Variation in Aboveground Cover Influences Soil Nitrogen Availability at Fine Spatial Scales Following Severe Fire in Subalpine Conifer Forests

Following fire, fine-scale variation in early successional vegetation and soil nutrients may influence development of ecosystem structure and function. We studied conifer forests burned by stand-replacing wildfire in Greater Yellowstone (Wyoming, USA) to address two questions: (1) How do the variability and spatial structure of aboveground cover and soil nitrogen availability change during the first 4 years following stand-replacing fire? (2) At fine scales (2–20 m), are postfire soil inorganic N pools and fluxes related to aboveground cover? Aboveground cover, soil N pools, and annual net N transformations were measured from 2001 to 2004 using a spatially explicit sampling design in four 0.25-ha plots that burned during summer 2000. Within-stand variability (coefficient of variation) in postfire live vegetative cover declined with time since fire, whereas variability in bare mineral soil, charred litter and fresh litter was greatest 2-3 years postfire. The soil nitrate pool was more variable than the soil ammonium pool, but annual net nitrification was less variable than annual net N mineralization. Spatial structure (quantified by semivariograms) was observed in some aboveground cover variables (for example, graminoids and fresh litter), but there was little spatial structure in soil N variables and no obvious congruence in spatial scales of autocorrelation for soil N and aboveground cover. Significant Spearman correlations (at the sample point) indicated that aboveground cover and soil N were coupled following severe fire, and the dominant influence was from aboveground cover to soil N, rather than from soil N to vegetation. Initial patterns of fire severity and re-vegetation contributed to fine-scale heterogeneity in soil N availability for at least 4 years after severe fire.     

Influence of food size and food quantity on the feeding of the mussel Dreissena polymorpha

Summary In common with many other suspension feeders, the freshwater mussel Dreissena polymorpha has a maximum filtration rate at low food concentrations and a maximum ingestion rate at high food concentrations. These high rates, which reflect the potential maximum food uptake of the animal, are called the filtration capacity and the ingestion capacity respectively. The ingestion capacity was attained without forming pseudofaeces with Chlamydomonas reinhardii as food. The incipient limiting level could be calculated as the quotient of these two values. A decrease of the filtration rate at high food concentrations was correlated with changes in pumping activity, which showed more frequent interruptions, or a lower level of water transport. Dreissena can filter out particles of diameter greater than 0.7 m from the water. Retention reaches a plateau at about 5 m particle diameter. Scanning electron micrographs of the arrangement of the cilia on the gill filaments are given.     

Purification and characterization of the enantioselective esterase from Kluyveromyces marxianus CBS 1553

An intracellular esterase from the yeast Kluyveromyces marxianus CBS 1553 with interesting enantioselective hydrolytic activity towards racemic esters of 1,2-O-isopropylidene glycerol (IPG) was purified and characterized. Optimal culture conditions for the obtainment of the enantioselective esterase on a 5 l-fermentation scale were investigated. Two esterase activities (EST1 and EST2) in the crude cell extract were identified by native PAGE with specific activity staining and separated from each other by anion-exchange chromatography. EST1 showed higher activity and enantioselectivity than EST2 in the resolution of racemic IPG acetate and was further purified by hydrophobic interaction chromatography and preparative electrophoresis (final specific activity approximately = 300 U mg(-1), showing a main protein band with a molecular mass of 29 kDa. EST1 showed optimal activity between pH 8.0 and 10.0 and was stable in the pH range 7.0-10.0. Moreover, it was rather thermostable and active up to 80 degrees C, and retained most of its activity in the presence of 15% (v/v) of various organic solvents. The enzyme showed similar Vmax in the hydrolysis of the acetate esters of IPG, whereas the Km value towards (S)-IPG acetate was significantly lower than the one towards the (R)-enantiomer (5.3 and 70 microM, respectively). Finally, comparison of EST1 activity in the presence of different glycerol esters and synthetic substrates with different chain lengths showed a strong preference of this biocatalyst for short-chain substrates.     

PHOTOSYNTHETIC FUNCTION IN DUNALIELLA TERTIOLECTA (CHLOROPHYTA) DURING A NITROGEN STARVATION AND RECOVERY CYCLE

Phytoplankton can be exposed to periods of N starvation with episodic N resupply. N starvation in Dunaliella tertiolecta (Butcher) measured over 4 days was characterized by slow reduction in cell chl and protein content and chl/carotenoid ratio and a decline in photosynthetic capacity and maximum quantum yield of photosynthesis (F_v/F_m). In the early stages of N starvation, cell division was maintained despite reduction in cellular chl. Chl content was more sensitive than carotenoids to N deprivation, and cellular chl a was maintained preferentially over chl b under N starvation. NO₃^? resupply stimulated rapid and complete recovery of F_v/F_m (from 0.4 to 0.7) within 24 h and commencement of cell division after 10 h, although N‐replete levels of cell chl and protein were not reestablished within 24 h. Recovery of F_v/F_m was correlated with increases in cell chl and protein and was more related to increases in F_m than to changes in F₀. Recovery of F_v/F_m was biphasic with a second phase of recovery commencing 4–6 h after resupply of NO₃^?. Uptake of NO₃^? from the external medium and the recovery of F_v/F_m, cell chl, and protein were inhibited when either cytosolic or chloroplastic protein synthesis was inhibited by cycloheximide or lincomycin, respectively; a time lag observed before maximum NO₃^? uptake was consistent with synthesis of NO₃^? transporters and assimilation enzymes. When both chloroplastic and cytosolic translation was inhibited, F_v/F_m declined dramatically. Dunaliella tertiolecta demonstrated a capacity to rapidly reestablish photosynthetic function and initiate cell division after N resupply, an important strategy in competing for limiting inorganic N resources.     

Comparative genomics of Salmonella enterica serovar Typhi strains Ty2 and CT18

We present the 4.8-Mb complete genome sequence of Salmonella enterica serovar Typhi strain Ty2, a human-specific pathogen causing typhoid fever. A comparison with the genome sequence of recently isolated S. enterica serovar Typhi strain CT18 showed that 29 of the 4,646 predicted genes in Ty2 are unique to this strain, while 84 genes are unique to CT18. Both genomes contain more than 200 pseudogenes; 9 of these genes in CT18 are intact in Ty2, while 11 intact CT18 genes are pseudogenes in Ty2. A half-genome interreplichore inversion in Ty2 relative to CT18 was confirmed. The two strains exhibit differences in prophages, insertion sequences, and island structures. While CT18 carries two plasmids, one conferring multiple drug resistance, Ty2 has no plasmids and is sensitive to antibiotics.     

Random amplified polymorphic DNA markers reveal genetic variation in the symbiotic fungus of leaf-cutting ants

RAPD markers were used to examine the degree of genetic variation within the putatively asexual basidiomycete fungus (Lepiotaceae: provisionally named Leucoagaricus gongylophorus) associated with the leaf-cutting ant species Atta cephalotes. We analyzed fungal isolates from ant nests in two geographically distant sites, two isolates from Panama and five isolates from Trinidad. Ten decamer primers were used to amplify total DNA from these seven fungal isolates, and RAPD banding patterns were compared. Genetic similarity among isolates was determined by pair-wise comparisons of the shared number of DNA bands on an agarose gel. There was considerable genetic variation among isolates of the symbiotic fungus even within sites. Pairs of fungal isolates from the two different sites shared an average of only 36% of the bands in their RAPD profiles, while pairs from the within sites shared an average of 72% of the bands. RAPD markers may be useful for further investigation of the genetic structure of the fungal symbiont within species of leaf-cutting ants.     

Human Papillomavirus Type 11 Recombinant L1 Capsomeres Induce Virus-Neutralizing Antibodies

The human papillomavirus type 11 (HPV-11) L1 major capsid protein can be trypsinized to generate recombinant capsomeres that retain HPV genotype-restricted capsid antigenicity (M. Li, T. P. Cripe, P. A. Estes, M. K. Lyon, R. C. Rose, and R. L. Garcea, J. Virol. 71:2988–2995, 1997). In the present study, HPV-11 virion-neutralizing monoclonal antibodies H11.F1 and H11.H3, previously characterized as recognizing two distinct HPV-11 capsid-neutralizing antigenic domains (S. W. Ludmerer, D. Benincasa, and G. E. Mark III, J. Virol. 70:4791–4794, 1996), were each found to be highly immunoreactive with trypsin-generated capsomeres in an enzyme-linked immunosorbent assay (ELISA). Capsomeres were used to generate high-titer polyclonal immune sera that demonstrated HPV genotype-restricted reactivity by ELISA. The capsomere antisera were then tested in an in vitro infectivity assay and found to neutralize HPV-11 virions. In this assay, HPV-11 capsomere polyclonal antisera exhibited neutralization titers (10⁻⁵ to 10⁻⁶) comparable to those obtained with a virion-neutralizing antiserum raised previously against intact HPV-11 VLPs (R. C. Rose, R. C. Reichman, and W. Bonnez, J. Gen. Virol. 75:2075–2079, 1994). These results indicate that highly immunogenic, genotype-restricted HPV capsid-neutralizing antigenic domains are contained entirely within capsomeres. Thus, capsomeres may be viable vaccine candidates for the prevention of HPV disease.     

Protein aggregation and amyloid fibril formation by an SH3 domain probed by limited proteolysis

The SH3 domains are small protein modules of 60-85 amino acid residues that are found in many proteins involved in intracellular signal transduction. The SH3 domain of the p85alpha subunit of bovine phosphatidylinositol 3'-kinase (PI3-SH3) under acidic solution adopts a compact denatured state from which amyloid fibrils are readily formed. This aggregation process has been found to be modulated substantially by solution conditions. Here, we have analyzed the conformational features of the native and acid denatured states of PI3-SH3 by limited proteolysis experiments using proteinase K and pepsin, respectively. Moreover, we have analyzed the propensity of PI3-SH3 to be hydrolyzed by pepsin at different stages in the process of aggregation and amyloid formation at pH 1.2 and 2.0 and compared the sites of proteolysis under these conditions with the conformational features of both native and aggregated PI3-SH3. The results demonstrate that the denatured state of PI3-SH3 formed at low pH is relatively resistant to proteolysis, indicating that it is partially folded. The long loop connecting beta-strands b and c in the native protein is the region in this structure most susceptible to proteolysis. Remarkably, aggregates of PI3-SH3 that are formed initially from this denatured state in acid solution display enhanced susceptibility to proteolysis of the long loop, suggesting that the protein becomes more unfolded in the early stages of aggregation. By contrast, the more defined amyloid fibrils that are formed over longer periods of time are completely resistant to proteolysis. We suggest that the protein aggregates formed initially are relatively dynamic species that are able readily to reorganize their interactions to enable formation of very well ordered fibrillar structures. In addition, the disordered and non-native character of the polypeptide chains in the early aggregates could be important in determining the high cytotoxicity that has been revealed in previous studies of these species.