FINE STRUCTURE OF BIFLAGELLATE ZOOSPORES OF ASTEROCOCCUS SUPERBUS (TETRASPORALES, CHLOROPHYCEAE), INCLUDING THE ABSOLUTE CONFIGURATION OF THE FLAGELLAR APPARATUS

The ultrastructure of zoospores of Asterococcus superbus (Cienk.) Scherffel was studied to provide ultrastructural data relevant to the systematic position of the genus. Our results demonstrated that the motile cells of A. superbus were similar to those of the tetrasporalean algae, such as Tetraspora sp. and Tetrasporidium javanicum Moebius . The flagellar apparatus of A. superbus had the same clock-wise orientation of basal bodies and the V-shaped alignment of basal bodies as Tetraspora cylindrica (Wahlb.) Ag. and T. lubrica (Roth) Ag., but differed by having rhizoplasts . The motile cells of A. superbus displayed chlamydomonadal ultrastructure, similar to Chlamydomonas reinhardtii Dangeard , including the absolute configuration of the flagellar apparatus. The pyrenoid matrix in A. superbus, however , showed a large lateral invagination occupied by chloroplast stroma, a characteristic that has never been observed in Chlorophyta.     

Studies in selected fragilarioid diatoms (Bacillariophyceae) from Lake Hovsgol, Mongolia

Three fragilarioid diatom taxa were studied in detail at the light microscopy and scanning electron microscopy levels from samples collected from Lake Hovsgol, Mongolia. Two of the taxa are new to science, described here as Staurosirella minuta Morales et M. B. Edlund and Pseudostaurosira tenuis Morales et M. B. Edlund, and may be endemic to Lake Hovsgol. The third taxon has been identified as Fragilaria polonica Witak et Lange‐Bertalot and it is transferred to the genus Pseudostaurosira (Grunow) D. M. Williams et Round as Pseudostaurosira polonica (Witak et Lange‐Bertalot) Morales et M. B. Edlund comb. nov. based on the ultrastructural features of its valves. The relationship of the above taxa to others reported in the literature is included herein, and the nomenclatural transfer, Pseudostaurosira naveana (Le Cohu) Morales et M. B. Edlund comb. nov., is proposed.     

The role of residues R97 and Y331 in modulating the pH optimum of an insect beta-glycosidase of family 1.

The activity of the digestive beta-glycosidase from Spodoptera frugiperda (Sfbetagly50, pH optimum 6.2) depends on E399 (pKa = 4.9; catalytic nucleophile) and E187 (pKa = 7.5; catalytic proton donor). Homology modelling of the Sfbetagly50 active site confirms that R97 and Y331 form hydrogen bonds with E399. Site-directed mutagenesis showed that the substitution of R97 by methionine or lysine increased the E399 pKa by 0.6 or 0.8 units, respectively, shifting the pH optima of these mutants to 6.5. The substitution of Y331 by phenylalanine increased the pKa of E399 and E187 by 0.7 and 1.6 units, respectively, and displaced the pH optimum to 7.0. From the observed deltapKa it was calculated that R97 and Y331 contribute 3.4 and 4.0 kJ.mol(-1), respectively, to stabilization of the charged E399, thus enabling it to be the catalytic nucleophile. The substitution of E187 by D decreased the pKa of residue 187 by 0.5 units and shifted the pH optimum to 5.8, suggesting that an electrostatic repulsion between the deprotonated E399 and E187 may increase the pKa of E187, which then becomes the catalytic proton donor. In short the data showed that a network of noncovalent interactions among R97, Y331, E399 and E187 controls the Sfbetagly50 pH optimum. As those residues are conserved among the family 1 beta-glycosidases, it is proposed here that similar interactions modulate the pH optimum of all family 1 beta-glycosidases.     

Retinoic acid receptor alpha2 is a growth suppressor epigenetically silenced in MCF-7 human breast cancer cells.

Retinoic acid (RA) receptor (RAR) beta2 has been shown to be underexpressed in human breast cancer cells, including MCF-7 cells, and recent reports have suggested that hypermethylation of the RAR beta2 promoter and 5'-UTR is the underlying cause. Here we show that RAR alpha2 is also underexpressed in MCF-7 breast cancer cells, at both the message and the protein level, relative to normal or nontumorigenic breast epithelial cells. Bisulfite sequencing of the CpG island in the RAR alpha2 promoter revealed highly penetrant and uniform cytosine methylation in MCF-7 cells. Pretreatment with the DNA methyltransferase inhibitor, azacytidine, followed by treatment with RA and a histone deacetylase inhibitor, trichostatin A, resulted in partial promoter demethylation and RAR alpha2 induction, which strongly suggested that promoter hypermethylation is responsible for RAR alpha2 underexpression. We compared the outcome of ectopic expression in MCF-7 cells of matched levels of RAR alpha2 and RAR beta2. On the basis of a clonogenic assay, RAR alpha2 displayed ligand-dependent growth-suppressive activity similar to that of RARb eta2; thus, 10 and 20 nM RA inhibited clonogenic growth by 52 and 80%, respectively, in RAR alpha2-transfected cells compared with 75 and 77%, respectively, in RAR beta2-transfected cells. We conclude that the silencing of the RAR alpha2 promoter by hypermethylation may play a contributory role in the dysregulation of RA signaling in mammary tumorigenesis.     

Host plant and natural enemy impact on cereal aphid competition in a seasonal environment

Competition between phytophagous insects could be mediated by differential impact of natural enemies or by induced changes in host plant quality. The interaction between the aphids Sipha flava and Rhopalosiphum maidis on the shared host plant, Sorghum halepense , was evaluated during the fall and spring seasons in the presence or absence of natural enemies in a garden experiment. During the fall, S. flava was negatively affected by R. maidis in all treatments. However, during the spring season, S. flava was not affected by R. maidis when natural enemies were excluded, and positively affected by R. maidis with natural enemies present. Rhopalosiphum maidis was negatively affected by S. flava during the fall and spring seasons under all treatments. Laboratory experiments demonstrated that changes in host plant quality induced by S. flava attack and the presence of the parasitoid Lysiphlebus testaceipes negatively affected R. maidis , but not S. flava . Under abiotic conditions simulating fall and spring (lower temperature and daylength in the fall than in the spring), S. flava showed lower performance than R. maidis under "fall" conditions, and both aphid species showed similar life history traits under "spring" conditions. For S. flava , the time to the first reproduction and longevity were longer, and the nymphal production was lower, under "fall" conditions than under "spring" conditions. Our results showed that S. flava is benefited when R. maidis and the parasitoid L. testaceipes are present but the abiotic conditions are not adverse.     

Key Role of Bacterial NH4+ Metabolism in Rhizobium-Plant Symbiosis

Symbiotic nitrogen fixation is carried out in specialized organs, the nodules, whose formation is induced on leguminous host plants by bacteria belonging to the family Rhizobiaceae. Nodule development is a complex multistep process, which requires continued interaction between the two partners and thus the exchange of different signals and metabolites. NH₄⁺ is not only the primary product but also the main regulator of the symbiosis: either as ammonium and after conversion into organic compounds, it regulates most stages of the interaction, from the production of nodule inducers to the growth, function, and maintenance of nodules. This review examines the adaptation of bacterial NH₄⁺ metabolism to the variable environment generated by the plant, which actively controls and restricts bacterial growth by affecting oxygen and nutrient availability, thereby allowing a proficient interaction and at the same time preventing parasitic invasion. We describe the regulatory circuitry responsible for the downregulation of bacterial genes involved in NH₄⁺ assimilation occurring early during nodule invasion. This is a key and necessary step for the differentiation of N₂-fixing bacteroids (the endocellular symbiotic form of rhizobia) and for the development of efficient nodules.     

2,3-dihydrojaborosalactone A,a withanolide from Acnistus breviflorus

From Acnistus breviflorus the new 27-hydroxy-5β,6β-epoxy-1-oxo-(22R)-witha-24-enolide (2,3-dihydrojaborosalactone A) as well as seven known withanolides, withaferin A, 2,3-dihydrowithaferin A, 6α-chloro-5β-hydroxywithaferin A, 5,6-deoxywithaferin A, jaborosalactone A, jaborosalactone D and jaborosalactone E were isolated and characterized by means of spectroscopic (¹H NMR, ¹³ C NMR and mass spectral) methods. Depending on the extraction solvent (methanol or ethanol), a known artifact (3β-methoxy-2,3-dihydrowithaferin A) and the new 5α-methoxy-4,5-dihydrojaborosalactone B and 5α-ethoxy-4,5-dihydrojaborosalactone B were also isolated and characterized.     

Ent-beyerene and ent-atisene diterpenes from viguiera insignis

The new diterpenes, ent-beyer-15-ene-12α,19-diol and ent-atis-13-en-3β,16α-diol, were isolated as minor constituents from the aerial part of Viguiera insignis. The structures were established by biomimetic transformation, spectroscopic means and comparison with closely related compounds.     

Metabolic Engineering of Corynebacterium glutamicum for Trehalose Overproduction: Role of the TreYZ Trehalose Biosynthetic Pathway

Trehalose has many potential applications in biotechnology and the food industry due to its protective effect against environmental stress. Our work explores microbiological production methods based on the capacity of Corynebacterium glutamicum to excrete trehalose. We address here raising trehalose productivity through homologous overexpression of maltooligosyltrehalose synthase and the maltooligosyltrehalose trehalohydrolase genes. In addition, heterologous expression of the UDP-glucose pyrophosphorylase gene from Escherichia coli improved the supply of glycogen. Gene expression effects were tested on enzymatic activities and intracellular glycogen content, as well as on accumulated and excreted trehalose. Overexpression of the treY gene and the treY/treZ synthetic operon significantly increased maltooligosyltrehalose synthase activity, the rate-limiting step, and improved the specific productivity and the final titer of trehalose. Furthermore, a strong decrease was noted in glycogen accumulation. Expression of galU/treY and galU/treYZ synthetic operons showed a partial recovery in the intracellular glycogen levels and a significant improvement in both intra- and extracellular trehalose content.