The restriction in ribonuclease T 2 catalysed hydrolysis of oligoribonucleotides after kethoxalation of guanine bases

Kethoxalation produces a restriction in splitting of phosphodiester bonds between modified Gp and 3′ neighbouring nucleotide irrespectively of the chemical nature of the latter. Possible applications of this observation for analysis of primary structure of ribonucleic acids and polynucleotides are discussed.     

Phase separation in dipalmitoyl phosphatidylcholine bilayers induced by ionophores and binary electrolytes

Thermotropic behavior of unsonicated aqueous dispersion of dipalmitoyl phosphatidylcholine (DPPC) has been studied by scanning microcalorimetry and fluorescent probe method. Phase separation in the lipid bilayers was observed for systems containing ionophores (valinomycin, dinactin) and 1 : 1 electrolytes (NaCl, KCl, RbCl, CsCl). The ratio of lipid phases coexisting in the systems appeared to be dependent on the concentration of the electrolytes. Changes in the thermotropic properties of the lipid phase induced by valinomycin were observed when K+ and Rb+ ions-forming complexes with the ionophore were present in the systems. The latter phenomenon was not found for the systems containing dinactin possessing a lower ability for complex formation with the cations.     

Early stages of formation and dispersal of the temperate flora in the Boreal Region

Budantsev, L. Yu. (Komarov Botanical Institute, Prof. Popov Str. 2, 197376 St. Petersburg, Russia). Early stages of formation and dispersal of the temperate flora in the Boreal Region. Bot. Rev.58(1): 1–48, 1992.—The thesis of this review is that, as stated as early as 1908 by V. L. Komarov, the composition of a flora can be understood only as a process, or separate stage, in the context of migration in time and space of various floristic assemblages and their isolation, as induced by transformation of continental and ocean shapes, changes in climate, and the environment as a whole. Thus the formation of geofloras of the past was influenced by gradually changing environments that determined the spread, patterning, and spatial differentiation of floras and their evolution. Parallel to the more commonly-seen names of eras—Paleozoic, Mesozoic, and Cenozoic—we can speak of the Paleophytic, Mesophytic, and Cenophytic eras. Eras defined in these two ways (by faunistic or by floristic criteria) do not completely coincide. Generally, changes in the flora have, necessarily, preceded changes in the fauna. It is the Cenophytic with which this review is mostly concerned, the era of Angiosperm dominance. The movement of early subtropical and warm temperate floras in the Early Cenophytic, followed by temperate or even boreal floras, as the climate changes, is traced in detail. The regions discussed most fully are the Boreal-Atlantic and Boreal-Pacific, with emphasis on the Angaro-Beringian flora. The disappearance of archaic forms (e.g., cycadophytes) and the gradual predominance of angiosperms is documented. The movements of the floral assemblages in response to environmental changes are mapped and described. The early development and diversification of the boreal temperate flora is considered to have taken place mainly in Angaro-Beringia, associated with the invasive migration of tropical angiosperms from southeastern Asia.     

Scanning and transmission electron microscopy of the squamose gill-filament epithelium from fresh- and seawater adaptedTilapia

Synopsis The architecture of the gill structure of variousTilapia species was studied in relation to their adaptability to hypersaline media. Using SEM and EM, it was shown that the squamose epithelial cells of the gills have species-typical patterns of ridges on their outer surfaces. These have previously been misinterpreted by other authors as microvilli or stereocillia. The ridges are more dense and better developed in euryhaline species, likeT. zillii, and less so in stenohaline species likeSarotherodon niloticus. Comparing freshwater and seawater-adapted individuals ofT. zillii, S. niloticus, S. galflaeus, andTristramella sacra, it was shown that in fresh water the surface cells are slightly swollen, extending over the openings of the chloride cells. During adaptation to sea water, these ridges become higher and denser and the cell surface shrinks, exposing the underlying orifices of the apical crypts of the chloride cells. The more euryhaline the species, the less change there is in the ridge pattern of the cells during passage from fresh to sea water. This evidence implicates the gill epithelium, together with the chloride cells, in the process of osmoregulation.     

Stop codon recognition in ciliates: Euplotes release factor does not respond to reassigned UGA codon

In eukaryotes, the polypeptide release factor 1 (eRF1) is involved in translation termination at all three stop codons. However, the mechanism for decoding stop codons remains unknown. A direct interaction of eRF1 with the stop codons has been postulated. Recent studies focus on eRF1 from ciliates in which some stop codons are reassigned to sense codons. Using an in vitro assay based on mammalian ribosomes, we show that eRF1 from the ciliate Euplotes aediculatus responds to UAA and UAG as stop codons and lacks the capacity to decipher the UGA codon, which encodes cysteine in this organism. This result strongly suggests that in ciliates with variant genetic codes eRF1 does not recognize the reassigned codons. Recent hypotheses describing stop codon discrimination by eRF1 are not fully consistent with the set of eRF1 sequences available so far and require direct experimental testing.     

Termination of translation in eukaryotes is mediated by the quaternary eRF1*eRF3*GTP*Mg2+ complex. The biological roles of eRF3 and prokaryotic RF3 are profoundly distinct

GTP hydrolysis catalyzed in the ribosome by a complex of two polypeptide release factors, eRF1 and eRF3, is required for fast and efficient termination of translation in eukaryotes. Here, isothermal titration calorimetry is used for the quantitative thermodynamic characterization of eRF3 interactions with guanine nucleotides, eRF1 and Mg²⁺. We show that (i) eRF3 binds GDP (K_d = 1.9 μM) and this interaction depends only minimally on the Mg²⁺ concentration; (ii) GTP binds to eRF3 (K_d = 0.5 μM) only in the presence of eRF1 and this interaction depends on the Mg²⁺ concentration; (iii) GTP displaces GDP from the eRF1•eRF3•GDP complex, and vice versa; (iv) eRF3 in the GDP-bound form improves its ability to bind eRF1; (v) the eRF1•eRF3 complex binds GDP as efficiently as free eRF3; (vi) the eRF1•eRF3 complex is efficiently formed in the absence of GDP/GTP but requires the presence of the C-terminus of eRF1 for complex formation. Our results show that eRF1 mediates GDP/GTP displacement on eRF3. We suggest that after formation of eRF1•eRF3•GTP•Mg²⁺, this quaternary complex binds to the ribosomal pretermination complex containing P-site-bound peptidyl-tRNA and the A-site-bound stop codon. The guanine nucleotide binding properties of eRF3 and of the eRF3•eRF1 complex profoundly differ from those of prokaryotic RF3.     

Cooperative binding of primycin and gramicidin on erythrocyte membranes. A cation transport study

In this paper the authors present a comparative study of the actions of the antibiotics primycin and gramicidin on the erythrocyte membrane permeability. It has been found that both antibiotics have a nonlinear effect on the membrane permeability. Above a threshold antibiotic concentration, which is characteristic of the type of the antibiotic, the cation permeability of the erythrocyte membranes increases sharply. In the range of nonlinearity the transport-kinetic curves level off before achieving the equilibrium radioactive ion distribution between the extra- and intracellular spaces. A stochastic model of the cooperative and aspecific incorporation of antibiotic molecules into the membrane explains the experimental findings. The authors conclude that membrane permeability increases at the places where two or more antibiotic molecules form aggregates in the membrane.     

The IAP-antagonist ARTS initiates caspase activation upstream of cytochrome C and SMAC/Diablo

ARTS (Sept4_i2) is a pro-apoptotic tumor suppressor protein that functions as an antagonist of X-linked IAP (XIAP) to promote apoptosis. It is generally thought that mitochondrial outer membrane permeabilization (MOMP) occurs before activation of caspases and is required for it. Here, we show that ARTS initiates caspase activation upstream of MOMP. In living cells, ARTS is localized to the mitochondrial outer membrane. In response to apoptotic signals, ARTS translocates rapidly to the cytosol in a caspase-independent manner, where it binds XIAP and promotes caspase activation. This translocation precedes the release of cytochrome C and SMAC/Diablo, and ARTS function is required for the normal timing of MOMP. We also show that ARTS-induced caspase activation leads to cleavage of the pro-apoptotic Bcl-2 family protein Bid, known to promote MOMP. We propose that translocation of ARTS initiates a first wave of caspase activation that can promote MOMP. This leads to the subsequent release of additional mitochondrial factors, including cytochrome C and SMAC/Diablo, which then amplifies the caspase cascade and causes apoptosis.