Stereoselectivity of the membrane potential-generating citrate and malate transporters of lactic acid bacteria.

The citrate transporter of Leuconostoc mesenteroides (CitP) and the malate transporter of Lactococcus lactis (MleP) are homologous proteins that catalyze citrate-lactate and malate-lactate exchange, respectively. Both transporters transport a range of substrates that contain the 2-hydroxycarboxylate motif, HO-CR(2)-COO(-) [Bandell, M., et al. (1997) J. Biol. Chem. 272, 18140-18146]. In this study, we have analyzed binding and translocation properties of CitP and MleP for a wide variety of substrates and substrate analogues. Modification of the OH or the COO(-) groups of the 2-hydroxycarboxylate motif drastically reduced the affinity of the transporters for the substrates, indicating their relevance in substrate recognition. Both CitP and MleP were strictly stereoselective when the R group contained a second carboxylate group; the S-enantiomers were efficiently bound and translocated, while the transporters had no affinity for the R-enantiomers. The affinity of the S-enantiomers, and of citrate, was at least 1 order of magnitude higher than for lactate and other substrates with uncharged R groups, indicating a specific interaction between the second carboxylate group and the protein that is responsible for high-affinity binding. MleP was not stereoselective in binding when the R groups are hydrophobic and as large as a benzyl group. However, only the S-enantiomers were translocated by MleP. CitP had a strong preference for binding and translocating the R-enantiomers of substrates with large hydrophobic R groups. These differences between CitP and MleP explain why citrate is a substrate of CitP and not of MleP. The results are discussed in the context of a model for the interaction between sites on the protein and functional groups on the substrates in the binding pockets of the two proteins.     

Identification of Culturable Oligotrophic Bacteria within Naturally Occurring Bacterioplankton Communities of the Ligurian Sea by 16S rRNA Sequencing and Probing

Abstract Typical marine bacteria (i.e., obligately oligotrophic) that were numerically dominant members of naturally occurring marine communities were identified by cloning and sequencing the amplified 16S rRNA genes obtained from dilution cultures of the original samples. The data reported here refer to two different habitats of a marine pelagic environment (28 miles offshore, in the northwestern Mediterranean Sea). The samples were taken from the water column at two representative layers, i.e., the 30-m depth, corresponding to the chlorophyll maximum layer, and the 1800-m depth, representative of a deep, oligotrophic environment. Three major lineages were found in the 16S rDNA clone libraries prepared from the two samples, two of which could be assigned to the Vibrio and the Rhodobacter groups. The third lineage was a distant relative of the genus Flavobacterium, but it was not closely related to any marine isolate. Six oligonucleotide probes, either complementary to the conserved sequence domains or selectively hybridizing to the clone sequences, were designed for use as hybridization group-specific and strain-specific probes. A single-mismatch discrimination between certain probes and nontarget sequences was demonstrated by detecting the probes' specificity at different hybridization and washing conditions. The screening of the clone libraries with the obtained probes revealed that neither the 30-m sample higher dilution nor the 1800-m one were pure cultures. While some representatives of the Vibrio group were found in both the surface and the deep sample, the members of the Flavobacterium and Rhodobacter lineages were detected only in the deep and the euphotic layers, respectively. We suggest an approach for analyzing autochthonous marine bacteria able to grow in unamended seawater. Received: 19 May 1998; Accepted: 29 October 1998     

New Gene Crt(Curly Roots) Controlling Pea (Pisum sativum L.) Root Development

Using ethyl methane sulfonate (EMS) treatment of the seeds ofline SGE, a new mutant of pea (Pisum sativum L.) with alterationsin root development was obtained. The mutant phenotype dependson the density of the growth substrate: on sand (a high densitysubstrate) the mutant forms a small compact curly root systemwhereas on vermiculite (a low density substrate) differencesbetween the root systems of the mutant and wild type plantsare less pronounced. Genetic analysis revealed that the mutantcarries a mutation in a new pea gene designedcrt (curly roots).Gene crt has been localized in pea linkage group V. The mutantline named SGEcrt showed increased sensitivity to exogenousauxin and an increased concentration of endogenous indole-3-aceticacid (IAA) in comparison with the wild type line SGE. Copyright2000 Annals of Botany Company Pisum sativum L., root development, garden pea mutant, curly roots, auxin, environmental stimulus response     

On Branching Processes and the Early Stages of the Spread of an Epidemic

Branchingprocess(BP)isusedtomodeltheearlystagesofthespreadofasexuallytransmitteddisease.TheearlystagesofAIDSspreadwhichistransmittedbothhomosexuallyandheterosexuallyarestudiedasaBP.     

Investigation of factors influencing drug release using calcium alginate polymer

The effects of mixing, the sodium alginate concentration, and calcium chloride concentration on the release of sulphamethoxazole (model drug) impregnated in calcium alginate beads were investigated and evaluated. The release behaviour of the sulphamethoxazole from the calcium alginate beads was studied in a 0.1N HCl aqueous solution at 37v°C. The release rate of the sulphamethoxazole depends heavily on the type of mixers during the formation of the drug-alginate beads. The highest release rate was achieved when four-bladed rectangular agitator was used while the lowest release was achieved when magnetic stirrer was used. The amount of the released sulphamethoxazole varies slightly with the variation of the alginate concentration. The total release of sulphamethoxazole when 1% w/v solution of sodium alginate was used found to be 80% of the total drug content while 72% and 68% of the total drug content for 1.5% and 2% sodium alginate solutions. Three different calcium chloride concentrations were used (i.e., 5%, 10%, and 15% CaCl₂). The effect of the calcium chloride concentration on the release of the sulphamethoxazole is very pronounced.     

Population cycles can maintain foraging polymorphism

The maintenance of genetic variability in morphological traits that affect fitness is poorly understood. We present a simple Mendelian model of genetic traits affecting foraging efficiency in grazing ungulates, based on a trade-off between rates of energy extraction at low versus high levels of plant abundance. The model suggests that variation in foraging efficiency could be maintained via lottery competition arising as a direct consequence of dynamically unstable interactions between consumers and their food resources. Lottery competition is a plausible mechanism for explaining wide variability in foraging efficiency, such as that documented in unstable Soay sheep populations on the St Kilda archipelago.