted on the addition of acyl carrier protein. This synthetase system required NADPH and preferentially utilized cyclohexanecarbonyl-CoA as a primer. The system was also able to use branched- and straight-chain acyl-CoAs with 4 to 6 carbon atoms effectively as primers but was unable to use acetyl-CoA. However, if acetyl acyl carrier protein was used as the priming substrate, the system produced straight-chain fatty acids. The results imply that the specificity of the initial acyl-CoA:acyl carrier protein acyltransferase dictates the structure of fatty acids synthesized and that the enzymes catalyzing the subsequent chain-elongation reactions do not have the same specificity restriction.     

Lipid shape as a determinant of lipid composition in Clostridium butyricum. The effects of incorporation of various fatty acids on the ratios of the major ether lipids

The lipid composition of Clostridium butyricum is strongly influenced by the aliphatic chain compositions of the membrane lipids. Growth on cis-monounsaturated fatty acids in the absence of biotin was shown to affect the relative proportions of phosphatidylethanolamine, plasmenylethanolamine, and the glycerol acetal of plasmenylethanolamine most strongly, with smaller effects on the acidic lipids, phosphatidylglycerol and cardiolipin. The ratio of the glycerol acetal of plasmenylethanolamine to total phosphatidylethanolamine in cells grown on a series of fatty acids is shown to decrease in the following order; cis-vaccenic acid greater than or equal to oleic acid = C19-cyclopropane fatty acid greater than linoleic acid greater than petroselinic acid greater than elaidic acid greater than 14-methylhexadecanoic acid (anteiso-C17) greater than 12-methyltridecanoic acid (iso-C14). All fatty acids were extensively incorporated into the lipid acyl, alkenyl, and alkyl chains. There was considerable chain-elongation of the iso-C14 to iso-C16. The results are consistent with the hypothesis that the membrane lipid composition is strongly influenced by lipid shape and that the observed changes in lipid composition serve to stabilize the bilayer arrangement of the cell membrane.     

On the interaction between intrinsic proteins and phosphatidylglycerol in the membrane of Acholeplasma laidlawii.

About 30% of the phosphatidylglycerol in oleic acid-enriched Acholeplasma laidlawii membranes are not hydrolyzed at temperatures below 10 °C by phospholipase A₂ from porcine pancreas. Removal of 53% of the membrane proteins by proteolysis did not reduce the size of this inaccessible phosphatidylglycerol pool. However, modification of the membrane proteins with 2,4,6-trinitrobenzenesulfonic acid or glutaraldehyde did make an additional 70% of this protected pool of phosphatidylglycerol accessible to phospholipase A₂. Complete hydrolysis of phosphatidylglycerol at low incubation temperatures was achieved only after heat treatment of the membranes which resulted in an extensive aggregation of intrinsic membrane proteins as visualized by freeze-etch electron microscopy. Phospholipase A₂ from bee venom was more effective in hydrolyzing phosphatidylglycerol at low temperature than the pancreatic enzyme. These results show that the inaccessibility of phosphatidylglycerol is not due to resealing of isolated membranes, the presence of a crystalline phase in the membrane lipids, or a shielding effect of surface proteins. The protection against hydrolysis may be due to an interaction of phosphatidylglycerol with intrinsic membrane proteins which is stabilized at low temperatures. Increasing the temperature favors the exchange of protein-bound phosphatidylglycerol with other membrane lipids resulting in complete hydrolysis.     

Lipid phase transitions in cat oocytes supplemented with deuterated fatty acids

Cryopreservation of oocytes has already been used to preserve genetic resources, but this technology faces limitations when applied to the species whose oocytes contain large amounts of cytoplasmic lipid droplets. Although cryoinjuries in such oocytes are usually associated with the lipid phase transition in lipid droplets, this phenomenon is still poorly understood. We applied Raman spectroscopy of deuterium-labeled lipids to investigate the freezing of lipid droplets inside cat oocytes. Lipid phase separation was detected in oocytes cryopreserved by slow-freezing protocol. For oocytes supplemented with stearic acid, we found that saturated lipids form the ordered phase being distributed at the periphery of lipid droplets. When an oocyte is warmed to physiological temperatures after cooling, a fraction of saturated lipids may remain in the ordered conformational state. The fractions of monounsaturated and polyunsaturated lipids redistribute to the core of lipid droplets. Monounsaturated lipids undergo the transition to the ordered conformational state below −10°C. Using deuterated fatty acids with a different number of double bonds, we reveal how different lipid fractions are involved in the lipid phase transition of a cytoplasmic lipid droplet and how they can affect cell survival. Raman spectroscopy of deuterated lipids has proven to be a promising tool for studying the lipid phase transitions and lipid redistributions inside single organelles within living cells.     

The effect of dietary lipids on the thermotropic behaviour of rat liver and heart mitochondrial membrane lipids

Diets supplemented with relatively high levels of either saturated fatty acids derived from sheep kidney fat (sheep kidney fat diet) or unsaturated fatty acids derived from sunflower seed oil (sunflower seed oil diet) were fed to rats for a period of 16 weeks and changes in the thermotropic behaviour of liver and heart mitochondrial lipids were determined by differential scanning calorimetry (DSC). The diets induced similar changes in the fatty acid composition in both liver and heart mitochondrial lipids, the major change being the omega 6 to omega 3 unsaturated fatty acid ratio, which was elevated in mitochondria from animals on the sunflower seed oil diet and lowered with the mitochondria from the sheep kidney fat dietary animals. When examined by DSC, aqueous buffer dispersions of liver and heart mitochondrial lipids exhibited two independent, reversible phase transitions and in some instances a third highly unstable transition. The dietary lipid treatments had their major effect of the temperature at which the lower phase transition occurred, there being an inverse relationship between the transition temperature and the omega 6 to omega 3 unsaturated fatty acid ratio. No significant effect was observed for the temperature of the higher phase transition. These results indicate that certain domains of mitochondrial lipids, probably containing some relatively higher melting-point lipids, independently undergo formation of the solidus or gel phase and this phenomenon is not greatly influenced by the lipid composition of the mitochondrial membranes. Conversely, other domains, representing the bulk of the membrane lipids and which probably contain the relatively lower melting point lipids, undergo solidus phase formation at temperatures which reflect changes in the membrane lipid composition which are in turn, a reflection of the nature of the dietary lipid intake. These lipid phase transitions do not appear to correlate directly with those events considered responsible for the altered Arrhenius kinetics of various mitochondrial membrane-associated enzymes.     

Lipid composition of Zymomonas mobilis: effects of ethanol and glucose

Zymomonas mobilis is an alcohol-tolerant microorganism which is potentially useful for the commercial production of ethanol. This organism was found to contain cardiolipin, phosphatidylethanolamine, phosphatidylglycerol, and phosphatidylcholine as major phospholipids. Vaccenic acid was the most abundant fatty acid, with lesser amounts of myristic, palmitic, and palmitoleic acids. No branched-chain or cyclopropane fatty acids were found. Previous studies in our laboratory have shown that ethanol induces the synthesis of phospholipids enriched in vaccenic acid in Escherichia coli (L. O. Ingram, J. Bacteriol. 125:670-678, 1976). The fatty acid composition of Z. mobilis, an obligately ethanol-producing microorganism, represents an extreme of the trend observed in E. coli. In Z. mobilis, vaccenic acid represents over 75% of the acyl chains in the polar membrane lipids. Glucose and ethanol had no major effect on the fatty acid composition of Z. mobilis. However, both glucose and ethanol caused a decrease in phosphatidylethanolamine and phosphatidylglycerol and an increase in cardiolipin and phosphatidylcholine. Ethanol also caused a dose-dependent reduction in the lipid-to-protein ratios of crude membranes. The lipid composition of Z. mobilis may represent an evolutionary adaptation for survival in the presence of ethanol.     

Lipid and protein composition and thermotropic lipid phase transitions in fatty acid-homogeneous membranes of Acholeplasma laidlawii B

The membrane composition and lipid physical properties have been systematically investigated as a function of fatty acid composition for a series of Acholeplasma laidlawii B membrane preparations made homogeneous in various fatty acids by growing cells on single fatty acids and avidin, a potent fatty acid synthetic inhibitor. The membrane protein molecular weight distribution is essentially constant as a function of fatty acid composition, but the lipid/protein ratio varies over a 2-fold range when different fatty acid growth supplements are used. The membrane lipid head-group composition varies somewhat under these conditions, particularly in the ratio of the two major neutral glycolipids. Differential thermal analytical investigations of the thermotropic phase transitions of various combinations of membrane components suggest that these compositional changes are unlikely to result in qualitative changes in the nature of lipid-protein or lipid-lipid interactions, although lesser changes of a quantitative nature probably do occur. The total lipids of membranes made homogeneous in their lipid fatty acyl chain composition exhibit sharper than normal gel-to-liquid-crystalline phase transitions of which midpoint temperatures correlate very well with the phase transition temperatures of synthetic hydrated phosphatidylcholines with like acyl chains. Our results indicate that using avidin and suitable fatty acids to grow A. laidlawii B, it is possible to manipulate the position and the sharpness of the membrane lipid phase transition widely and independently without causing major modifications in other aspects of the membrane composition. This fact makes the fatty acid-homogeneous A. laidlawii B membrane a very useful biological membrane preparation in which to study lipid physical properties and their functional consequences.     

Lipid phase transitions in fatty acid-homogeneous membranes of Acholeplasma laidlawii B

The thermotropic behaviour of fatty acid-homogeneous membranes of Acholeplasma laidlawii B was investigated by Fourier transform infrared spectroscopy. The organism was grown at 37°C in the presence of avidin, an inhibitor of fatty acid synthesis, in a medium supplemented with pentadecanoic acid-d₂₉; the enrichment of the membranes with this fatty acid was 95%. The temperature-dependent phase behaviour of the membranes was studied via the C–D stretching vibrational modes of the membrane lipids and was compared with that of the lipid extract. The high level of fatty acid homogeneity results in a sharp (for natural membranes) gel to liquid crystalline phase transition. The transition, in both the membranes and extracted lipids, is centered at about 6°C above the growth temperature. During the transition two principal liquid states are evident, one being more conformationally ordered than the other. The effect of proteins on the principal lipid phase transition is minimal. However, in the intact membranes there is evident a weaker, lower temperature transition, which is not evident in the extracted lipids.     

Influence of lipid membranes on the conformational transitions of nucleic acids

The conformational transitions of nucleic acids which were enclosed in reverse phase evaporation vesicles (REV) were studied by thermal denaturation with optical recording. Cloned fragments of double-stranded DNA containing 179 base pairs and 187 base pairs, respectively, and polyA.polyU were enclosed in REV with a yield up to every vesicle containing 50 nucleic acid molecules. With the 179 base pairs DNA enclosed in the vesicle from egg lecithin two well resolved helix-coil transitions could be measured; one is very similar in the midpoint-temperature Tm and halfwidth delta T1/2 to the transition of the free nucleic acid, and the other transition occurs stabilized at a 3.5 degrees C higher Tm-value and with a broader delta T1/2, 2.7 degrees C instead of 0.6 degree C. Both transitions are from nucleic acids inside the vesicles. Varying the surface charge of the lipid membrane by adding the negatively charged phosphatidylserine or phosphatidylglycerol, an optimum in the yield of enclosure and a maximum in the increase in Tm (4.5 degrees C) and delta T1/2 (5.5 degrees C instead of 1.0 degrees C) was obtained at 20% phosphatidylserine or phosphatidylglycerol. In vesicles from pure negatively charged lipids no second population of nucleic acids was observed. Qualitatively, similar effects were observed with polyA.polyU. Stabilization and broadening of the second transition is higher for nucleic acids inside vesicles from lipids with unsaturated fatty acids, as dioleoyl-phosphatidylcholine, than with saturated fatty acids, dipalmitoyl-phosphatidylcholine. Stabilization and broadening decrease with increasing ionic strength, whereas the relative contributions of both transitions to the total hypochromicity remain unchanged; the second transition coincides with the first at 90 mM Na+. From the experimental results it was concluded that the interaction of nucleic acids and lipid membranes is mainly of electrostatic nature. The nucleic acids exist inside the vesicles in two populations, one behaving like nucleic acid free in solution and one influenced by the contact with the membrane. All results are in accordance with a model in which the interaction between the nucleic acid and the membrane is in competition with the dipole-dipole interaction inside the membrane surface.     

Modification of membrane lipid: physical properties in relation to fatty acid structure.

Differential scanning calorimetry (DSC) and electron spin resonance (ESR) measurements were made to characterize how modifications in the fatty acid composition of Escherichia coli affected the thermotropic phase transition(s) of the membrane lipd. When the fatty acid composition contained between 20 and 60% saturated fatty acids, the DSC curves for isolated phospholipids and cytoplasmic membranes showed a broad (15-25 degree C) gel to liquid-crystalline phase transition, the position of which depended on the particular fatty acid composition. Utilizing multiple lipid mutants, enrichment of the membrane phospholipids with a single long-chain cis-monoenoic fatty acid in excess of that possible in a fatty acid levels less than 20% and gradually replaced the broad peak as the cis-monoenoic fatty acid content increased. These results were obtained with phospholipids, cytoplasmic membranes, and whole cells. With these same phopholipids, plots of 2,2,6,6-tetramethylpiperidinyl-1-oxy partitioning and ESR order parameters vs. 1/T revealed discontinuities at temperatures 40-60 degrees C above the calorimetrica-ly measured gel to liquid-crystalline phase transitions. Moreover, when the membrane phospholipids were enriched with certain combinations of cis-monenoic fatty acids (e.g., cis-delta 9-16:1 plus cis-delta 11-18:1) the DSC curve showed a broad gel to liquid crystalline phase change below 0 degrees C but the ESR studies revealed no discontinuities at temperatures above those of the gel to liquid-crystalline transition. These results demonstrated that enrichment of the membrane lipids with molecules in which both fatty acyl chains are identical cis-monoenoic residues led to a distinct type of liquid-crystalline phase. Furthermore, a general conclusion from this study is that Escherichia coli normally maintains a heterogeneous mixture of lipid molecules and, by so doing, prevents strong lipid-lipid associations that lead to the formation of lipid domains in the membrane.     

Impact of free hydroxylated and methyl-branched fatty acids on the organization of lipid membranes

Differential scanning calorimetry (DSC) has been applied to study the effect of free hydroxylated and methyl-branched fatty acids on the physico-chemical properties of lipid membranes. First, the impact of free hydroxy fatty acids (HFAs) on dimyristoylphosphatidylcholine (DMPC) model membranes was monitored only as a function of chain length and position of the attached hydroxyl group. Second, racemic vs. enantiopure anteiso fatty acids (AFAs) and HFAs were investigated to address the question of which role does a fatty acid's chirality play on its membrane pertubing effect. The DSC thermograms revealed that the main gel to liquid-crystalline phase transition of the DMPC bilayers which results in a disordering effect of the lipid hydrocarbon chains was affected in different ways depending on the nature of the incorporated fatty acid. Long-chain 2- and 3-HFAs stabilized the gel phase by reducing the phase transition temperature (T(m)), whereas short-chain HFAs and long-chain HFAs with the hydroxy group remote from the head group stabilized the more disordered liquid-crystalline state. Additionally, we observed that enantiopure (S)-14-methylhexadecanoic acid ((S)-a17:0) and (R)-2-hydroxy octadecanoic acid and the corresponding racemates had contrary effects upon incorporation into DMPC bilayers. In both cases, the pure enantiomers alleviated the liquid-crystalline state of the biological model membrane.     

Lipid- and temperature-dependent structural changes in Acholeplasma laidlawii cell membranes

The lipids in cell membranes of Acholeplasma laidlawii were enriched with different fatty acids selected to produce membranes showing molecular motion discontinuities at temperatures between 10 and 35 °C. Molecular motion in these membranes was probed by ESR after labelling with 12-nitroxide stearate, and structure in these membranes was examined by electron microscopy after freeze-etching.Freeze-etching and electron microscopy showed that under certain conditions the particles in the A. laidlawii membranes aggregated, resulting in particle-rich and particle-depleted regions in the cell membrane. Depending upon the lipid content of the membrane, this aggregation could begin at temperatures well above the ESR-determined discontinuity. Aggregation increased with decreasing temperature but was completed at or near the discontinuity. However, cell membranes grown and maintained well below their ESR-determined discontinuity did not show maximum particle aggregation until after they had been exposed to temperatures at or above the discontinuity.The results show that temperatures at or near a phase transition temperature can induce aggregation of the membrane particles. This suggests that temperature-induced changes in the lipid phase of a biological membrane can induce phase separations which affect the topography of associated proteins.     

Temperature-dependent phase behavior of phosphatidylglycerols from chilling-sensitive and chilling-resistant plants

Seven major lipid classes were isolated from leaves of chilling-sensitive and chilling-resistant plants, and the temperature-dependent phase behaviors of their aqueous dispersions were studied by a fluorescence polarization method using trans-parinaric acid and its methyl ester. Phosphatidylglycerols from the chilling-sensitive plants went from the liquid crystalline state into the phase separation state at about 30°C in 100 mm NaCl and at about 40°C in 5 mm MgCl₂. In contrast, phosphatidylglycerols from the chilling-resistant plants went into the phase separation state at a much lower temperature. The other classes of lipids remained in the liquid crystalline state at all temperatures between 5°C and 40°C regardless of the chilling sensitivity of the plants, except sulfoquinovosyl diacylglycerol from sponge cucumber in which phase separation seemed to begin at about 15°C. Compositions and positional distributions of fatty acids of the lipids suggest that the phosphatidylglycerols from the chilling-sensitive plants, but no other lipids, contained large proportions of molecular species which undergo phase transition at room temperature or above. The thermotropic phase behaviors and the fatty acid compositions suggest that, among the major lipid classes from leaves of the chilling-sensitive plants, only phosphatidylglycerol can induce a phase transition. Since a major part of this lipid in leaves originates from the chloroplasts, phase transition probably occurs in the chloroplast membranes.     

Genomic and biochemical analysis of lipid biosynthesis in the unicellular rhodophyte Cyanidioschyzon merolae: lack of a plastidic desaturation pathway results in the coupled pathway of galactolipid synthesis

The acyl lipids making up the plastid membranes in plants and algae are highly enriched in polyunsaturated fatty acids and are synthesized by two distinct pathways, known as the prokaryotic and eukaryotic pathways, which are located within the plastids and the endoplasmic reticulum, respectively. Here we report the results of biochemical as well as genomic analyses of lipids and fatty acids in the unicellular rhodophyte Cyanidioschyzon merolae. All of the glycerolipids usually found in photosynthetic algae were found, such as mono- and digalactosyl diacylglycerol, sulfolipid, phosphatidylglycerol, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol. However, the fatty acid composition was extremely simple. Only palmitic, stearic, oleic, and linoleic acids were found as major acids. In addition, 3-trans-hexadecanoic acid was found as a very minor component in phosphatidylglycerol. Unlike the case for most other photosynthetic eukaryotes, polyenoic fatty acids having three or more double bonds were not detected. These results suggest that polyunsaturated fatty acids are not necessary for photosynthesis in eukaryotes. Genomic analysis suggested that C. merolae lacks acyl lipid desaturases of cyanobacterial origin as well as stearoyl acyl carrier protein desaturase, both of which are major desaturases in plants and green algae. The results of labeling experiments with radioactive acetate showed that the desaturation leading to linoleic acid synthesis occurs on phosphatidylcholine located outside the plastids. Monogalactosyl diacylglycerol is therefore synthesized by the coupled pathway, using plastid-derived palmitic acid and endoplasmic reticulum-derived linoleic acid. These results highlight essential differences in lipid biosynthetic pathways between the red algae and the green lineage, which includes plants and green algae.     

An estimate of the minimum amount of fluid lipid required for the growth of escherichia coli

The lipid phase transition of Escherichia coli was studied by high sensitivity differential scanning calorimetry. A temperature sensitive unsaturated fatty acid auxotroph was used to obtain lipids with subnormal unsaturated fatty acid contents. From these studies it was concluded that E. coli can grow normally with as much as 20% of its membrane lipids in the ordered state but that if more than 55% of the lipids are ordered, growth ceases. Studies with wild-type cells show that the phase transition ends more than 10°C below the growth temperature when the growth temperature when the growth temperature is either 25°C or 37°C.