Force-induced melting of DNA—evidence for peeling and internal melting from force spectra on short synthetic duplex sequences

Overstretching of DNA occurs at about 60–70 pN when a torsionally unconstrained double-stranded DNA molecule is stretched by its ends. During the transition, the contour length increases by up to 70% without complete strand dissociation. Three mechanisms are thought to be involved: force-induced melting into single-stranded DNA where either one or both strands carry the tension, or a B-to-S transition into a longer, still base-paired conformation. We stretch sequence-designed oligonucleotides in an effort to isolate the three processes, focusing on force-induced melting. By introducing site-specific inter-strand cross-links in one or both ends of a 64 bp AT-rich duplex we could repeatedly follow the two melting processes at 5 mM and 1 M monovalent salt. We find that when one end is sealed the AT-rich sequence undergoes peeling exhibiting hysteresis at low and high salt. When both ends are sealed the AT sequence instead undergoes internal melting. Thirdly, the peeling melting is studied in a composite oligonucleotide where the same AT-rich sequence is concatenated to a GC-rich sequence known to undergo a B-to-S transition rather than melting. The construct then first melts in the AT-rich part followed at higher forces by a B-to-S transition in the GC-part, indicating that DNA overstretching modes are additive.     

Phylogenetic analysis of 1.5 Mbp and platypus EST data refute the Marsupionta hypothesis and unequivocally support Monotremata as sister group to Marsupialia/Placentalia

The extant mammalian groups Monotremata, Marsupialia and Placentalia are, according to the 'Theria' hypothesis, traditionally classified into two subclasses. The subclass Prototheria includes the monotremes and subclass Theria marsupials and placental mammals. Based on some morphological and molecular data, an alternative proposition, the Marsupionta hypothesis, favours a sister group relationship between monotremes and marsupials to the exclusion of placental mammals. Phylogenetic analyses of single genes and even multiple gene alignments have not yet been able to conclusively resolve this basal mammalian divergence. We have examined this problem using one data set composed of expressed sequence tags (EST) and another containing 1 510 509 nucleotide (nt) sites from 1358 inferred cDNA genomic sequences. All analyses of the concatenated sequences unambiguously supported the Theria hypothesis. The Marsupionta hypothesis was rejected with high statistical confidence from both data sets. In spite of the strong support for Theria, a non-negligible number of single genes supported either of the two alternative hypotheses. The divergence between monotremes and therian mammals was estimated to have taken place 168–178 Mya, a dating compatible with the fossil record. Considering the long common evolutionary branch of therians, it is surprising that sequence data from many thousand amino acid sites were needed to conclusively resolve their relationship to monotremes. This finding draws attention to other mammalian divergences that have been taken as unequivocally settled based on much smaller alignments. EST data provide a comprehensive random sample of protein coding sequences and an economic way to produce large amounts of data for phylogenetic analysis of species for which genomic sequences are not yet available.     

Superiority of radiotelemetry over the tail-cuff method in evaluating control and drug-induced data in cardiovascular functions

The aim of the study was to compare an often-used method to measure blood pressure (BP) using a tail-cuff (TC) device, with radiotelemetry (RT) which allows to sample data on heart rate (HR) and BP in freely moving rodents without any restraint in behaviour. Data were collected in male normotensive Wistar-Kyoto rats and in spontaneously hypertensive rats. Experiments were performed under 12:12 h light–dark conditions (lights on at 07:00 h) with simulated dawn and dusk for 45 min. Experiments were performed at 08:00–10:00 h (rest phase) and at 20:00–22:00 h (activity phase) under control conditions and after treatment with the beta-adrenoceptor blocker metoprolol (8 mg/kg). For TC, the Harvard BP Monitoring System (Edenbrigde, England) and for RT radio transmitters (Dataquest IV system, TA11PA-C40, DSI, St. Paul, Minnesota, USA) were used. Rats bearing the TC device were also monitored under RT. The experiments show that TC significantly increased HR and both systolic and diastolic BP in both strains and both at L and D. Metoprolol reduced TC-induced HR but left BP increase uneffected. The study shows that RT is the method of choice to monitor BP and HR in rodents, TC is not suitable.     

Environmental stability affects phenotypic evolution in a globally distributed marine picoplankton

Marine phytoplankton can evolve rapidly when confronted with aspects of climate change because of their large population sizes and fast generation times. Despite this, the importance of environment fluctuations, a key feature of climate change, has received little attention—selection experiments with marine phytoplankton are usually carried out in stable environments and use single or few representatives of a species, genus or functional group. Here we investigate whether and by how much environmental fluctuations contribute to changes in ecologically important phytoplankton traits such as C:N ratios and cell size, and test the variability of changes in these traits within the globally distributed species Ostreococcus. We have evolved 16 physiologically distinct lineages of Ostreococcus at stable high CO₂ (1031±87 μatm CO₂, SH) and fluctuating high CO₂ (1012±244 μatm CO₂, FH) for 400 generations. We find that although both fluctuation and high CO₂ drive evolution, FH-evolved lineages are smaller, have reduced C:N ratios and respond more strongly to further increases in CO₂ than do SH-evolved lineages. This indicates that environmental fluctuations are an important factor to consider when predicting how the characteristics of future phytoplankton populations will have an impact on biogeochemical cycles and higher trophic levels in marine food webs.     

The distinctness and diversity of Ethiopian barleys

 The relative diversity and distinctness of Ethiopian barleys has been investigated using (1) morphology/isozyme/hordein polymorphisms and (2) RFLP markers. In the former a set of 51 landraces from over the whole of Ethiopia was compared with Iranian landraces based on data from previous studies and new hordein data. The two sets of landraces were found to have a comparable diversity. The Ethiopian ones are more diverse morphologically (5 traits), are similar in numbers of alleles per protein locus (17 loci) and in genetic differentiation, but are less diverse in average heterozygosity per locus and degree of polymorphism. However, on the basis of the hordein data the two sources of germplasm are very distinct. The correlation between morphological and protein diversity is very low. In the RFLP study 28 probes evenly distributed across the genome were used to analyse 43 Ethiopian landraces (represented by one single genotype) and to compare them with modern cultivars from North America, Europe and Japan, as well as 3 landraces from Iran, 1 from Nepal, and 1 accession of H. spontaneum from Afghanistan. The major finding was that the Ethiopian germplasm appears to be significantly less diverse than the modern germplasm but that it is genotypically very distinct. The apparent contradiction between a high diversity of useful genes coming from Ethiopia and an apparently low diversity at the DNA level is discussed. Received: 22 July 1996 / Accepted: 26 July 1996     

Physiology of the ECL cells.

The enterochromaffin-like (ECL) cells of the oxyntic mucosa (fundus) of the stomach produce, store and secrete histamine, chromogranin A-derived peptides such as pancreastatin, and an unanticipated but as yet unidentified peptide hormone. The cells are stimulated by gastrin and pituitary adenylate cyclase activating peptide and suppressed by somatostatin and galanin. Choline esters and histamine seem to be without effect on ECL cell secretion. The existence of a gastrin-ECL cell axis not only explains how gastrin stimulates acid secretion but also may help to explore the functional significance of the ECL cells with respect to the nature and bioactivity of its peptide hormone. From the results of studies of gastrectomized/fundectomized and gastrin-treated rats, it has been speculated that the anticipated ECL-cell peptide hormone acts on bone metabolism.