Chemical and ultrastructural studies on the cell walls of the yeastlike and mycelial forms of Histoplasma farciminosum

The cell wall of the yeast form of Histoplasma farciminosum contains 13.2% beta-1,3-glucan, 1.0% galactomannan, and 25.8% chitin, whereas the cell wall of mycelial form has 21.8, 4.5, and 40%, respectively, for the same polymers. Also, the cell wall of the yeast form contains alpha-1,3-glucan (13.5%) and an unidentified polymer (21.5%). Chitin, one of the structural polymers of both yeast and mycelial cell walls, is identified as thin isolated fibers (4 nm wide) or in thick bundles (50 nm wide) of fibers. beta-(1-3)-Glucan is also found as thin isolated fibers indistinguishable from isolated fibers of chitin. Fibers 14 nm wide and resembling alpha-(1-3)-glucan fibers of other fungi are found in the yeast form. The results reported here do not give support to the proposal for a different taxonomic classification.     

The cell wall of fungal human pathogens: Its possible role in host-parasite relationshipsA review

The ability of some fungi to cause disease in animals, and particularly in humans, appears to be related to some peculiar trait of their metabolism not shared by taxonomically related species, so that among thousands of species of fungi, only some 100 are considered pathogenic for humans. Recently, there has been an increase in the basic research aimed at elucidating the physiology, biochemistry, and mechanisms of pathogenicity of the fungi in addition to the responses of the infected host. The results from these studies are often difficult to compare because of the lack of reference cultures, and because variability among laboratories are found for many parameters such as origin of isolates, growth conditions, and fractionation procedures used to obtain chemically defined components of cell walls. This review deals with cell wall structures and their influence in host-parasite relationships in those relatively few pathogenic fungi on which substantial literature exists. Serological properties of the walls are included when available. Readers interested in medical and biological aspects of fungi may consult Rippon's (140) and Emmonset al.'s (51) textbooks, among others. Some reviews on microbial surfaces and their relationships to pathogenicity have been published (160–162). The pathogenic fungi included in this review are classified according to their cell wall structure, following Bartnicki-García's criteria (15) on the distribution of the two major polysaccharides within the fungal cell wall. This classification is based on the fact that fungi may be subdivided into various categories according to the chemical nature of their walls, and that these categories closely parallel conventional taxonomic boundaries. As seen in Table 1, this classification is based on dual combinations of those polysaccharides which appear to be the principal component of vegetative walls, disregarding the presence of small amounts of other classifying polysaccharides. The few pathogenic fungi whose cell walls have been studied to some extent belong to Bartnicki-García's categories 5 (chitin-glucan) and 6 (mannan-glucan) (Table 2). Besides these main polysaccharides, some minor components of the cell wall may also play important roles in pathogenicity, as antigens, or as virulence factors, and they will be mentioned in this review.     

Mechanisms of degradation of the fungal ornithine decarboxylase

Ornithine decarboxylase (ODC) is the first enzyme in polyamine biosynthesis in numerous living organisms, from bacteria to mammalian cells. Its control is under negative feedback regulation by the end products of the pathway. In dimorphic fungi, ODC activity and therefore polyamine concentrations are related to the morphogenetic process. From the fission yeast Schizosaccharomyces pombe to human, polyamines induce antizyme synthesis which in turn inactivates ODC. This is hydrolyzed by the 26S proteasome without ubiquitination. The regulatory mechanism of antizyme on polyamines is conserved, although to date no antizyme homology has been identified in some fungal species. The components that are responsible for regulating polyamine levels in cells and the current knowledge of ODC regulation in dimorphic fungi are presented in this review. ODC degradation is of particular interest because inhibitors of this pathway may lead to the discovery of novel antifungal drugs.     

Cyclization of the Urokinase Receptor-Derived Ser-Arg-Ser-Arg-Tyr Peptide Generates a Potent Inhibitor of Trans-Endothelial Migration of Monocytes

The receptor for the urokinase-type plasminogen activator (uPAR) is a widely recognized master regulator of cell migration and uPAR_88-92 is the minimal sequence required to induce cell motility. We and others have previously documented that the uPAR_88-92 sequence, even in the form of synthetic linear peptide (SRSRY), interacts with the formyl peptide receptor type 1 (FPR1), henceforth inducing cell migration of several cell lines, including monocytes. FPR1 is mainly expressed by mammalian phagocytic leukocytes and plays a crucial role in chemotaxis. In this study, we present evidence that the cyclization of the SRSRY sequence generates a new potent and stable inhibitor of monocyte trafficking. In rat basophilic leukaemia RBL-2H3/ETFR cells expressing high levels of constitutively activated FPR1, the cyclic SRSRY peptide ([SRSRY]) blocks FPR1 mediated cell migration by interfering with both internalization and ligand-uptake of FPR1. Similarly to RBL-2H3/ETFR cells, [SRSRY] competes with fMLF for binding to FPR1 and prevents agonist-induced FPR1 internalization in human monocyte THP-1 cells. Unlike scramble [RSSYR], [SRSRY] inhibits fMLF-directed migration of monocytes in a dose-dependent manner, with IC₅₀ value of 0.01 nM. PMA-differentiated THP-1 cell exposure to fMLF gradient causes a marked cytoskeletal re-organization with the formation of F-actin rich pseudopodia that are prevented by the addition of [SRSRY]. Furthermore, [SRSRY] prevents migration of human primary monocytes and trans-endothelial migration of monocytes. Our findings indicate that [SRSRY] is a new FPR1 inhibitor which may suggest the development of new drugs for treating pathological conditions sustained by increased motility of monocytes, such as chronic inflammatory diseases.     

Topological change of Andean plant–pollinator networks along an altitudinal gradient

Pollination interaction networks exhibit structural regularities across a wide range of natural environments. Long-tailed degree distribution, nestedness, and modularity are the most prevalent topological patterns found in most bipartite networks analyzed up to day. In this work we evaluate the variation of these topological properties along an altitudinal gradient. To this end, we examined four plant–pollinator networks from the Chilean Andes at 33°S, in range from 1800 to 3600 m elevation. Our results indicate that network topology is strongly and systematically affected by elevation. At increasing altitude, the number of potential visitors per plant decreased, and species’ degree distributions are closer to random expectations. On the other hand, the nested structure of mutualistic interactions systematically decreased with elevation, and network modularity was significantly higher than random expectations over the entire altitudinal range. In addition, at increasing elevations the pollination networks were organized in fewer and more strongly connected modules. Our results suggest that the severe abiotic conditions found at increased elevations translate into less organized pollination networks.     

The actin gene in Paracoccidioides brasiliensis: organization, expression and phylogenetic analyses

PbrACT1, the gene responsible for the synthesis of actin in Paracoccidioides brasiliensis, was found as a single copy, organized into six exons and five introns. Its open reading frame (ORF) codes for a putative protein of 375 amino acids, with a molecular mass of 41.5 kDa and an isoelectric point of 5.6. Analysis of the nucleotide sequence revealed a high homology to other fungal actins, the presence of characteristic fungal actin sequences, and heat shock elements at the 5′ untranslated region (UTR). Phylogenetic analyses with deduced amino acid sequences of fungal actins grouped P. brasiliensis within the phylum Ascomycota, order Onygenales, in concordance with a few previous reports. Patterns of expression through the temperature-induced morphological transitions from mycelial to yeast-like shapes and reverse, suggests that PbrACT1 is regulated in this process. The PbrACT1 gene sequence is available at the GenBank database under accession number AY383732.     

Biosurfactant production by two isolates ofPseudomonas aeruginosa

Two strains of biosurfactant-producing bacteria, identified asPseudomonas aeruginosa, were isolated from injection water and crude oil-associated water in Venezuelan oil fields. Both biosurfactants resembled rhamnolipids and produced stable emulsions of heavy and extra-heavy crude oils, reducing the surface tension of water from 72 to 28 dynes/cm. Tenso-active properties of the biosurfactants were not affected by pH, temperature, salinity or Ca²⁺ or Mg²⁺ at concentrations in excess of those found in many oil reservoirs in Venezuela.     

Refinements of calcareous nannofossil biostratigraphy at the Miocene/Pliocene Boundary in the Mediterranean region

The high-resolution quantitative study of the calcareous nannofossil assemblages in two Mediterranean deep-sea successions (ODP Sites 969B and 975B) encompassing the Miocene/Pliocene boundary allows the recognition of a set of reliable bioevents useful to detecting the base of the Zanclean stage. The results have been successfully compared with two on-land sections (Cava Serredi, Tuscany, and Montepetra borehole, Marche Region, Central Italy). This study confirms that Ceratolithus acutus and Triquetrorhabdulus rugosus, the markers traditionally used for identifying the Miocene/Pliocene Boundary are very rare in the Mediterranean area and cannot be used for biostratigraphic correlation. Conversely, the absence interval (paracme) of Reticulofenestra pseudoumbilicus and the distribution range of a new species belonging to the Reticulofenestra genus (Reticulofenestra zancleana nov. sp.) show high biostratigraphical potential. The position of the new biohorizons has been compared to those of the planktonic foraminifers events, and correlated to the CaCO₃ cycles reconstructed for the two sections. On the basis of these new nannofossil bioevents, Rio et al.’s (1990) MNN12 biozone is subdivided into three subzones, thus improving the biostratigraphic resolution of the Early Pliocene.