Ultrastructure comparée de la spermiogenèse et du spermatozoïde de trois espèces de Paravortex (Rhabdocoela, "Dalyellioida", Graffillidae), Turbellariés parasites intestinaux de Mollusques

Three species of the genus Paravortex were studied. Spermiogenesis involves progressive lengthening of the spermatid. No centriole nor axoneme were found. Thc mature spermatozoon is threadlike, 30μm long and aflagellate. It contains a nucleus, two longitudinal parallel mitochondria, numerous dense bodies limited by a membrane, a row of cortical microtubules and granules. The spatial arrangement of the microtubules wah deduced from a statistical analysis of their number in transverse section. The microtubules are continuous along the length of the spermatozoon and are arranged along a long cone. The granules, not limited by a membrane, are about 25 nm in diameter. They are arranged in a semicircle and represent a new structure for sperm of Platyhclminthes. A statistical comparison demonstrated that two species may be distinguished by the number of their cortical sperm microtubules. This is the tirst ultrastructural study of an aflagellate spermatozoon in the Rhabdocoela. A rickettsia was found in P. cardii .     

Life history predicts advancement of avian spring migration in response to climate change

An increasing number of studies demonstrate that plant and animal phenologies such as the timing of bird migration have been advancing over the globe, likely as a result of climate change. Even closely related species differ in their phenological responses, and the sources of this variation are poorly established. We used a large, standardized dataset of first arrival dates (FAD) of migratory birds to test the effects of phylogenetic relationships and various life-history and ecological traits on the degree to which different species adapt to climate change by earlier migration in spring. Using the phylogenetic comparative method, we found that the advancement of FAD was greater in species with more generalized diet, shorter migration distance, more broods per year, and less extensive prebreeding molt. In turn, we found little evidence that FAD trends were influenced by competition for mating (polygamy or extra-pair paternity) and breeding opportunities (cavity nests). Our findings were robust to several potentially confounding effects. These evolutionary correlations, coupled with the low levels of phylogenetic dependence we found, indicate that avian migration phenology adapts to climate change as a species-specific response. Our results suggest that the degree of this response is fundamentally shaped by constraints and selection pressures of the species' life history, and less so by the intensity of sexual selection.     

Extent of sequence homology required for bacteriophage lambda site-specific recombination

Bacteriophage lambda integration and exicision occur by reciprocal recombination within a 15-base homologous core region present in the recombining attachment (att) sites. Strand exchange within the core occurs at precise nucleotide positions, which define an overlap region in which the products of recombination contain DNA strands derived from different parents. In order to define the role of sequence homology during recombination we have constructed point mutations within the core and assayed their effects in vivo and in vitro on site-specific recombination. Two of the mutations are located at position ?3 of the core, which is one base-pair outside of the overlap region where strand exchange occurs. These mutations do not affect integrative or excisive recombination, thereby suggesting that homology outside the overlap region is not required for recombination. Two other mutations are located at position ?2 of the core, which is one base-pair within the overlap region. These mutations show severely depressed integrative and excisive recombination activities in vitro and in vivo when recombined against wild-type att sites. However, the ?2 mutations show normal recombination activity when recombined against att sites containing the homologous mutation, thereby suggesting that homology-dependent DNA interactions are required within the overlap region for effective recombination. In vitro recombination between homoduplex attP sites and heteroduplex attB sites demonstrated that the DNA interactions require only one strand of the attB overlap region to be homologous to attP in order to promote recombination.     

Mutations in an integration host factor-binding site: effect on lambda site-specific recombination and regulatory implications.

The manner in which integration host factor (IHF) regulates lambda site-specific recombination has been analyzed by examining the behavior of both wild-type and mutant DNAs in integrative and excisive recombination as well as in protein binding. While integrative recombination of an attP with two base changes in the H1 site required 8-fold more IHF than did wild type, binding to this site was lowered at least 500-fold, suggestive of cooperative interactions. A mutant attP with nine base changes did not integrate at all in vitro, with the defect being less severe in vivo. IHF inhibition of excisive recombination was relieved by both mutations in vitro and in vivo. These results imply that occupancy of the H1 site is critical for determining the direction of recombination. It is proposed that IHF inhibition of excision provides a monitor of the strength of the induction stimulus and the nutritional state of the cell; this would allow the prophage to excise selectively in conditions which favor successful completion of the lytic cycle.     

Mutational analysis of integrase arm-type binding sites of bacteriophage lambda. Integration and excision involve distinct interactions of integrase with arm-type sites

Integrative recombination between specific attachment (att) regions of the bacteriophage lambda genome (attP) and the Escherichia coli genome (attB) results in a prophage flanked by the hybrid recombinant sites attL and attR. Each att site contains sequences to which proteins involved in recombination bind. Using site-directed mutagenesis, we have constructed a related set of point mutations within each of the five Int "arm-type" binding sites located within attP, attL and attR. Footprint analyses of binding demonstrate that mutating the arm-type sites significantly disrupts the binding of Int. Recombination analyses of mutant att sites in vivo and in vitro demonstrate that only three wild-type arm-type sites within attP are required for efficient integrative recombination. Similar analyses demonstrate that efficient excision can occur with two other different sets of wild-type arm-type sites in attL and attR. These results demonstrate that integrative and excisive recombination may involve interactions of Int with distinct and different subsets of arm-type sites.     

Activation of phosphates to substitution(II). A comparison of catalyses initiated directly by VO2+ and by oxidation of VO2+ by MnO4−

Vanadium(V)-induced hydrolyses of triphosphates in aqueous solutions were initiated in two ways: (1) oxidizing vanadium(IV)-polyphosphate complexes to produce metastable vanadium(V) complexes; (2) forming VO₂⁺-polyphosphate complexes by acidification of solutions of VO₄^3? and polyphosphate to yield equilibrium mixtures of V(V), polyphosphate, and their complexes. Hydrolysis rates for the complexes formed at 40°C ? T ? 25°C follow the order V₂PPP_i = 2(VPPP_i) ≌ (VO₂) ATP ? V(PPP_i)₂ ≌ PPP_i. The hydrolysis of (V(V))₂(PPP_i) was not very temperature sensitive; the activation enthalpy appears small and the activation entropy large and negative. Mechanistic studies reveal that requirements for the activated state in metal-ion-catalyzed hydrolysis of polyphosphates include monodentate polyphosphate ligated cis to H₂O or OH^? in the coordination sphere of the metal ion:

     

The preparative synthesis of oligodeoxyribonucleotides using RNA ligase.

The synthesis of nmol quantities of defined sequences of oligodeoxyribonucleotides using T4 RNA ligase has been demonstrated. Reacting using from 18 to 200 nmol of substrates in which a single 2'-deoxyribonucleoside 3',5'-bisphosphate was added to an oligodeoxyribonucleotide resulted in yields from 13 to 95%. When two oligodeoxyribonucleotides were similarly joined using RNA ligase, the yields ranged from 10 to 50%. Although the reactions contained high concentrations of enzyme and were incubated from 5 to 21 days, there was little degradation of either substrates or products. We have also characterized an unusual product which arises when 3'-phosphate terminated oligodeoxyribonucleotides are incubated with RNA ligase and high concentrations of ATP. This product has an adenylyl group linked to the 3'-phosphate by an anhydride bond. The mechanistic and synthetic implications of forming this product are discussed.     

Determining the DNA sequence elements required for binding integration host factor to two different target sites.

Binding sites for the Escherichia coli protein integration host factor (IHF) include a set of conserved bases that can be summarized by the consensus sequence WATCAANNNNTTR (W is dA or dT, R is dA or dG, and N is any nucleotide). However, additional 5'-proximal bases, whose common feature is a high dA+dT content, are also thought to be required for binding at some sites. We examine the relative contribution of these two sequence elements to IHF binding to the H' and H1 sites in attP of bacteriophage lambda by using the bacteriophage P22-based challenge-phage system. IHF was unable to act as a repressor in the challenge-phage assay at H' sites containing the core consensus element but lacking the dA+dT-rich element. This indicates that both elements are required for IHF to bind to the H' site. In contrast, the core consensus determinant alone is sufficient for IHF binding to the H1 site, which lacks an upstream dA+dT-rich region. Fifty mutants that decreased or eliminated IHF binding to the H1 site were isolated. Sequence analysis showed changes in the bases in the core consensus element only, further indicating that this determinant is sufficient for IHF binding to the H1 site. We found that placement of a dA+dT-rich element upstream of the H1 core consensus element significantly increased the affinity, suggesting that the presence of a dA+dT-rich element enhances IHF binding.