Cooperativity in ligand binding: a new graphic analysis.

When analyzing binding of ligands to macromolecules, the existence of site-site interactions complicates a straightforward interpretation of the binding parameters obtained through classical analytical methods, such as the Scatchard plot. For describing site-site interactions, we propose a new parameter, the $\text{average affinity}$ of the receptor sites, $\text{K}$, calculated as $\text{(}\text{B}\text{F}\text{)/(R}{}_{\mathrm{o}}\text{?B)}$. Plotting $\text{K}$ as a function of fractional occupancy ($\text{B}\text{R}{}_{\mathrm{o}}$), reveals that: (1) at very low occupancy a limiting high $\text{K}$ is obtained ($\text{K}$_e) (“empty sites” conformation); (2) when the fraction of sites filled increases above a certain threshold, $\text{K}$ begins to fall due to increasing site-site interactions until (3) a limiting low $\text{K}$ ($\text{K}$_f) is obtained (“filled sites” conformation). This method has been successfully applied to the negative cooperativity of insulin receptors.     

Structure and invasive behaviour of Plasmodium knowlesi merozoites in vitro.

The structure and invasive behaviour of extracellular erythrocytic merozoites prepared by a cell sieving method have been studied with the electron microscope. Free merozoites contain organelles similar to those described in late schizonts of Plasmodium knowlesi. Their surface is lined by a coat of short filaments. On mixing with fresh red cells, merozoites at first adhere, then cause the red cell surface to invaginate rapidly, often with the formation of narrow membranous channels in the red cell interior. As the merozoite enters the invagination it forms an attachment by its cell coat to the rim of the pit, and finally leaves this coat behind as it is enclosed in a red cell vacuole. Dense, rounded intracellular bodies then move to the merozoite periphery, and apparently rupture to cause further localized invagination of the red cell vacuole. The merozoite finally loses its rhoptries, the pellicle is reduced to a single membrane and the parasite becomes a trophozoite. Invasion is complete by 1 min after adhesion, and the trophozoite is formed by 10 min.     

Xanthine dehydrogenase accumulation in developing Drosophila eyes

Xanthine dehydrogenase activity is assayed by following the oxidation of pterin to isoxanthopterin by spectrofluorometry at the reaction's wavelength peaks: excitation, 344 nm; emission, 412 nm. The method is sensitive to less than 0·1 μU of activity (0·1 pmol/min) and allows the assay of Drosophila imaginal disk homogenates.While the larval eye disk contains less than 0·1 per cent of the individual's XDH, the developing eye becomes a major store, with 30 per cent of the individual's activity by the time of eye pigmentation. The data suggest a basis for the well-known non-autonomous action of the gene rosy, the structural gene for XDH: the enzyme is synthesized in an organ of primary gene expression, and transported through the haemolymph to the eye of the pupa and pharate adult.     

Evolution of sex I. Primitive sex

Sex is a process of fusion of separate hereditary determinants. The advantages which could accrue from such fusions are protection against deleterious mutations and the possibility of combining favorable alleles into a single individual. If the fitness of the aggregate resulting from fusion is greater than its parts there will be strong selective pressure to perpetuate the aggregate in all progeny. Continued fusion presents problems though and new environmental conditions may occur which favor segregation. Segregation is also favored because of the existence of favorable recessive mutations. It is argued that the balance between these alternative goals of phenotype stability versus variety achieved an effective compromise with the development of the meiosis-fusion-mitosis cycle.     

Nonhomologous recombination in mammalian cells: role for short sequence homologies in the joining reaction.

Although DNA breakage and reunion in nonhomologous recombination are poorly understood, previous work suggests that short sequence homologies may play a role in the end-joining step in mammalian cells. To study the mechanism of end joining in more detail, we inserted a polylinker into the simian virus 40 T-antigen intron, cleaved the polylinker with different pairs of restriction enzymes, and transfected the resulting linear molecules into monkey cells. Analysis of 199 independent junctional sequences from seven constructs with different mismatched ends indicates that single-stranded extensions are relatively stable in monkey cells and that the terminal few nucleotides are critical for cell-mediated end joining. Furthermore, these studies define three mechanisms for end joining: single-strand, template-directed, and postrepair ligations. The latter two mechanisms depend on homologous pairing of one to six complementary bases to position the junction. All three mechanisms operate with similar overall efficiencies. The relevance of this work to targeted integration in mammalian cells is discussed.     

Hybrids of sugars and aromatics: A Pd-catalyzed modular approach to chromans and isochromans

Herein we describe the synthesis of highly substituted chromans and isochromans using carbohydrates as starting materials. The key step of our synthetic approach is the annelation of the benzene moiety via a highly efficient Pd-catalyzed domino reaction. This powerful approach led to a small library of highly substituted chromans and isochromans by making use of a variety of different diynes and bromoglycals. We investigated several Pd-catalysts in order to improve the yields and to enlarge the scope of the domino reaction. Furthermore, we elucidated the mechanistic picture of the reaction with isotope-labelling experiments. Most probably the reaction proceeds via an oxidative addition followed by two carbopalladation steps and a final cyclization reaction.     

Phylogenetic island disequilibrium: evidence for ongoing long‐term population dynamics in two Mediterranean butterflies

Aim Our aims were to verify the existence of phylogenetic disequilibrium between butterfly lineages at the subcontinental scale for islands and the nearest mainland and to test the capacity of islands for hosting ancestral populations of butterflies and the significance of such relict populations. Location The western Mediterranean continental area of Europe and North Africa together with several large and small islands (Balearics, Tuscan Archipelago, Aeolian Archipelago, Capri, Sardinia, Sicily, Corsica). Methods Using geometric morphometrics, the shape of male genitalia was analysed in two common butterflies (Pyronia cecilia and Pyronia tithonus), whose spatial heterogeneity in the Mediterranean region has recently been described. Observed patterns in genital shapes were compared with shapes predicted for islands and fossil islands to assess the contribution of historical and current events in accounting for the transition from a refugial model to an equilibrium model. Measurements were taken for 473 specimens in 90 insular and mainland sites. Results The shape of the genitalia of populations of most islands differed substantially from that predicted by the equilibrium hypothesis while closely fitting the refugial hypothesis. The comparison between different models strongly suggests that islands maintain ancestral lineages similar to those living in Spain (P. cecilia) and France (P. tithonus). A high correlation between observed and predicted patterns on islands and fossil islands occurs during the first steps of modelled introgressive hybridization while the following steps exposed a successively lower fit, suggesting that the process from a refugial to an equilibrium situation is highly skewed towards an earlier non‐equilibrium. Main conclusions The observed non‐equilibrium pattern supports the refugial hypothesis, suggesting that an ancestral lineage was originally distributed from Spain to Italy, and also occupied offshore islands. This lineage, replaced in Italy, has persisted on the islands owing to their isolation. A comparison of the distribution patterns for genetic and morphometric markers in several species indicates that the situation highlighted for Pyronia may represent a common biogeographic feature for many Mediterranean butterflies.