Head-group contributions to bilayer stability: monolayer and calorimetric studies on synthetic, stereochemically uniform glucolipids

Monolayer and differential scanning calorimetry studies have been performed on synthetic, stereochemically uniform glyceroglucolipids having saturated, ether-linked alkyl chains. The limiting area, A0 = 40 A2 X molecule-1, resulting from the monolayer measurements of the glucolipids is comparable to the A0 value found for phosphatidylethanolamine lipids. The area corresponds to twice the value observed with saturated straight chain fatty acids, which indicates that at high surface pressure the space requirement of the glucose head group does not exceed that of the two alkyl chains. The apparent specific heat capacities of the glucolipid dispersions have been found to be higher than those of corresponding phospholipids. They can be approximated from group parameters with the additional assumption that the experimental partial molar heat capacity of glucose is valid for the glucose head groups of the lipids. The transition enthalpies of the C16 and C18 glyceroglucolipids are clearly larger than the delta H values of corresponding phospholipids, while the C14 glyceroglucolipid has the same transition enthalpy as dimyristoylphosphatidylethanolamine or ditetradecylphosphatidylethanolamine. Glucolipids exhibit gel to liquid-crystalline phase transition temperatures which are only slightly lower than those of their phosphatidylethanolamine analogues, although they are uncharged molecules. Like phosphatidylethanolamine the glucolipids do not show a pretransition; however, with the C14 glucolipid a highly cooperative posttransition, approximately 5 deg above the main transition, has been found. Calorimetric experiments with a C14 glucolipid, in which the hydroxyl protons of the glucose moiety have been exchanged by deuterium, suggest that the posttransition might reflect structural changes of the head group.(ABSTRACT TRUNCATED AT 250 WORDS)     

Bovine Nucleus Caudatus Acetylcholinesterase: Active Site Determination and Investigation of a Dimeric Form Obtained by Selective Proteolysis

Abstract: The number of catalytic subunits of purified bovine nucleus caudatus acetylcholinesterase (E.C. 3.1.1.7) has been determined by active site labelling with [³H]diisopropyl fluorophosphate ([³H]DFP). The 10.5 S, 16 S, and 20 S forms were estimated to contain two, four, and six active sites, respectively, per molecule. A 4.8 S form, which showed a weak amphiphile-dependent activity behavior, was obtained by selective proteolytic digestion with pronase. The inability of the purified 4.8 S form to aggregate after detergent removal, and the molecular mass in the range of 130-165 kD under nondenaturating conditions, indicate that this form is a dimeric form, lacking those hydrophobic regions responsible for aggregation.     

What is truth: Consensus and discordance in next‐generation phylogenetic analyses of Daucus

High‐throughput (next‐generation) DNA sequencing has removed barriers to data quantity and quality, and it has produced phylogenies with high statistical support. Such data are useful to address phylogenetic congruence among individual genes. Concatenated analyses of unlinked genes often produce well‐resolved phylogenetic trees with bootstrap support on major nodes at or approaching 100%, but they have been criticized for providing incorrect phylogenies for various reasons to include a history of hybridization, introgression, and incomplete lineage sorting. The present study compares next‐generation sequencing results of the same accessions of Daucus with different genomic regions, of which three have been reported before: (i) the entire plastid genome, (ii) 47 mitochondrial genes, and (iii) 94 conserved nuclear orthologs. Here, we report a fourth dataset, (iv) 564 895 nuclear SNPs. There are areas of discordance in all four results using the same accessions analyzed with maximum parsimony, maximum likelihood, and with the nuclear data species trees through a coalescent analysis. The nuclear results show significant areas of discordance that were unexpected, because these studies used the same DNA samples, the nuclear studies were generated from large and high‐quality datasets with the SNPs distributed on all nine linkage groups of Daucus carota, and the results were supported by high bootstrap values. These results raise questions concerning the best data and analytical methods to reconstruct and understand the “truth” of a phylogeny.     

Phylogeny and assemblage composition of Frankia in Alnus tenuifolia nodules across a primary successional sere in interior Alaska

In nitrogen (N) fixing symbioses, host‐symbiont specificity, genetic variation in bacterial symbionts and environmental variation represent fundamental constraints on the ecology, evolution and practical uses of these interactions, but detailed information is lacking for many naturally occurring N‐fixers. This study examined phylogenetic host specificity of Frankia in field‐collected nodules of two Alnus species (A. tenuifolia and A. viridis) in interior Alaska and, for A. tenuifolia, distribution, diversity, spatial autocorrelation and correlation with specific soil factors of Frankia genotypes in nodules collected from replicated habitats representing endpoints of a primary sere. Frankia genotypes most commonly associated with each host belonged to different clades within the Alnus‐infective Frankia clade, and for A. tenuifolia, were divergent from previously described Frankia. A. tenuifolia nodules from early and late succession habitats harboured distinct Frankia assemblages. In early succession, a single genotype inhabited 71% of nodules with no discernable autocorrelation at any scale, while late succession Frankia were more diverse, differed widely among plants within a site and were significantly autocorrelated within and among plants. Early succession Frankia genotype occurrence was strongly correlated with carbon/nitrogen ratio in the mineral soil fraction, while in late succession, the most common genotypes were correlated with different soil variables. Our results suggest that phylogenetic specificity is a significant factor in the A. tenuifolia‐Frankia interaction and that significant habitat‐based differentiation may exist among A. tenuifolia‐infective genotypes. This is consistent with our hypothesis that A. tenuifolia selects specific Frankia genotypes from early succession soils and that this choice is attenuated in late succession.     

Partial-trisomy syndromes

Summary One of the autosomes of the C group (6–12), C, can sometimes be identified by a secondary constriction.A mother and her daughter, both afflicted with the oral-facialdigital (OPD) syndrome, are shown to be partially trisomic for an inner segment of C, this segment having been inserted into chromosome 1. The other chromosome 1, the two chromosomes C, and the rest of the complement of 46 chromosomes appear normal.In four other OFD patients no chromosomal abnormality was detected. Nonetheless, it is concluded that the OFD syndrome is generally caused by partial trisomy for a specific region of C. The presence of the inserted extra segment would usually escape microscopical detection.The present work was supported by grant C-3313 from the U.S. Public Health Service.     

Effects of fungal food quality and starvation on the fatty acid composition of Protaphorura fimata (Collembola)

The lipid pattern of animals is influenced by species, life stage, environmental conditions and diet. We investigated the effects of food quality and starvation on the phospholipid (PLFA) and neutral lipid (NLFA) fatty acid pattern of the collembolan Protaphorura fimata. Collembolans were fed with two common soil fungi, Agrocybe gibberosa and Chaetomium globosum, of which the cellular lipid composition was analysed. A. gibberosa was grown on agar with different nitrogen contents, resulting in altered fatty acid patterns and C:N ratios, i.e. fungi of different food quality. Collembolans did not mirror the lipid composition of the fungal diet as the pattern of major NLFAs in P. fimata was vice versa. Presumably, altered food quality of fungi caused compensatory responses by the collembolans, thereby diminishing the fungal signal. In a further experiment P. fimata (previously maintained with C. globosum) was kept without food for up to 4 weeks. Starvation resulted in a decline in the total amount of NLFAs; however, it did not affect the fatty acid pattern, indicating that NLFAs were degraded indiscriminately. Generally, the PLFA profile of the collembolans changed only slightly due to variations in diet quality or starvation.     

Glycine mineralization in situ closely correlates with soil carbon availability across six North American forest ecosystems

Free amino acids (FAA) constitute a significant fraction of dissolved organic nitrogen (N) in forest soils and play an important role in the N cycle of these ecosystems. However, comparatively little attention has been given to their role as labile carbon (C) substrates that might influence the metabolic status of resident microbial populations. We hypothesized that the residence time of simple C substrates, such as FAA, are mechanistically linked to the turnover of endogenous soil C pools. We tested this hypothesis across a latitudinal gradient of forested ecosystems that differ sharply with regard to climate, overstory taxon, and edaphic properties. Using a combined laboratory and field approach, we compared the turnover of isotopically labeled glycine in situ to the turnover of mineralizable soil C (C_min) at each site. The turnover of glycine was rapid (residence times <2 h) regardless of soil type. However, across all ecosystems glycine turnover rates were strongly correlated with indices of soil organic matter quality. For example, C:N ratios for the upper soil horizons explained ~80% of the variability observed in glycine turnover, and there was a strong positive correlation between in situ glycine-C turnover and C_min measured in the laboratory. The turnover of glycine in situ was better explained by changes in soil C availability than cross-ecosystem variation in soil temperature or concentrations of dissolved inorganic N and FAA-N. This suggests the consumption of these low-molecular-weight substrates by soil microorganisms may be governed as much by the overall decomposability of soil C as by N limitation to microbial growth.     

Variation in carbohydrate source–sink relations of forest and treeline white spruce in southern, interior and northern Alaska

Two opposing hypotheses have been presented to explain reduced tree growth at the treeline, compared with growth in lower elevation or lower latitude forests: the carbon source and sink limitation hypotheses. The former states that treeline trees have an unfavorable carbon balance and cannot support growth of the magnitude observed at lower elevations or latitudes, while the latter argues that treeline trees have an adequate carbon supply, but that cold temperatures directly limit growth. In this study, we examined the relative importance of source and sink limitation in forest and treeline white spruce (Picea glauca) in three mountain ranges from southern to northern Alaska. We related seasonal changes in needle nonstructural carbohydrate (NSC) content with branch extension growth, an approach we argue is more powerful than using needle NSC concentration. Branch extension growth in the southernmost Chugach Mountains was much greater than in the White Mountains and the Brooks Range. Trees in the Chugach Mountains showed a greater seasonal decline in needle NSC content than trees in the other mountain ranges, and the seasonal change in NSC was correlated with site-level branch growth across mountain ranges. There was no evidence of a consistent difference in branch growth between the forest and treeline sites, which differ in elevation by approximately 100 m. Our results point to a continuum between source and sink limitation of growth, with high-elevation trees in northern and interior Alaska showing greater evidence of sink limitation, and those in southern Alaska showing greater potential for source limitation.