Sexual Development of Cellular Slime Molds

Macrocyst formation represents sexual development of cellular slime molds and begins with fusion between cells of compatible mating types. Homothallic and heterothallic strains as well as bisexual and asexual ones have been described. Macrocyst development requires certain environmental conditions such as darkness and excessive humidity. Sexual cell fusion has been analyzed at a molecular level in Dictyostelium discoideum , and several cell surface proteins related to it have been identified. Some of them are common to both mating types, while others are specific to one or other type. The involvement of cell-surface carbohydrates has also been suggested, though direct evidence for this is still lacking. Macrocyst formation is regulated by diffusible, pheromone like substances. Genetic studies on sexual development are scarce, probably because no suitable mutants have been available. However, several asexual mutants, as well as antibody and nucleotide probes, have recently been obtained, so mechanisms of sexual cell fusion may be understood in the near future. Considering the unique phylogenical position of cellular slime molds, analysis of sexual development in these organisms should contribute to the understanding of the mechanism and evolution of sexual reproduction systems.     

Effect of Bacteria on Chemotaxis in the Cellular Slime Molds

The effect of chemotactic substances, secreted by Escherichia coli, on the cellular slime molds was studied by deposition of bacteria near myxamoebae populations. Droplets of a bacterial suspension and a myxamoebae suspension were placed separately, at predetermined distances from each other, on a hydrophobic agar surface of low rigidity. Myxamoebae remained confined inside the droplets, except when they were activated by the bacterial products. The sphere of attraction increased at higher bacterial concentrations. Myxamoebae could be attracted over distances as great as 5 mm. Myxamoebae in droplets close to dense bacterial populations not only were attracted toward the bacteria but also moved out in an opposite direction from the bacteria. There was a gradual decrease of attraction at increasing distances between amoebae and bacteria. The attraction by bacteria or bacterial products was reduced at lower temperatures. Light did not affect the distance over which attraction could be observed. Myxamoebae close to their aggregation phase were most sensitive to the bacterial attractants. Bacterial attractants at high concentrations could disperse aggregates, even when they were in an advanced stage. At still higher concentrations of the bacterial products, cells stopped moving altogether. The bacterial attractants activated different species of cellular slime molds. They appeared to be present not only in E. coli but also in all other bacterial species that were tested. These results are discussed in the light of earlier observations on the attraction of cells by aggregates of myxamoebae.     

河南省网柄细胞状黏菌的研究

从在河南省采集的75份基物中共分离得到14株网柄细胞状黏菌,其中网柄菌属Dictyostelium的圆头网柄菌D.sphaerocephalum、棒形网柄菌D.clavatum和紫网柄菌D.purpureum和轮柄菌属Polysphondylium的亮白轮柄菌P.candidum、紫轮柄菌P.violaceum和纤细轮柄菌P.tenuissium,均为河南省新记录种。文中对该6种网柄菌进行了形态学描述,并附有生长发育部分阶段形态和显微照片。     

Dictyostelid Cellular Slime Molds in Canopy Soils of Tropical Forests1

The occurrence and distribution of dictyostelid cellular slime molds (CSM) in the mantle of dead organic matter (literally a “canopy soil”) at the bases of large epiphytes were studied in the Luquillo Experimental Forest of north-eastern Puerto Rico. CSM were isolated from 18 of 50 samples collected from this microhabitat, and four different species were recovered. Dictyostelium purpureum was the single most abundant species and represented almost half (48%) of all clones isolated during the study. Total densities (clones/g) averaged only 38 in the five forest types examined, but densities > 75 were recorded for two forest types. Relative abundance of CSM in canopy soils of the five forest types followed the same general pattern displayed by these organisms in forest floor litter, but a particular species was not necessarily common to both microhabitats in a given forest type.     

Deoxyribonucleic Acid Base Composition of Some Cellular Slime Molds*

SYNOPSIS. Deoxyribonucleic acids (DNAs) isolated from 12 representatives of cellular slime molds grown in monoxenic cultures were analyzed by density gradient centrifugation in CsC1. The unique contribution of the DNA of each food organism was subtracted and the DNA base composition of each cellular slime mold determined. The guanine plus cytosine contents range 22–37 moles % within the representatives of the order. Minor bands (satellite bands) of DNA have been observed in preparations from Dictyostelium spp. and from Polysphondylium pallidum.     

Discadenine Distribution in Cellular Slime Molds and Its Inhibitory Activity on Spore Germination

We constructed a hybrid plasmid to allow controlled expression of a gene and the subsequent secretion into the culture medium of the gene product in Escherichia coli. This was achieved by the use of five trp promoter-operator regions in tandem followed by the DNA fragment coding for the signal peptide and the N-terminus of the OmpF protein, and the trpR gene coding for the Trp repressor. Multiplication of the trp promoter-operator appreciably enhanced expression of the gene that followed. A single copy of the trpR gene on the chromosome was insufficient for controlling the enhanced expression. The expression was, however, completely controlled when the trpR gene was cloned onto the same plasmid. When the multiple trp promoter-operator was followed by the DNA fragment coding for the signal peptide and the N-terminus of the OmpF protein that was further followed by the gene for human β-endorphin, a β-endorphin-containing polypeptide was synthesized under the complete control of the trp promoter-operator, and secreted to the culture medium across both the cytoplasmic membrane and the outer membrane. Controlled expression of a foreign gene and subsequent secretion into the medium of the product were thus achieved.     

The Role of Surface Water and Light on Differentiation in the Cellular Slime Molds

A key environmental factor in inducing final fruiting body differentiation in the migrating slugs of cellular slime molds is loss of contact with a surface film of water. If the slug tip reaches above the water film, fruiting tends to occur, but if it stays in contact with the surface water, continued migration is favored. Light is effective in promoting fruiting only if the phototactic slugs orient away from surface (as with overhead light) and therefore, only indirectly affects differentiation.     

网柄细胞状黏菌的化学成分及药理作用研究进展

文中综述了从网柄细胞状黏菌中分离得到的DIF-1及其衍生物、氯代苯并呋喃、三联苯化合物等芳香族化合物,脂肪酸和磷酸化合物、酶以及糖类等,并介绍了各成分的药理或生理作用等。表明网柄细胞状黏菌可以成为某些活性成分及药物的新来源。     

Use of a Fluorescent Brightener to Demonstrate Cellulose in the Cellular Slime Molds

The presence and location of cellulose in different stages of the life cycles of the cellular slime molds can be demonstrated by use of the disodium salt of 4,4'-bis(4-anilino-6-bis (2-hydroxyethyl)-amino-s-triazin-2-ylamino)-2,2' -stilbene disulfonic acid, a fluorescent brightener. It may be used successfully as a direct stain at a concentration of 0.1% in half-normal saline at pH 6; and it may be incorporated into growth media as a vital stain at a concentration of 0.0025% with no inhibitory effect at any developmental stage. Vegetative myxamoebae contain no cellulose and show no fluorescence in the presence of this brightener when viewed with ultraviolet light. In later stages of the life cycle, the time and sites of cellulose formation can be demonstrated with the brightener because of its fluorescence. e.g., in the slime covering of the pseudoplasmodia, in the sorophore sheath, in the walls of stalk cells and spores, in the walls of microcysts, and in the walls and sheath material of macrocysts. The brightener appears to be a very sensitive indicator for cellulose, and it has certain advantages over other cellulose stains, since the staining reaction (fluorescence) is very intense, long-lasting, and not obscured by unstained cellulose-free myxamoebae if such are present.     

An Examination of the Methods for Isolating Cellular Slime Molds (Dictyosteliida) from Soil Samples*

SYNOPSIS. Present methods for extracting cellular slime molds from soil samples are shown to be biased in favor of spores and microcysts. To alleviate this problem, a modified method is proposed together with a method for partitioning active (amebae) and inactive forms (spores and microcysts).     

Protein Secretion and Membrane Insertion Systems in Gram-Negative Bacteria

In contrast to other organisms, gram-negative bacteria have evolved numerous systems for protein export. Eight types are known that mediate export across or insertion into the cytoplasmic membrane, while eight specifically mediate export across or insertion into the outer membrane. Three of the former secretory pathway (SP) systems, type I SP (ISP, ABC), IIISP (Fla/Path) and IVSP (Conj/Vir), can export proteins across both membranes in a single energy-coupled step. A fourth generalized mechanism for exporting proteins across the two-membrane envelope in two distinct steps (which we here refer to as type II secretory pathways [IISP]) utilizes either the general secretory pathway (GSP or Sec) or the twin-arginine targeting translocase for translocation across the inner membrane, and either the main terminal branch or one of several protein-specific export systems for translocation across the outer membrane. We here survey the various well-characterized protein translocation systems found in living organisms and then focus on the systems present in gram-negative bacteria. Comparisons between these systems suggest specific biogenic, mechanistic and evolutionary similarities as well as major differences.     

Development of Cellular Magnetic Dipoles in Magnetotactic Bacteria

Magnetotactic bacteria benefit from their ability to form cellular magnetic dipoles by assembling stable single-domain ferromagnetic particles in chains as a means to navigate along Earth's magnetic field lines on their way to favorable habitats. We studied the assembly of nanosized membrane-encapsulated magnetite particles (magnetosomes) by ferromagnetic resonance spectroscopy using Magnetospirillum gryphiswaldense cultured in a time-resolved experimental setting. The spectroscopic data show that 1), magnetic particle growth is not synchronized; 2), the increase in particle numbers is insufficient to build up cellular magnetic dipoles; and 3), dipoles of assembled magnetosome blocks occur when the first magnetite particles reach a stable single-domain state. These stable single-domain particles can act as magnetic docks to stabilize the remaining and/or newly nucleated superparamagnetic particles in their adjacencies. We postulate that docking is a key mechanism for building the functional cellular magnetic dipole, which in turn is required for magnetotaxis in bacteria.     

Studies of Membrane Formation in Tetrahymena pyriformis. VIII. On the Origin of Membranes Surrounding Food Vacuoles*†

SYNOPSIS. A procedure was devised for the isolation of purified food vacuoles from Tetrahymena pyriformis fed particles of ferric oxide. Phospholipids extracted from vacuolar membranes were more similar in composition to the lipids of microsomes than to lipids of whole cells, cilia or post-microsomal supernatant. Fractionation of cells grown in the presence of [¹⁴C]palmitic acid or [³²P]inorganic phosphate also revealed similarities in the specific radioactivities of microsomes and vacuolar membranes. The data suggested that vacuolar membranes arise from a pool of cytoplasmic membranes.     

Ultrastructure of Acrasis rosea,a Cellular Slime Mold,During Development*

SYNOPSIS. An ultrastructural study of the myxamoebae of Acrasis rosea in the vegetative, aggregative and culminative stages was made. An intracytoplasmic system of microfibrillar bundles develops as the cells enter the aggregative stage and commence the morphogenetic sequence leading to the construction of a fruiting body. The fibrillar bundles disappear in the cells of the mature fruiting body. No relevant ultrastructural differences were observed between spores, stalk cells and microcysts. Each of these cells is surrounded by a single-layered coat of fibrillar material that is oriented parallel to the cell surface. Tubular structures were observed between the plasma membrane and the cell coat. The tubules may be layered along the cell periphery or they may be recessed in pockets formed by the plasma membrane. They resemble lomasomes typical of fungal cells. The myxamoebae of A. rosea clearly differ from the Dictyostelium-type myxamoebae in mitochondrial structure, the presence of lamellate structures in the nucleolus and the absence of prespore vacuoles.     

An Electron Microscope Study of Food Vacuoles in Paramecium aurelia

The foed vacuoles of Paramecium aurelia , when examined in the electron microscope, are seen to be surrounded by small secondary vacuoles 0.05 - 0.2 μ. in diameter. Similar small vacuoles also surround the deepest part of the buccal cavity. Young focd vacuoles, i.e. those containing well preserved bacteria, are encircled by a smooth. vacuolar membrane. In older food vacuoles the vacuolar membrane in a transverse section often appears more wavy with small gulfs and protuberances. It is suggested that the small surrounding vacuoles are formed by the vacuolar membrane of older vacuoles by means of a process similar to pinocytosis. There is no evidence, however, that formation of small surrounding vacuoles takes place by pinocytosis in young food vacuoles. Examination of the cytoplasmic membrane of the deepest parts of the buccal cavity shows a similar prccess of vacuole formation by pinocytosis.     

Inhibition of Spore Germination in the Cellular Slime Moulds

The phenomenon of auto-inhibition of spore germination has beendemonstrated in the slime moulds Dictyostelium purpureum, strainno. 2; Dictyostelium discoideum, strain no. 1; Dictyosteliummucoroides, strain no. 2; and Polysphondy-Iium violeceum, strainno. 6. A water-soluble substance present in spores was shownto cause this self-inhibition. The substance was isolated fromeach species studied. Evidence as presented indicates that thesame substance occurs in each species of Dictyostelium, butthat it differs from that found in Pv6. An attempt was alsomade to relate this inhibitory substance to the phenomenon of'differential inhibition' in Dp2, Dd, Dm2, and Pu6. An explanationbased on evidence for differential sensitivity to a common inhibitorof spore germination in Dictyostelium and an inhibition in otherthan the spore stage in Pu6 is presented. Apparent differentialproduction of a common inhibitor in Dictyostelium is also reported.