Ammonium Triggers Uptake of NO-3 by Chenopodium rubrum Suspension Culture Cells and Remobilization of their Vacuolar Nitrate Pool

Photoautotrophic cell suspension cultures of Chenopodium rubrumrequire high concentrations of nitrate and ammonium. Duringthe growth phase total NH₄⁺ and the greater portion of NH₃^–were consumed. During the stationary phase nitrate uptake continuedbut at a substantially smaller rate than during the growth phase.During growth the bulk of the absorbed N was incorporated intoprotein, the amount of which was then maintained constant untilsenescence. NH₃^– was accumulated upon transition betweenthe growth and the stationary phase. NH₃^–, like the freeamino acids, was deposited in the vacuole but, unlike thesecompounds, could not be remobilized upon transfer of the cellsinto N-free medium. Readdition of NH₄⁺ to the medium, however,resulted in a mobilization of the vacuolar NH₃^–-pool.Reutilization of both vacuolar N-storage pools must have beenaccomplished by recycling from the vacuole to the cytoplasmbecause N-metabolizing enzymes could not be detected in isolatedvacuoles. Transfer of the cells of the stationary phase intomedium containing NH₃^– and NH₄⁺ resulted in an inductionof nitrate uptake by the cells, but only after a lag phase of4–5 days. It is conceivable that NH₄⁺ induces NH₃^–-translocatingsystems in the plasmalemma and in the tonoplast. (Received December 19, 1988; Accepted March 2, 1989)     

Procedures for the Generation of Mature Chlamydomonas reinhardtii Zygotes for Molecular and Biochemical Analyses

Zygotes represent an important stage in the sexual cycle of the unicellular green alga Chlamydomonas reinhardtii. To study zygote germination at a molecular level, a protocol was elaborated for the generation of zygotes in large quantities and a method was developed for the extraction from zygotes of RNA that could be translated in vitro.     

Line segregation

Six experiments investigated the preattentive segregation of line-like patterns composed of discrete elements in a background of distractors. The results indicate that other factors in addition to spatial density influence line segregation. Edge alignment, edge length and principal axis orientation also affect line segregation. Differences in the outputs of Gabor filters fail to account for the perceived segregation of the lines. Possible models of line segregation based on element grouping, feature density and search are briefly discussed.     

Structure and composition of savanna vegetation in northern Bolivia: A preliminary report

The vegetation of seasonally flooded savannas iin the tropical lowland of the Departamento La Paz, northern Bolivia, was studied. A first collection of vascular plants in this area includes about 600 species, at least three of which are new to the Bolivian flora and four are new to science. The eight major vegetation types distinguished within the savannas are dry bunchgrass savanna (“altura”), seasonally flooded bunch-grass community (“sartenejal”), palm swamp (“palmar”), two types of river floodplains (”bajíos”), a tall shrub community (“chaparral”), the vegetation of termite mounds, and gallery forest. The study area is among the least known in Bolivia. There is a strong need for further studies of these savannas.     

Chloride binding to photosystem II in the dark is in slow exchange

We have studied the 35Cl- NMR line broadening in the presence of photosystem II (PS II) membranes from spinach in the dark. In the presence of NH3 (which other work has shown to competitively inhibit chloride binding to PS II) we observed no decrease in 35 Cl- linewidths. We conclude that binding of Cl- to the O2 evolving center in PS II in the dark (previously demonstrated by EPR) is in slow exchange on the NMR timescale. We assign the observed line broadening to interaction with non-specific binding sites and with free paramagnetics.     

Names and naming: Instances from the ORU-IGBO

Conclusion People have many names. But, changing one's name is no small thing. It can signify a major step in life or an embarrassing story, as in the last case.A person has a complex individual identity. His collective identity is clear from the moment he is conceived. He is one with a body of people, his kinsmen, who share one common ancestor and through him access to land and resources. The continuation of this group and its name is perpetuated through paternal descent.Kingroup membership ensures a person's place in life, his right to exist, eat, live and enjoy. But, who is he, apart from being a kinsman? Which kinsman is he? How are people going to interact with him? His individual identity will become clear through time. He is a re-incarnation of ancestor x. The circumstances of his birth were such. He behaves like this. He has these personal traits. This has happened to him. He has achieved that. There is no permanent role attached to an individual. He is neither bound to class nor caste. He is dynamic and changes constantly. His life is one of achievement, constant striving and upward mobility till death — when it all begins again. His name signals his state of being in time and existence.Sabine Jell-Bahlsen is an anthropologist and filmmaker.     

Fanconi's anaemia: correlation of genetic complementation group with psoralen/UVA response

Summary The correlation found by Moustacchi (1987) between cellular response to a crosslinking challenge and genetic heterogeneity in Fanconi's anaemia is confirmed for an earlier set of complementation groups (Zakrzewski and Sperling 1980). This allows the matching of the two independently established complementation groupings and better characterization of their DNA repair-related biochemical properties.     

Tenascin interferes with fibronectin action

Primary chick embryo fibroblasts attach to a tenascin substrate, but remain rounded and do not spread out. The proportion between tenascin and fibronectin in mixtures used to coat the substrate determines the shape of the cells. Tenascin inhibits integrin-mediated chick fibroblast attachment to fibronectin, laminin, and the GRGDS peptide. Rat fibroblast attachment to fibronectin, but not to laminin, is inhibited by tenascin. A monoclonal antibody against tenascin, as well as its Fab fragments, is able to neutralize the inhibitory activity on cell attachment and is therefore assumed to mask the cell-binding site of tenascin. On electron micrographs showing this monoclonal antibody bound to tenascin, its epitope can be localized to the terminal knob at the distal ends of the tenascin arms.