cAMP controls cytosolic Ca<Superscript>2+</Superscript> levels in <Emphasis Type="Italic">Dictyostelium discoideum</Emphasis>

Background  

Differentiating Dictyostelium discoideum amoebae respond upon cAMP-stimulation with an increase in the cytosolic free Ca²⁺ concentration ([Ca²⁺]_i) that is composed of liberation of stored Ca²⁺ and extracellular Ca²⁺-influx. In this study we investigated whether intracellular cAMP is involved in the control of [Ca²⁺]_i.     

Comparative genomics of the social amoebae <Emphasis Type="Italic">Dictyostelium discoideum</Emphasis> and <Emphasis Type="Italic">Dictyostelium purpureum</Emphasis>

Background

The social amoebae (Dictyostelia) are a diverse group of Amoebozoa that achieve multicellularity by aggregation and undergo morphogenesis into fruiting bodies with terminally differentiated spores and stalk cells. There are four groups of dictyostelids, with the most derived being a group that contains the model species Dictyostelium discoideum.

Results

We have produced a draft genome sequence of another group dictyostelid, Dictyostelium purpureum, and compare it to the D. discoideum genome. The assembly (8.41 × coverage) comprises 799 scaffolds totaling 33.0 Mb, comparable to the D. discoideum genome size. Sequence comparisons suggest that these two dictyostelids shared a common ancestor approximately 400 million years ago. In spite of this divergence, most orthologs reside in small clusters of conserved synteny. Comparative analyses revealed a core set of orthologous genes that illuminate dictyostelid physiology, as well as differences in gene family content. Interesting patterns of gene conservation and divergence are also evident, suggesting function differences; some protein families, such as the histidine kinases, have undergone little functional change, whereas others, such as the polyketide synthases, have undergone extensive diversification. The abundant amino acid homopolymers encoded in both genomes are generally not found in homologous positions within proteins, so they are unlikely to derive from ancestral DNA triplet repeats. Genes involved in the social stage evolved more rapidly than others, consistent with either relaxed selection or accelerated evolution due to social conflict.

Conclusions

The findings from this new genome sequence and comparative analysis shed light on the biology and evolution of the Dictyostelia.

     

Subcellular localization of ammonium transporters in <Emphasis Type="Italic">Dictyostelium discoideum</Emphasis>

Background  

With the exception of vertebrates, most organisms have plasma membrane associated ammonium transporters which primarily serve to import a source of nitrogen for nutritional purposes. Dictyostelium discoideum has three ammonium transporters, Amts A, B and C. Our present work used fluorescent fusion proteins to determine the cellular localization of the Amts and tested the hypothesis that the transporters mediate removal of ammonia generated endogenously from the elevated protein catabolism common to many protists.     

Ca<Superscript>2+</Superscript> reduces the effect of hypoxia in mosses <Emphasis Type="Italic">Mnium undulatum</Emphasis> and <Emphasis Type="Italic">Polytrichum commune</Emphasis>

Gametophores of mosses Mnium undulatum and Polytrichum commune were submerged in distilled water or in calcium chloride solution (0.9 mM Ca²⁺) to induce hypoxia. The net photosynthetic (P_N) and dark respiration rate (R_D) were measured in the air containing 300–400 μmol(CO₂)·mol⁻¹(air) and 0.21 mol(O₂)·mol⁻¹(air). P_N of M. undulatum gametophores decreased to 58 % of the control after 1-h submersion in water, whereas to 80 % of the control in P. commune gametophores. A smaller decrease in P_N was observed when the gametophores were immersed in CaCl₂ solution. In hypoxia, R_D in the tested mosses species was a little higher than in the control.     

Antagonistic effect of divalent cations Ca<Superscript>2+</Superscript> and Mg<Superscript>2+</Superscript> on the morphological development of <Emphasis Type="Italic">Streptomyces hygroscopicus</Emphasis> var. <Emphasis Type="Italic">geldanus</Emphasis>

The automated docking program DOCK 5.3.0 was applied to screening for quorum sensing inhibitors (QSIs) of Peudomonus aeruginosa from a database containing 51 active components of Traditional Chinese Medicines with antibacterial activity. Five potential QSIs were revealed by the computer-based virtual screening. The compounds 3, 4, 5, 6, 7 inhibit biofilm formation of P. aeruginosa at a concentration of 200 μM. Compound 4 (baicalein) does not inhibit the growth of P. aeruginosa; however, it significantly inhibits biofilm formation of the bacteria at a lower concentration of 20 μM and promoted proteolysis of the signal receptor TraR protein in Escherichia coli at 4–40 mM. Baicalein and ampicillin showed synergistic activity against P. aeruginosa. These results suggested that baicalein can interfere with quorum sensing system of P. aeruginosa and will be developed as antibacterial agent with novel target.     

Membrane Stretching Triggers Mechanosensitive Ca<Superscript>2+</Superscript> Channel Activation in <Emphasis Type="Italic">Chara</Emphasis>

In order to confirm that mechanosensitive Ca²⁺ channels are activated by membrane stretching, we stretched or compressed the plasma membrane of Chara by applying osmotic shrinkage or swelling of the cell by varying the osmotic potential of the bathing medium. Aequorin studies revealed that treatments causing membrane stretching induced a transient but large increase in cytoplasmic concentration of Ca²⁺ (Δ[Ca²⁺]_c). However, the observed Δ[Ca²⁺]_c decreased during the treatments, resulting in membrane compression. A second experiment was carried out to study the relationship between changes in membrane potential (ΔE _m) and stretching or compression of the plasma membrane. Significant ΔE _m values, often accompanied by an action potential, were observed during the initial exchange of the bathing medium from a hypotonic medium to a hypertonic one (plasmolysis). ΔE _m appears to be triggered by a partial stretching of the membrane as it was peeled from the cell wall. After plasmolysis, other exchanges from hypertonic to hypotonic media, with their accompanying membrane stretching, always induced large ΔE _m values and were often accompanied by an action potential. By contrast, action potentials were scarcely observed during other exchanges from hypotonic to hypertonic solutions (=membrane compression). Thus, we concluded that activation of the mechanosensitive channels is triggered by membrane stretching in Chara.     

Understanding Streaming in <Emphasis Type="Italic">Dictyostelium discoideum</Emphasis>: Theory Versus Experiments

Recent experimental work involving Dictyostelium discoideum seems to contradict several theoretical models. Experiments suggest that localization of the release of the chemoattractant cyclic adenosine monophosphate to the uropod of the cell is important for stream formation during aggregation. Yet several mathematical models are able to reproduce streaming as the cells aggregate without taking into account localization of the chemoattractant. A careful analysis of the experiments and the theory suggests the two major features of the system which are important to stream formation are random cell motion and chemotaxis to regions of higher cell density. Random cell motion acts to reduce streaming, whereas chemotaxis to regions of higher cell density reinforces streaming. With this understanding, the experimental results can be explained in a manner consistent with the theoretical results. In all the experiments, alterations in the two main factors of random motion and chemotaxis to regions of higher cell density, not the localization of the release of the chemoattractant, can explain the results as they relate to streaming. Additionally, a comparison of results from a mathematical model that simulates cells which localize the chemoattractant and cells which do not shows little difference in the streaming patterns.     

Enhanced intracellular Ca<Superscript>2+</Superscript> concentrations in <Emphasis Type="Italic">Escherichia coli</Emphasis> and <Emphasis Type="Italic">Bacillus subtilis</Emphasis> after addition of oligosaccharide elicitors

Elicitation can lead to overproduction of secondary metabolites in plants and microbes. Potential changes in cytosolic Ca²⁺ levels in bacteria were studied in response to elicitation. We report, for the first time, the effect of oligosaccharide elicitors on intracellular Ca²⁺ levels. The apoaequorin gene was cloned into Escherichia coli DH5α and Bacillus subtilis 1604 cultures. Addition of elicitors, oligoguluronate and mannan oligosaccharides, to the cultures caused up to 11-fold increase in cytosolic Ca²⁺ in E. coli and tenfold increase in B. subtilis. These increases in Ca²⁺ levels could therefore contribute to the enhancement of secondary metabolite levels.     

Arachidonic acid is a chemoattractant for <Emphasis Type="Italic">Dictyostelium discoideum</Emphasis> cells

Cyclic AMP (cAMP) is a natural chemoattractant of the social amoeba Dictyostelium discoideum. It is detected by cell surface cAMP receptors. Besides a signalling cascade involving phosphatidylinositol 3,4,5-trisphosphate (PIP₃), Ca²⁺ signalling has been shown to have a major role in chemotaxis. Previously, we have shown that arachidonic acid (AA) induces an increase in the cytosolic Ca²⁺ concentration by causing the release of Ca²⁺ from intracellular stores and activating influx of extracellular Ca²⁺. Here we report that AA is a chemoattractant for D. discoideum cells differentiated for 8–9 h. Motility towards a glass capillary filled with an AA solution was dose-dependent and qualitatively comparable to cAMP-induced chemotaxis. Ca²⁺ played an important role in AA chemotaxis of wild-type Ax2 as ethyleneglycolbis(b-aminoethyl)-N,N,N′,N′-tetraacetic acid (EGTA) added to the extracellular buffer strongly inhibited motility. In the HM1049 mutant whose iplA gene encoding a putative Ins(1,4,5)P₃-receptor had been knocked out, chemotaxis was only slightly affected by EGTA. Chemotaxis in the presence of extracellular Ca²⁺ was similar in both strains. Unlike cAMP, addition of AA to a cell suspension did not change cAMP or cGMP levels. A model for AA chemotaxis based on the findings in this and previous work is presented.     

Isolation and Hybridization Kinetics of Messenger RNA from <Emphasis Type="Italic">Dictyostelium discoideum</Emphasis>

Messenger RNA from the cellular slime mould Dictyostelium discoideum contains a sequence of poly A residues and more than 95% hybridizes to the non-reiterated DNA.     

Autonomous and non-autonomous traits mediate social cooperation in <Emphasis Type="Italic">Dictyostelium discoideum</Emphasis>

In the trishanku (triA ⁻ ) mutant of the social amoeba Dictyostelium discoideum, aggregates are smaller than usual and the spore mass is located mid-way up the stalk, not at the apex. We have monitored aggregate territory size, spore allocation and fruiting body morphology in chimaeric groups of (quasi-wild-type) Ax2 and triA ⁻ cells. Developmental canalisation breaks down in chimaeras and leads to an increase in phenotypic variation. A minority of triA ⁻ cells causes largely Ax2 aggregation streams to break up; the effect is not due to the counting factor. Most chimaeric fruiting bodies resemble those of Ax2 or triA ⁻ . Others are double-deckers with a single stalk and two spore masses, one each at the terminus and midway along the stalk. The relative number of spores belonging to the two genotypes depends both on the mixing ratio and on the fruiting body morphology. In double-deckers formed from 1:1 chimaeras, the upper spore mass has more Ax2 spores, and the lower spore mass more triA ⁻ spores, than expected. Thus, the traits under study depend partly on the cells’ own genotype and partly on the phenotypes, and so genotypes, of other cells: they are both autonomous and non-autonomous. These findings strengthen the parallels between multicellular development and behaviour in social groups. Besides that, they reinforce the point that a trait can be associated with a genotype only in a specified context.     

Ca<Superscript>2+ </Superscript>and Cu<Superscript>2+ </Superscript>supplementation increases mannitol production by <Emphasis Type="Italic">Candida magnoliae</Emphasis>

Supplementation with CaCl₂·2H₂O (50 mg l⁻¹) or CuSO₄·5H₂O (10 mg l⁻¹) improved mannitol production by Candida magnoliae by 14.5 and 18.6% (25 and 32 g/L), respectively. When used in combination, they acted synergistically: Ca²⁺ decreased the intracellular concentration of mannitol 30%, whereas Cu²⁺ increased the intracellular activity of mannitol dehydrogenase 1.6-times more than control. Ca²⁺ probably works by altering the permeability of cells to mannitol, whereas, Cu²⁺ increases the activity of an enzyme responsible for mannitol biosynthesis.     

Examination of the putative Ca<Superscript>2+</Superscript>-binding site in the light-harvesting complex 1 of thermophilic purple sulfur bacterium <Emphasis Type="Italic">Thermochromatium</Emphasis><Emphasis Type="Italic">tepidum</Emphasis>

The core light-harvesting complex (LH1) of purple sulfur photosynthetic bacterium Thermochromatium tepidum exhibits an unusual absorption maximum at 915 nm for the Q _y transition, and is highly stable when copurified with reaction center (RC) in a LH1–RC complex form. In previous studies, we demonstrated that the calcium ions are involved in both the large red shift and the enhanced thermal stability, and possible Ca²⁺-binding sites were proposed. In this study, we further examine the putative binding sites in the LH1 polypeptides using purified chromatophores. Incubation of the chromatophores in the presence of EDTA revealed no substantial change in the absorption maximum of LH1 Q _y transition, whereas further addition of detergents to the chromatophores-EDTA solution resulted in a blue-shift for the LH1 Q _y peak with the final position at 892 nm. The change of the LH1 Q _y peak to shorter wavelengths was relatively slow compared to that of the purified LH1–RC complex. The blue-shifted LH1 Q _y transition in chromatophores can be restored to its original position by addition of Ca²⁺ ions. The results suggest that the Ca²⁺-binding site is exposed on the inner surface of chromatophores, corresponding to the C-terminal region of LH1. An Asp-rich fragment in the LH1 α-polypeptide is considered to form a crucial part of the binding network. The slow response of LH1 Q _y transition upon exposure to EDTA is discussed in terms of the membrane environment in the chromatophores.     

<Emphasis Type="Italic">Ty</Emphasis>1<Emphasis Type="Italic">/copia</Emphasis>- and <Emphasis Type="Italic">Ty</Emphasis>3<Emphasis Type="Italic">/gypsy</Emphasis>-like DNA sequences in <Emphasis Type="Italic">Helianthus </Emphasis>species

Two repeated DNA sequences isolated from a partial genomic DNA library of Helianthus annuus, p HaS13 and p HaS211, were shown to represent portions of the int gene of a Ty3 /gypsy retroelement and of the RNase-Hgene of a Ty1 /copia retroelement, respectively. Southern blotting patterns obtained by hybridizing the two probes to BglII- or DraI-digested genomic DNA from different Helianthus species showed p HaS13 and p HaS211 were parts of dispersed repeats at least 8 and 7 kb in length, respectively, that were conserved in all species studied. Comparable hybridization patterns were obtained in all species with p HaS13. By contrast, the patterns obtained by hybridizing p HaS211 clearly differentiated annual species from perennials. The frequencies of p HaS13- and p HaS211-related sequences in different species were 4.3x10(4)-1.3x10(5) copies and 9.9x10(2)-8.1x10(3) copies per picogram of DNA, respectively. The frequency of p HaS13-related sequences varied widely within annual species, while no significant difference was observed among perennial species. Conversely, the frequency variation of p HaS211-related sequences was as large within annual species as within perennials. Sequences of both families were found to be dispersed along the length of all chromosomes in all species studied. However, Ty3 /gypsy-like sequences were localized preferentially at the centromeric regions, whereas Ty1/ copia-like sequences were less represented or absent around the centromeres and plentiful at the chromosome ends. These findings suggest that the two sequence families played a role in Helianthusgenome evolution and species divergence, evolved independently in the same genomic backgrounds and in annual or perennial species, and acquired different possible functions in the host genomes.     

Revision of the <Emphasis Type="Italic">Serrulati</Emphasis> species of <Emphasis Type="Italic">Penstemon</Emphasis> section <Emphasis Type="Italic">Saccanthera</Emphasis> subsection <Emphasis Type="Italic">Serrulati</Emphasis>

A revision of Penstemon sect. Saccanthera subsect. Serrulati includes a new species (P. salmonensis), a new variety (P. triphyllus var. infernalis), and the elevation of a subspecies to species (P. curtiflorus), bringing the total number of species to eight, which are keyed and described, complete with nomenclature and type citations.