Electrostatic coupling of ion pumps.

In this paper the electrostatic interactions between membrane-embedded ion-pumps and their consequences for the kinetics of pump-mediated transport processes have been examined. We show that the time course of an intrinsically monomolecular transport reaction can become distinctly nonexponential, if the reaction is associated with charge translocation and takes place in an aggregate of pump molecules. First we consider the electrostatic coupling of a single dimer of ion-pumps embedded in the membrane. Then we apply the treatment to the kinetic analysis of light-driven proton transport by bacteriorhodopsin which forms two-dimensional hexagonal lattices. Finally, for the case of nonordered molecules, we also consider a model in which the pumps are randomly distributed over the nodes of a lattice. Here the average distance is equal to that deduced experimentally and the elemental size of the lattice is the effective diameter of one single pump. This latter model is applied to an aggregate of membrane-embedded Na, K- and Ca-pumps. In all these cases the electrostatic potential considered is the exact solution calculated from the method of electrical images for a plane membrane of finite thickness immersed in an infinite aqueous solution environment. The distributions of charges (ions or charged binding sites) are considered homogeneous or discrete in the membrane and/or in the external solution. In the case of discrete distributions we compare the results from a mean field approximation and a stochastic simulation.     

Life history patterns of river invertebrates

The initiation of invertebrate distribution patterns in rivers occurs by choice of oviposition sites and is influenced by the evolved reproductive strategies of the individual species. Subsequent redistribution by migration or drifting establishes patterns which are then modified by environmental influences on growth and mortality. Continuity of life cycles is sustained by variations on a number of defined life history strategies combined with evolved behavioural responses.     

Primary sequence and functional expression of a novel ouabain-resistant Na,K-ATPase. The beta subunit modulates potassium activation of the Na,K-pump.

In order to understand the molecular mechanism of ouabain resistance in the toad Bufo marinus, Na,K-ATPase alpha and beta subunits have been cloned and their functional properties tested in the Xenopus laevis oocyte expression system. According to sequence comparison between species, alpha 1, beta 1, and beta 3 isoforms were identified in a clonal toad urinary bladder cell line (TBM 18-23). The sequence of the alpha 1 isoform is characterized by two positively charged amino acids (Arg, Lys) at the N-terminal border of the H1-H2 extracellular loop and no charged amino acid at the C terminus, a pattern distinct from the ouabain-resistant rat alpha 1 isoform. The coexpression of alpha 1 beta 1 or alpha 1 beta 3 TBM subunits in the Xenopus oocyte resulted in the expression of identical maximum Na,K-pump currents with identical inhibition constant for ouabain (Ki) (alpha 1 beta 1: 53 +/- 3 microM; n = 7 vs. alpha 1 beta 3: 57 +/- 3.0 microM; n = 8) but distinct potassium half activation constant (K1/2) (alpha 1 beta 1: 0.87 +/- 0.08 mM, n = 16; alpha 1 beta 3: 1.29 +/- 0.07 mM, n = 17; p less than 0.005). We conclude that (i) the TBM alpha 1 isoform is necessary and sufficient to confer the ouabain resistant phenotype; (ii) the beta 3 or beta 1 subunit can associate with the alpha 1 equally well without affecting the ouabain-resistant phenotype; (iii) some specific sequence of the beta subunit can modulate the activation of the Na,K-pump by extracellular potassium ions.     

The P2 element of the td intron is dispensable despite its normal role in splicing.

The P2 region of group I introns has been proposed to be involved in the correct positioning of the P1 5'-splice site duplex in the catalytic core (Michel, F., and Westhof, E. (1990) J. Mol. Biol. 216, 585-610). The behavior of delta P2 deletion mutants of the td intron is consistent with this hypothesis. The delta P2 mutants are capable of site-specific hydrolysis, indicating that the conformation of the ribozyme is not grossly altered, but they are incapable of transesterification reactions at the splice sites, as would be predicted if P1 is not appropriately aligned within the catalytic core. Nevertheless, the function of the P2 element can be bypassed in specific pseudorevertants isolated by genetic selection from the delta P2 mutants. These results, together with phylogenetic data, support the existence of alternate strategies to create a functional P1-core interaction.     

Immunizations for foreign travel.

One of the most important aspects of preparing travelers for destinations throughout the world is providing them with immunizations. Before administering any vaccines, however, a careful health and immunization history and travel itinerary should be obtained in order to determine vaccine indications and contraindications. There are three categories of immunizations for foreign travel. The first category includes immunizations which are routinely recommended whether or not the individual is traveling. Many travelers are due for primary vaccination or boosting against tetanus-diphtheria, measles-mumps-rubella, pneumococcal pneumonia, and influenza, for example, and the pre-travel visit is an ideal time to administer these. The second category are immunizations which might be required by a country as a condition for entry; these are yellow fever and cholera. The final category contains immunizations which are recommended because there is a risk of acquiring a particular disease during travel. Typhoid fever, meningococcal disease, rabies, and hepatitis are some examples. Travelers who are pregnant or who are infected with the human immunodeficiency virus require special consideration. Provision of appropriate immunizations for foreign travel is an important aspect of preventing illness in travelers.     

Electrical impedance of white spruce shoots in relation to pressure-volume analysis and free sugar content

Electrical impedance measurements made on white spruce, Picea glauca (Moench) Voss, stems were related to shoot free sugar contents and to osmotic, turgor and water potential. During seasonal dormancy induction, there were commensurate increases in free sugar contents, osmotic potential at full turgor and impedance which resulted in linear relationships among these variables. When measured over the course of laboratory drying, impedance increased curvilinearly with decreasing relative water content. There was a linear increase in impedance with decreasing water potential, with a break point coincident with the turgor loss point, possibly attributed to disruption to current flow through broken plasmodesmatal connections between adjacent cells. This technique offers a non-destructive method to measure tissue free sugar content, and therefore, short- and long-term shifts in parameters historically derived from pressure-volume analysis.