The effect of growth temperature on the thermotropic behavior of the membranes of a thermophilic Bacillus. Composition-structure-function relationships

The following study was carried out with the aim of widening our understanding of the thermoadaptive mechanisms of the membrane of thermophiles, using Bacillus stearothermophilus var. nondiastaticus as test-organism. The phospholipids and their acyl chain composition of this Bacillus studied in relation to the physical properties of its membrane from bacteria grown at various temperatures. Phospholipids account for 68-75 weight% of the total lipid in cells grown at 45, 55 or 65 degrees C. Phosphatidylglycerol and diphosphatidylglycerol constitute up to 90% of the total phospholipids; no amino phospholipids were found. Increasing the growth temperatures from 45 degrees to 65 degrees C caused an approximately 4-fold decrease in the proportion of the branched-chain fatty acids and a 2-fold increase in the amount of the saturated acyl chains. The reduced proportion of the branched fatty acids was mainly due to a decrease in their anteiso forms. Unsaturated fatty acids were not produced by cells grown at 65 degrees C. In accordance with the fatty acid composition, the molecular packing of phospholipids in monolayers was more expanded with phospholipids from 45 degrees C grown cells as compared with cultures grown at 55 degrees C. The thermotropic gel to liquid-crystalline phase transition of the membrane lipids was monitored by differential scanning calorimetry and fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene. With increase of the growth temperature the phase transition was progressively shifted to higher but narrower range of temperatures. Completion of the lipid melting occurred always at temperatures below those employed for growth. A constructed phase diagram enabled to relate the growth temperature, the fatty acid composition and the lipid apparent microviscosity at temperatures not used in the present study for growth of the thermophile. The minimum temperature for growth and the upper boundary temperature of the least saturated lipid crystallization were extrapolated in this manner; they correspond to the experimentally determined minimal growth temperature. The apparent microviscosity, a measure of membrane order, decreased gradually and conspicuously as the growth temperature was elevated. The delimiting apparent microviscosity values, at the maximal (65 degrees C) and minimal (41 degrees C) growth temperatures were 0.8 and 1.8 poise, respectively. This lack of rigorous homeostatic control of the bulk lipid viscosity prompted reevaluation of the physiological significance of 'homeoviscous adaptation' in Bacillus stearothermophilus.     

Effect of Polyunsaturated Fatty Acids on Fetal Mouse Brain Cells in Culture in a Chemically Defined Medium

Abstract: The biochemical and morphological effects of polyunsaturated fatty acids on fetal brain cells grown in a chemically defined medium were studied. Fetal brain cells were dissociated from mouse cerebral hemispheres taken on the 16th day of gestation. After cells had grown in chemically defined medium for 8 days, the proportion of polyunsaturated fatty acids of cultured cells was only one-half of that observed at day 0 and about 1.5 times less than that of cells grown in serum-supplemented medium. Fatty acid 20:3(n-9) was present in cultured cells grown in either chemically defined or serum-supple-mented medium. demonstrating the deficiency of essential fatty acids. The reduced amount of polyunsaturated fatty acids in cells grown in the chemically defined medium was balanced by an increase in monounsaturated fatty acids. The saturated fatty acids were not affected. When added at the seeding time, linoleic, linolenic, arachidonic, or docosahexaenoic acid stimulated the proliferation of small dense cells. Besides, we demonstrate that each of the four fatty acids studied was incorporated into phospholipids. Adding fatty acids of the n-6 series increased the content of n-6 fatty acids in the cells, but also provoked an increase in the n-3 fatty acids. Among several combinations of fatty acids, only 20:4 and 22:6, when added to the culture in a ratio of 2:1, restored a fatty acid profile similar to controls (i.e. in vivo tissue taken at post- natal dav 5).     

Temperature- and surfactant-induced membrane modifications that alter Listeria monocytogenes nisin sensitivity by different mechanisms

Nisin interacts with target membranes in four sequential steps: binding, insertion, aggregation, and pore formation. Alterations in membrane composition might influence any of these steps. We hypothesized that cold temperatures (10 degrees C) and surfactant (0.1% Tween 20) in the growth medium would influence Listeria monocytogenes membrane lipid composition, membrane fluidity, and, as a result, sensitivity to nisin. Compared to the membranes of cells grown at 30 degrees C, those of L. monocytogenes grown at 10 degrees C had increased amounts of shorter, branched-chain fatty acids, increased fluidity (as measured by fluorescence anisotropy), and increased nisin sensitivity. When 0.1% Tween 20 was included in the medium and the cells were cultured at 30 degrees C, there were complex changes in lipid composition. They did not influence membrane fluidity but nonetheless increased nisin sensitivity. Further investigation found that these cells had an increased ability to bind radioactively labeled nisin. This suggests that the modification of the surfactant-adapted cell membrane increased nisin sensitivity at the binding step and demonstrates that each of the four steps can contribute to nisin sensitivity.     

Effects of temperature and nutritional changes on the fatty acids of agmenellum quadruplicatum.

The fatty acid composition of the blue-green bacterium Agmenellum quadruplicatum was examined under a wide variety of growth conditions. The fatty acid composition was found to undergo significant changes with variations in temperature, media composition, and growth phase (log versus stationary). With increasing growth temperature (20 to 43 C) log-phase cells exhibited an increase in saturated fatty acids (38.4% at 20 C to 63.6% at 43 C). Striking changes were seen with some of the individual fatty acids such as 18.3, which made up 16.0% of the total fatty acid at 20 C but was not neasurable at 43 C. Fatty acid 12:0 was not measurable at 20 C but made up 16.3% of the total fatty acids at 43 C. Cell lipids were separated into neutral lipid, glycolipid, and very polar liquid fractions. The neutral lipid fraction was composed almost entirely of 12 carbon fatty acids (12:0, 12:1). Glycolipid and very polar lipids were more similar in their fatty acid composition when compared to the total cellular fatty acids, although they did lack 12 carbon fatty acids. The total of 12 carbon fatty acids in the cell can be used as an indicator of the amount of neutral lipid present.     

Induction of polyunsaturated fatty-acid synthesis enhances tolerance of a cyanobacterium, Cylindrospermopsis raciborskii, to low-temperature photoinhibition

Acyl-lipid desaturation introduces double bonds (unsaturated bonds) at specifically defined positions of fatty acids that are esterified to the glycerol backbone of membrane glycerolipids. Desaturation pattern of the glycerolipids of Cylindrospermopsis raciborskii (C. raciborskii), a filamentous cyanobacterial strain, was determined in cells grown at 35 degrees C and 25 degrees C. The lowering of the growth temperature from 35 degrees C to 25 degrees C resulted in a considerable accumulation of polyunsaturated octadecanoic fatty acids in all lipid classes. Lipid unsaturation of C. raciborskii was also compared to Synechocystis PCC6803. In C. raciborskii cells, a shift in growth temperature induced a much more pronounced alteration in the desaturation pattern of all lipid classes than in Synechocystis PCC6803. The tolerance to low-temperature photoinhibition of the C. raciborskii cells grown at 25 degrees C and 35 degrees C was also compared to the tolerance of Synechocystis cells grown at the same temperatures. Lower growth temperature increased the tolerance of C. raciborskii cells but not that of Synechocystis cells. These results strengthen the importance of polyunsaturated glycerolipids in the tolerance to environmental stresses and may give a physiological explanation for the determinative role of C. raciborskii strain in algal blooming in the Lake Balaton (Hungary).     

Effect of Temperature on the Fatty Acid Composition of Thermus aquaticus

Thermus aquaticus contains four major fatty acids, iso-C(15) (28%), iso-C(16) (9%), normal-C(16) (13%), and iso-C(17) (48%), when grown at 70 C, as determined by gas chromatography and mass spectrometry. Small amounts of iso-C(12), normal-C(12:1), iso-C(13), normal-C(14), iso-C(14), and normal-C(15:1) were also detected. A change in growth temperature (50 to 75 C at 5-C intervals) affects a shift in the proportions of some of the fatty acids. The proportions of the monoenoic and branched-C(17) fatty acids decreased and the proportions of the higher-melting iso-C(16) and normal-C(16) fatty acids increased. Cells grown at 75 C contained 70% more total fatty acids than cells grown at 50 C. The largest increases, in absolute amounts, were in the content of iso-C(16) and normal-C(16) fatty acids, with only a 1.6-fold increase in the major iso-C(15) and iso-C(17) fatty acids. There was a 2.5-fold decrease in normal-C(15:1) and at least a 24-fold decrease in anteiso-C(17), which is present at 50 and 55 C but not at higher temperatures. There was no difference in proportion or amount of fatty acids between exponential and stationary-phase cells grown at 70 C. When cells were grown on glutamate instead of yeast-extract and tryptone at 70 C, the total fatty acid content remained constant, but there was an increase in the proportions of iso-C(16) and normal-C(16) fatty acids concomitant with a decrease in the proportions of the iso-C(15) and iso-C(17) fatty acids.     

Low-temperature-induced desaturation of fatty acids and expression of desaturase genes in the cyanobacterium Synechococcus sp. PCC 7002

Changes in response to temperature of lipid classes, fatty acid composition and mRNA levels for acyl-lipid desaturase genes were studied in the marine unicellular cyanobacterium, Synechococcus sp. PCC 7002. The degree of unsaturation of C₁₈ fatty acids increased in cells grown at lower temperature for all lipid classes, and ω3 desaturation occurred specifically in cells grown at low temperature. While the level of 18:1(9) fatty acids declined, desaturation at the ω3 position of C₁₈ fatty acids increased gradually during a 12-h period after a temperature shift-down to 22°C. However, the mRNA levels of the desA (Δ12 desaturase), desB (ω3 desaturase) and desC (Δ9 desaturase) genes increased within 15 min after a temperature shift-down to 22°C; the desaturase gene mRNA levels also rapidly declined within 15 min after a temperature shift-up to 38°C. Therefore, the elevation of mRNA levels for the desaturase genes is not the rate-limiting event for the increased desaturation of membrane lipids after a temperature shift-down. The rapid, low-temperature-induced changes in mRNA levels occurred even when cells were grown under light-limiting conditions for which the growth rates at 22°C and 38°C were identical. These studies indicate that the ambient growth temperature, and not some other growth rate-related process, regulates the expression of acyl lipid desaturation in this cyanobacterium.     

Growth-mediated variations in fatty acids of Xenorhabdus sp

The bacterium Xenorhabdus sp. is symbiotically associated with the entomopathogenic nematode Steinernema riobravis. This nematode is produced in monoxenic culture with Xenorhabdus sp. and is sold as a biological insecticide. Acceptable yields in fermentors can only be achieved in the presence of vigorous growth of the bacterium. We investigated the fatty acid composition of Xenorhabdus species when grown at 15, 20, 25 or 30 degrees C on media containing one of two primary carbon sources: glucose or lipids from the insect host, Galleria mellonella. Both temperature and primary carbon source significantly affected lipid quantity and quality in Xenorhabdus sp. Bacteria grown with insect lipids as a primary carbon source accumulated more lipids with greater proportion of longer chain fatty acids than bacteria grown with glucose as a primary carbon source. Cells grown with insect lipids at 15 degrees C had a lower lipid content than cells grown on the same media at 20, 25 or 30 degrees C. Increasing growth temperature increased saturated fatty acids and decreased unsaturated fatty acids, irrespective of carbon source. We recommend addition of complex fatty acid sources that resemble natural host lipids to growth medium for mass producing entomopathogenic nematodes. This could provide nematode quality similar to in vivo-produced nematodes.     

Temperature- and growth-phase-dependent changes in membrane fatty acid compositions of Brevibacterium ammoniagenes

Fatty acids newly synthesized by Brevibacterium ammoniagenes grown at different temperatures were analyzed. The assay temperature, not the growth temperature, was found to be the major factor affecting the unsaturated/saturated ratio of newly synthesized fatty acids in logarithmic-phase cells. However, in the stationary-phase cells the growth temperature also affected the product profile significantly; cells grown at 7 degrees C produced relatively more oleate and stearate and less palmitate and hexadecenoate when shifted up to 37 degrees C than did cells grown and assayed at 37 degrees C. The unsaturated/saturated ratio as well as average chain length of fatty acids also varied along with the progress of isothermal growth phase. These changes in fatty acid product profiles observed in vivo could be mimicked in vitro assays of the fatty acid synthetase by changing malonyl-CoA concentrations. Our results suggest that the malonyl-CoA concentration is a factor which, in addition to temperature, determines growth-phase-dependent and growth-temperature-dependent changes in the unsaturated/saturated ratios of fatty acids.     

Effects of the temperature range and the lack of beta-ketoacyl acyl-carrier protein synthase II on fatty acid synthesis in Escherichia coli K12 after shifts in temperature

Escherichia coli K12 cells grown at higher temperatures and then subjected to lower temperatures produce fatty acids with higher unsaturated/saturated ratios than cells completely adapted to the lower temperatures (Okuyama et al. (1982) J. Biol. Chem. 257, 4812-4817). This hyper-response was not an artefact of chloramphenicol treatment and was observed when the shift-down was more than 20 degrees C in the cells grown at either 40 degrees C or 35 degrees C. In contrast, cells grown at either 25 degrees C or 30 degrees C showed no appreciable hyper-response in terms of unsaturated/saturated ratio on temperature shifts to as low as 10 degrees C. By combining shift-down and shift-up experiments, we could show the presence of different types of temperature dependency in the fatty acid-synthesizing systems of cells grown at various temperatures. Contrary to wild-type cells which synthesized mainly cis-vaccenate on down-shift to 10 degrees C, a mutant strain lacking beta-ketoacyl acyl-carrier protein synthase II synthesized more palmitoleate (16:1) and less palmitate at 10 degrees C than at 40 degrees C. The average chain lengths of saturated and unsaturated fatty acids also changed, but differently, between the mutant and wild-type cells on shifts of temperature. Thus, the mutant strain has a temperature-dependent fatty acid-synthesizing system qualitatively different from that seen in a wild-type strain.     

Characterization of highly purified ornithine decarboxylase from rat heart

The fatty acid composition of cultured Friend erythroleukemia cells was modified by supplementation of the medium with oleic or linoleic acid. There was a 30% reduction in saturated and a 35% reduction in polyunsaturated fatty acids in microsomal phospholipids when the cells were grown in media supplemented with oleic acid, and a 3-fold increase in polyunsaturated fatty acids when the cells were grown in linoleic acid-supplemented media. Electron-spin resonance studies with the 5- nitroxystearate probe demonstrated that there was no appreciable change in microsomal lipid mobility as measured by the order parameters. In contrast, changes in lipid mobility were detected with the spin-label probe when microsomes were first isolated from Friend erythroleukemia cells and subsequently modified by incubation with liposomes composed of either dioleoyl- or dilinoleoylphosphatidylcholine plus bovine liver phospholipid-exchange protein. The fatty acid compositional changes produced in these microsomes were similar to those obtained when the intact cells were grown in media containing supplemental fatty acids. These findings indicate that the lipid mobility of Friend cell microsomes can be altered by phospholipid replacements in vitro, but that this does not occur when similar microsomal fatty acid modifications are produced during culture of the intact cell.     

Heterologous production of dihomo-gamma-linolenic acid in Saccharomyces cerevisiae

To make dihomo-gamma-linolenic acid (DGLA) (20:3n-6) in Saccharomyces cerevisiae, we introduced Kluyveromyces lactis Delta12 fatty acid desaturase, rat Delta6 fatty acid desaturase, and rat elongase genes. Because Fad2p is able to convert the endogenous oleic acid to linoleic acid, this allowed DGLA biosynthesis without the need to supply exogenous fatty acids on the media. Medium composition, cultivation temperature, and incubation time were examined to improve the yield of DGLA. Fatty acid content was increased by changing the medium from a standard synthetic dropout medium to a nitrogen-limited minimal medium (NSD). Production of DGLA was higher in the cells grown at 15 degrees C than in those grown at 20 degrees C, and no DGLA production was observed in the cells grown at 30 degrees C. In NSD at 15 degrees C, fatty acid content increased up until day 7 and decreased after day 10. When the cells were grown in NSD for 7 days at 15 degrees C, the yield of DGLA reached 2.19 microg/mg of cells (dry weight) and the composition of DGLA to total fatty acids was 2.74%. To our knowledge, this is the first report describing the production of polyunsaturated fatty acids in S. cerevisiae without supplying the exogenous fatty acids.     

Relationships between heat resistance and phospholipid fatty acid composition of Vibrio parahaemolyticus.

Vibrio parahaemolyticus was grown in tryptic soy broth (TSB) containing NaCl levels of 0.5, 3.0, and 7.5% (wt/vol). Cultures incubated at 21, 29, and 37 C were harvested in late exponential phases and thermal death times at 47 C (D47 c; time at 47 C required to reduce the viable population by 90%) were determined in phosphate buffer containing 0.5, 3.0, and 7.5% NaCl. At a given NaCl concentration in the growth medium, D47 c values increased with elevated incubation temperatures and with elevated levels of NaCl in the heating menstrua. Differences in thermal resistance of cells cultured at a particular temperature were greater between those grown in TSB containing 0.5 and 3.0% NaCl than between those grown in TSB containing 3.0 and 7.5% NaCl. D47c values ranged from 0.8 min (grown at 21 C in TSB with 0.5% NaCl) to 6.5 min (grown at 37 C in TSB with 7.5%, heated in 7.5% NaCl buffer). Methyl esters of major phospholipid fatty acids extracted from cells were quantitated. The ratio of saturated to unsaturated fatty acids in cells grown at a given NaCl concentration increased with elevated incubation temperature. At a particular growth temperature, however, saturated to unsaturated fatty acids ratios were lowest for cells grown in TSB containing 3.0% NaCl.     

Electron-spin resonance studies of lipid-modified microsomes from Friend erythroleukemia cells

The fatty acid composition of cultured Friend erythroleukemia cells was modified by supplementation of the medium with oleic or linoleic acid. There was a 30% reduction in saturated and a 35% reduction in polyunsaturated fatty acids in microsomal phospholipids when the cells were grown in media supplemented with oleic acid, and a 3-fold increase in polyunsaturated fatty acids when the cells were grown in linoleic acid-supplemented media. Electron-spin resonance studies with the 5-nitroxystearate probe demonstrated that there was no appreciable change in microsomal lipid mobility as measured by the order parameters. In contrast, changes in lipid mobility were detected with the spin-label probe when microsomes were first isolated from Friend erythroleukemia cells and subsequently modified by incubation with liposomes composed of either dioleoyl- or dilinoleoylphosphatidylcholine plus bovine liver phospholipid-exchange protein. The fatty acid compositional changes produced in these microsomes were similar to those obtained when the intact cells were grown in media containing supplemental fatty acids. These findings indicate that the lipid mobility of Friend cell microsomes can be altered by phospholipid replacements in vitro, but that this does not occur when similar microsomal fatty acid modifications are produced during culture of the intact cell.     

Effect of variou cultural conditions on the fatty acid and lipid composition of Choanephora cucurbitarum.

The fatty acid composition of the total and polar lipid fractions of Choanephora cucurbitarum grown under different cultural conditions were analyzed by thin-layer and gas-liquid chromatography. It was observed that temperature, age, pH, and light influenced the degree of unsaturation, this being due mainly to changes in the gamma-linolenic acid concentration. The conditions used in this study did not alter the qualitative profile of fatty acids normally present in the organism. Neither did these conditions stimulate the production of further long-chain fatty acids (C20-C26) beyond gamma-linolenic acid (C18:3) as reported earlier using growth media containing glutamic acid. The fatty acid pattern of lipid fractions though the same qualitatively, differed quantitatively. The polar lipid fractions, phosphatidyl choline, phosphatidyl ethanolamine, and diphosphatidyl glycerol showed an appreciable variation in gamma-linolenic acid content under different cultural conditions. The degree of unsaturation of the various lipid fractions decreased with increases in temperature, light intensity, and pH, but within each treatment the same pattern of decreasing degree of unsaturation with increasing age was observed. The significance of these observations is discussed.     

Pseudomonas putida NCTC 10936 balances membrane fluidity in response to physical and chemical stress by changing the saturation degree and the trans/cis ratio of fatty acids

This study explored the capability of Pseudomonas putida NCTC 10936 to maintain homeoviscosity after changing the growth temperature, incubating resting cells at different temperatures or at a constant temperature in the presence of 4-chlorophenol (4-CP). After raising the growth temperature from 20 to either 30 or 35 degrees C, the degree of saturation of the organism's fatty acids increased and the ratio of trans to cis unsaturated fatty acids decreased somewhat. In contrast, after the incubation temperature of resting cells was raised (grown at 30 degrees C) from 20 to 30 or 35 degrees C the degree of saturation of the fatty acids remained nearly constant, while the ratio of trans to cis unsaturated fatty acids increased. Incubating resting cells (grown at 30 degrees C) at 20 degrees C in the presence of 4-CP again caused no major changes in the degree of saturation, but cis to trans conversion of unsaturated fatty acids was induced, with a corresponding increase in the trans/cis ratios. Increases in both the saturation degree of the fatty acids and the trans/cis ratio of the unsaturated fatty acids correlated with increases in the fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene intercalated in the bilayers of liposomes prepared from the cells of P. putida NCTC 10936. Electron transport phosphorylation (ETP) could be stabilized by adaptive adjustments in the fluidity of the cytoplasmic membrane mediated by changes in fatty acid composition such as those observed. Whether changes in the degree of saturation or in the trans/cis ratio are more effective can be decided by studying P. putida NCTC 10936.     

Lipids of Pseudomonas aeruginosa Cells Grown on Hydrocarbons and on Trypticase Soy Broth

Lipids were extracted from cells of Pseudomonas aeruginosa grown on a pure hydrocarbon (tridecane), mixed hydrocarbons (JP-4 jet fuel), and on Trypticase Soy Broth. Total lipids produced from each substrate represented from 7.1 to 8.2% of cellular dry weight, of which 5.0 to 6.4% were obtained before cellular hydrolysis (free lipids) and 1.7 to 2.0% were extracted after cellular hydrolysis (bound lipids). Free lipids from cells grown on each medium were separated into four fractions by thin-layer chromatography. All fractions were present in cells from each type of medium, and the "neutral fraction" constituted the largest fraction. The fatty acid composition of free lipids was determined by gas-liquid chromatography. Cells grown on each medium contained saturated and unsaturated C(14) to C(20) fatty acids. Trace amounts of C(13) fatty acids were found in tridecane-grown cells. Saturated C(16) and C(18) were the major acids present in all cells. Quantitative differences were found in fatty acids produced on the three media, but specific correlations between substrate carbon sources and fatty acid content of cells were not evident. Tridecane-grown cells contained only traces of C(13) acid and small amounts of C(15) and C(17) acids, suggesting that the organism's fatty acids were derived from de novo synthesis rather than by direct incorporation of the hydrocarbon.     

Effect of growth temperature and media composition on the fatty acid composition of Bacillus stearothermophilus AN 002

The influence of growth temperature, media composition and cell age on the chemical composition of Bacillus stearothermophilus strain AN 002 has been determined. The total cellular protein decreased and the free amino acid content increased with growth temperature, in both exponential and stationary growth phase. The protein and free amino acid contents of cells were higher in the stationary phase than in the exponential phase, irrespective of growth temperature and media composition. The RNA content was only reduced in cells grown at 55° C. No significant variations were observed in the DNA and carbohydrate contents with respect to growth temperature and cell age. The total lipid and fatty acid compositions on the other hand varied as a function of growth temperature, cell age and media composition. Differences in the relative concentrations of even, odd and branched chain fatty acids were noticed. Novariation was observed in the antiiso and unsaturated fatty acids with respect to growth temperature. The unique variations in the fatty acid composition and total lipids at the growth temperature of 50° C and their variations in the stationary growth phase seem to be characteristic for B. stearothermophilus AN 002.     

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.     

Thermal regulation of the fatty acid composition of lipopolysaccharides and phospholipids of Proteus mirabilis.

The fatty acid composition of the lipid A moiety of the lipopolysaccharide and phospholipid fractions of Proteus mirabilis changed significantly on varying the growth temperature. A decrease in the growth temperature from 43 degrees C to 15 degrees C resulted in a decrease in the palmitic acid content of the lipopolysaccharide from 19.4% of total fatty acids at 43 degrees C to 1.4% at 15 degrees C, and by the appearance of an unsaturated fatty acid residue, hexadecenoic acid. Changes in the 3-hydroxy-myristic acid content of the lipid A were minimal. The decrease in the growth temperature also resulted in a decrease in the saturated fatty acid content of the phospholipid fraction, which was accompanied by an increase in their fluidity, as measured by the freedom of motion of spin-labeled fatty acids incorporated into dispersions made of the phospholipids. Nevertheless, the fluidity obtained with membrane phospholipids extracted from the cells grown at various temperatures were essentially the same when fluidity was determined at the growth temperature, supporting the hypothesis that variations in the fatty acid composition of membrane phospholipids serve to produce membranes having a constant fluidity at different temperatures of growth.