Catálogo de publicaciones - revistas

<jats:p>The long-term effect of water-level regulation on riparian plant communities was assessed for storage reservoirs and run-of-river impoundments. Soon after the onset of regulation, there were few species and sparse vegetation cover, regardless of whether the new water level intersected former upland or riparian vegetation. In the longer term, an impoverished vegetation was maintained by storage reservoirs, whereas in run-of-river impoundments, some community characteristics deteriorated and others recovered compared to adjacent free-flowing rivers.</jats:p>

<jats:p> Cardiac hypertrophy and heart failure caused by high blood pressure were studied in single myocytes taken from hypertensive rats (Dahl SS/Jr) and SH-HF rats in heart failure. Confocal microscopy and patch-clamp methods were used to examine excitation-contraction (EC) coupling, and the relation between the plasma membrane calcium current ( <jats:italic>I</jats:italic> <jats:sub>Ca</jats:sub> ) and evoked calcium release from the sarcoplasmic reticulum (SR), which was visualized as “calcium sparks.” The ability of <jats:italic>I</jats:italic> <jats:sub>Ca</jats:sub> to trigger calcium release from the SR in both hypertrophied and failing hearts was reduced. Because <jats:italic>I</jats:italic> <jats:sub>Ca</jats:sub> density and SR calcium-release channels were normal, the defect appears to reside in a change in the relation between SR calcium-release channels and sarcolemmal calcium channels. β-Adrenergic stimulation largely overcame the defect in hypertrophic but not failing heart cells. Thus, the same defect in EC coupling that develops during hypertrophy may contribute to heart failure when compensatory mechanisms fail. </jats:p>

<jats:p> DNA replication origins in chromosomes of eukaryotes are activated according to a temporal program. In the yeast <jats:italic>Saccharomyces cerevisiae</jats:italic> , activation of origins in early S phase appears to be a default state. However, <jats:italic>cis</jats:italic> -acting elements such as telomeres can delay origin activation until late S phase. Site-specific recombination was used to separate origin from telomere in vivo, thereby demonstrating that the signal for late activation is established between mitosis and START in the subsequent G <jats:sub>1</jats:sub> phase. Once set, the signal can persist through the next S phase in the absence of the telomere. Establishment of the temporal program and of initiation competence of origins may be coincident events. </jats:p>

<jats:p> To study nucleosome mobility and positioning, the R3 <jats:italic>lac</jats:italic> repressor was used with an adenosine triphosphate (ATP)–dependent chromatin assembly system to establish the positioning of five nucleosomes, with one nucleosome located between two R3 <jats:italic>lac</jats:italic> operators. When R3 protein was dissociated from DNA with isopropyl β- <jats:sc>d</jats:sc> -thiogalactopyranoside, the R3-induced nucleosome positions remained unchanged for at least 60 minutes in the absence of ATP but rearranged within 15 minutes in the presence of ATP. These results suggest that nucleosomes are dynamic and mobile rather than static and that a DNA binding factor is continuously required for the maintenance of nucleosome positioning. </jats:p>

<jats:p> High resolution x-ray diffraction data from crystals of the <jats:italic>Rhodobacter sphaeroides</jats:italic> photosynthetic reaction center (RC) have been collected at cryogenic temperature in the dark and under illumination, and the structures were refined at 2.2 and 2.6 angstrom resolution, respectively. In the charge-separated D <jats:sup>+</jats:sup> Q <jats:sub>A</jats:sub> Q <jats:sub>B</jats:sub> <jats:sup>−</jats:sup> state (where D is the primary electron donor (a bacteriochlorophyll dimer), and Q <jats:sub>A</jats:sub> and Q <jats:sub>B</jats:sub> are the primary and secondary quinone acceptors, respectively), Q <jats:sub>B</jats:sub> <jats:sup>−</jats:sup> is located approximately 5 angstroms from the Q <jats:sub>B</jats:sub> position in the charge-neutral (DQ <jats:sub>A</jats:sub> Q <jats:sub>B</jats:sub> ) state, and has undergone a 180° propeller twist around the isoprene chain. A model based on the difference between the two structures is proposed to explain the observed kinetics of electron transfer from Q <jats:sub>A</jats:sub> <jats:sup>−</jats:sup> Q <jats:sub>B</jats:sub> to Q <jats:sub>A</jats:sub> Q <jats:sub>B</jats:sub> <jats:sup>−</jats:sup> and the relative binding affinities of the different ubiquinone species in the Q <jats:sub>B</jats:sub> pocket. In addition, several water channels (putative proton pathways) leading from the Q <jats:sub>B</jats:sub> pocket to the surface of the RC were delineated, one of which leads directly to the membrane surface. </jats:p>

<jats:p> Mutations in the tumor suppressor gene <jats:italic>PATCHED</jats:italic> ( <jats:italic>PTC</jats:italic> ) are found in human patients with the basal cell nevus syndrome, a disease causing developmental defects and tumors, including basal cell carcinomas. Gene regulatory relationships defined in the fruit fly <jats:italic>Drosophila</jats:italic> suggest that overproduction of Sonic hedgehog (SHH), the ligand for PTC, will mimic loss of <jats:italic>ptc</jats:italic> function. It is shown here that transgenic mice overexpressing SHH in the skin develop many features of basal cell nevus syndrome, demonstrating that SHH is sufficient to induce basal cell carcinomas in mice. These data suggest that <jats:italic>SHH</jats:italic> may have a role in human tumorigenesis. </jats:p>

<jats:p>The visual system separates processing of an object’s form and color (“what”) from its spatial location (“where”). In order to direct action to objects, the identity and location of those objects must somehow be integrated. To examine whether this process occurs within the prefrontal (PF) cortex, the activity of 195 PF neurons was recorded during a task that engaged both what and where working memory. Some neurons showed either object-tuned (what) or location-tuned (where) delay activity. However, over half (52 percent, or 64/123) of the PF neurons with delay activity showed both what and where tuning. These neurons may contribute to the linking of object information with the spatial information needed to guide behavior.</jats:p>