Shading results in depletion of the soil seed bank

To what extent is the decline of characteristic dry alvar species related to the decline in the soil seed bank during scrub encroachment? We recorded the number of flower stems in the vegetation in relation to light attenuation along an encroachment series progressing from open alvar through small gaps inside dense scrub of Juniperus communis (cover of 60%) to intact dense scrub (cover of 100%) on the island of Öland, Sweden. This measurement of potential reproduction (number of flower stems) was then compared to the number of species in the soil seed bank at each site along the alvar encroachment series. Scrub encroachment results in light attenuation between and under the shrubs. The total number of flower stems averaged over all species was similar between gaps and open alvar, but that of alvar species was significantly lower in the gaps, indicating that light attenuation could reduce their seed set. Shading in gaps is related to depletion of the soil seed bank and loss of alvar species, particularly those that do not form a persistent soil seed bank.     

Novel Staphylococcal Glycosyltransferases SdgA and SdgB Mediate Immunogenicity and Protection of Virulence-Associated Cell Wall Proteins

Infection of host tissues by Staphylococcus aureus and S. epidermidis requires an unusual family of staphylococcal adhesive proteins that contain long stretches of serine-aspartate dipeptide-repeats (SDR). The prototype member of this family is clumping factor A (ClfA), a key virulence factor that mediates adhesion to host tissues by binding to extracellular matrix proteins such as fibrinogen. However, the biological siginificance of the SDR-domain and its implication for pathogenesis remain poorly understood. Here, we identified two novel bacterial glycosyltransferases, SdgA and SdgB, which modify all SDR-proteins in these two bacterial species. Genetic and biochemical data demonstrated that these two glycosyltransferases directly bind and covalently link N-acetylglucosamine (GlcNAc) moieties to the SDR-domain in a step-wise manner, with SdgB appending the sugar residues proximal to the target Ser-Asp repeats, followed by additional modification by SdgA. GlcNAc-modification of SDR-proteins by SdgB creates an immunodominant epitope for highly opsonic human antibodies, which represent up to 1% of total human IgG. Deletion of these glycosyltransferases renders SDR-proteins vulnerable to proteolysis by human neutrophil-derived cathepsin G. Thus, SdgA and SdgB glycosylate staphylococcal SDR-proteins, which protects them against host proteolytic activity, and yet generates major eptopes for the human anti-staphylococcal antibody response, which may represent an ongoing competition between host and pathogen.     

Identification of Bacillus thuringiensis delta-endotoxin Cry1C domain III amino acid residues involved in insect specificity.

Cry1C domain III amino acid residues involved in specificity for beet armyworm (Spodoptera exigua) were identified. For this purpose, intradomain III hybrids between Cry1E (nontoxic) and Cry1E-Cry1C hybrid G27 (toxic) were made. Crossover points of these hybrids defined six sequence blocks containing between 1 and 19 of the amino acid differences between Cry1E and G27. Blocks B, C, D, and E of G27 were shown to be required for optimal activity against S. exigua. Block E was also required for optimal activity against the tobacco hornworm (Manduca sexta), whereas block D had a negative effect on toxicity for this insect. The mutagenesis of individual amino acids in block B identified Trp-476 as the only amino acid in this block essential, although not sufficient by itself, for full S. exigua activity. In block D, we identified a seven-amino-acid insertion in G27 that was not in Cry1E. The deletion of either one of two groups of four consecutive amino acids in this insertion completely abolished activity against S. exigua but resulted in higher activity against M. sexta. Alanine substitutions of the first group had little effect on toxicity, whereas alanine substitutions of the second group had the same effect as its deletion. These results identify groups of amino acids as well as some individual residues in Cry1C domain III, which are strongly involved in S. exigua-specific activity as well as sometimes involved in M. sexta-specific activity.     

Habitat differentiation vs. isolation-by-distance: the genetic population structure of Elymus athericus in European salt marshes

We investigated genetic differentiation among populations of the clonal grass Elymus athericus, a common salt-marsh species occurring along the Wadden Sea coast of Europe. While E. athericus traditionally occurs in the high salt marsh, it recently also invaded lower parts of the marsh. In one of the first analyses of the genetic population structure in salt-marsh species, we were interested in population differentiation through isolation-by-distance, and among strongly divergent habitats (low and high marsh) in this wind- and water-dispersed species. High and low marsh habitats were sampled at six sites throughout the Wadden Sea. Based on reciprocal transplantation experiments conducted earlier revealing lower survival of foreign genotypes we predicted reduced gene flow among habitats. Accordingly, an analysis with polymorphic cross-species microsatellite primers revealed significant genetic differentiation between high and low marsh habitats already on a very small scale (< 100 m), while isolation-by-distance was present only on larger scales (60-443 km). In an analysis of molecular variance we found that 14% of the genetic variance could be explained by the differentiation between habitats, as compared to only 8.9% to geographical (isolation-by-distance) effects among six sites 2.5-443 km distant from each other. This suggests that markedly different selection regimes between these habitats, in particular intraspecific competition and herbivory, result in habitat adaptation and restricted gene flow over distances as small as 80 m. Hence, the genetic population structure of plant species can only be understood when considering geographical and selection-mediated restrictions to gene flow simultaneously.     

NAD+ binding to the Escherichia coli K+-uptake protein TrkA and sequence similarity between TrkA and domains of a family of dehydrogenases suggest a role for NAD+ in bacterial transport

The nucleotide sequence of trkA, a gene encoding a surface component of the constitutive K⁺-uptake systems TrkG and TrkH from Escherichia coli, was determined. The structure of the TrkA protein deduced from the nucleotide sequence accords with the view that TrkA is peripherally bound to the inner side of the cytoplasmic membrane. Analysis by a dot matrix revealed that TrkA is composed of similar halves. The M-terminal part of each TrkA half (residues 1–130 and 234–355, respectively) is similar to the complete NAD⁺-binding domain of NAD⁺-dependent dehydrogenases. The C-terminal part of each TrkA half (residues 131–233 and 357–458, respectively) aligns with the first 100 residues of the catalytic domain of glyceraldehyde-3-phosphate dehydrogenase. Strong u.v. illumination at 252 nm led to cross-linking of NAD⁺ or NADH, but not of ATP to the isolated TrkA protein.     

Hereditary nonpolyposis colorectal cancer families not complying with the Amsterdam criteria show extremely low frequency of mismatch-repair-gene mutations.

Hereditary nonpolyposis colorectal cancer (HNPCC) is a common autosomal dominant cancer-susceptibility condition characterized by early onset colorectal cancer. Germ-line mutations in one of four DNA mismatch repair (MMR) genes, hMSH2, hMLH1, hPMS1, or hPMS2, are known to cause HNPCC. Although many mutations in these genes have been found in HNPCC kindreds complying with the so-called Amsterdam criteria, little is known about the involvement of these genes in families not satisfying these criteria but showing clear-cut familial clustering of colorectal cancer and other cancers. Here, we applied denaturing gradient-gel electrophoresis to screen for hMSH2 and hMLH1 mutations in two sets of HNPCC families, one set comprising families strictly complying with the Amsterdam criteria and another set in which at least one of the criteria was not satisfied. Interestingly, hMSH2 and hMLH1 mutations were found in 49% of the kindreds fully complying with the Amsterdam criteria, whereas a disease-causing mutation could be identified in only 8% of the families in which the criteria were not satisfied fully. In correspondence with these findings, 4 of 6 colorectal tumors from patients belonging to kindreds meeting the criteria showed microsatellite instability, whereas only 3 of 11 tumors from the other set of families demonstrated this instability. Although the number of tumors included in the study admittedly is small, the frequencies of mutations in the MMR genes show obvious differences between the two clinical sets of families. These results also emphasize the practical importance of the Amsterdam criteria, which provide a valid clinical subdivision between families, on the basis of their chance of carrying an hMSH2 or an hMLH1 mutation, and which bear important consequences for genetic testing and counseling and for the management of colorectal cancer families.     

Body composition and energetic efficiency in two lines of mice selected for rapid growth rate and their F1 crosses

Summary Correlated responses to selection for increased growth rate were compared in two mouse populations (M16 and H₆) of distinct genetic origin. Traits studied were body composition, feed intake, constituent gains and energetic efficiency. When compared with their respective controls (ICR and C₂) at 6 and 9 weeks of age, body weight increased more in M16 (57%and 69 % of the control mean) than in H₆ (40 % and 34%). The M16 showed correlated responses in fat percent of 2.6% (P <.05), 8.4% (P <.01) and 11.2% (P <.01) at 3, 6 and 9 weeks, respectively, whereas corresponding values in H₆ were –2.4% (P <.05), 3.3% (P <.05) and 2.09 % (P >.05). The correlated responses in fat percent were 2.7 and 4.7 times higher in M16 than H₆ at 6 and 9 weeks. The regression of ln fat weight on ln empty body weight was larger in M16 (P <.05) compared to ICR and larger (P <.01) in H₆ compared to C₂. Both M16 and H₈ exhibited positive correlated responses from 3 to 6 weeks of age in feed intake and gain and efficiency in fat, protein, calories and ash; fat and caloric gain and efficiency exhibited higher correlated responses in M16 than H₆. During the 6- to 9-week interval, the M16 population continued to evince positive correlated responses in gains and efficiencies of fat, protein and calories, whereas H₆ did not. Several possible explanations are presented to account for the differences in correlated responses between the selected populations. Partitioning of correlated response differences between M16 and H₆ into average direct and average maternal genetic effects indicated that average direct genetic effects, favoring M16, were responsible for the major difference between the selected populations. Direct heterosis in F₁ crosses of the selected populations were generally not significant, although there was a tendency for fat percent and fat weight to show heterosis.Paper No. 4929 of the Journal Series of the North Carolina Agricultural Experiment Station, Raleigh, North Carolina 27607, Animal Research Institute Contribution No. 624 and Agricultural University at Wageningen Contribution No. 654-490-10. The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Experiment Station of the Products named, nor criticism of similar ones not mentioned.On leave from Department of Animal Husbandry, Agricultural University, Wageningen, The Netherlands.On leave from Animal Research Institute, Agriculture Canada, Ottawa, Ontario K1A OC6.