1,721,062 research outputs found
Logica immaginaria
No full textMedico di formazione, dopo alcuni tentativi come poeta, storico e critico letterario, Nikolaj Aleksandrovic Vasil’ev (1880-1940) decide di dedicarsi alla filosofia, in particolare alla logica. Attraverso un’attenta riflessione sui quantificatori e sulla modalità dei giudizi, egli giunge a proporre una “logica del concetto”, in cui non vale il principio del terzo escluso. Di qui, grazie anche alle suggestioni che gli venivano dalla lettura di Lobacevskij, procede a elaborare una “logica immaginaria”, in cui non vale nemmeno il principio di contraddizione – una logica per oggetti incompleti e contraddittori. Ponendo in evidenza le basi ontologiche della logica e introducendo in questa l’uso della finzione, Vasil’ev si fa sostenitore di un pluralismo logico, secondo cui la logica formale contiene elementi che riflettono la nostra comprensione del mondo e dei tipi di oggetti che trattiamo; pertanto, per mondi diversi dal nostro, per “mondi immaginari”, valgono altre logiche. Le sue teorie, considerate di volta in volta anticipatrici delle logiche polivalenti o delle paraconsistenti, delle logiche intensionali o delle teorie dei mondi impossibili, hanno trovato riscontro nella logica contemporanea e ispirato, in alcuni casi, anche nuovi indirizzi di ricerca. Il volume, corredato da tre saggi introduttivi e da una ricca bibliografia, presenta la prima traduzione in una lingua occidentale di tutta la produzione logica e di filosofia della logica di Vasil’ev
Molecular and functional properties of an archaeal phenylalanyl synthetase from the hyperthermophile Sulfolobus solfataricus
No full textThe interaction between the archaeal elongation factor 1alpha and its nucleotide exchange factor 1beta
No full textIn Sulfolobus solfataricus the binding of the exchange factor 1b (SsEF-1b) to SsEF-1aGDP displaces the nucleotide and the SsEF-1aSsEF-1b complex is formed. The complex itself is stable, but it dissociates upon the addition of GDP or Gpp(NH)p but not ATP. Since the rate of the formation of the SsEF-1aSsEF-1b complex is significatively slower than the rate of the nucleotide exchange catalyzed by SsEF-1b it can be inferred that in vivo the GDP/GTP exchange reaction proceeds via an SsEF-1aSsEF-1b interaction without involving the formation of a stable binary complex as an intermediate
Resistance of archaebacterial aEF-1A/GDP against denaturation by heat and urea
No full textThe elongation factor aEF-1 alpha, isolated as aEF-1 alpha.GDP from the thermoacidophilic archaebacterium Sulfolobus solfataricus, exchanges GDP for [3H]GDP at a rate which reaches a maximum at 95 degrees C. The rate constants at different temperatures of the heat inactivation of aEF-1 alpha.GDP are considerably lower compared to those referred to Escherichia coli EF-Tu.GDP. The Tm values determined for both aEF-1 alpha.GDP and EF-Tu.GDP are 97 and 53 degrees C, respectively. The addition of GDP during the heat treatment protects significantly EF-Tu.GDP but only slightly aEF-1 alpha.GDP. The ability of aEF-1 alpha.GDP to exchange GDP for [3H]GDP is impaired at 70 degrees C by urea at concentrations which are greater compared to those required to inactivate E. coli EF-Tu.GDP at 45 degrees C; apparently both factors are not protected by GDP against inactivation by urea
Studies on the polypeptide elongation factor 2 from Sulfolobus solfataricus
No full textThe elongation factor 2 from the thermoacidophilic archaeon Sulfolobus solfataricus (SsEF-2) binds [3H]GDP at 1:1 molar ratio. The bound [3H]GDP is displaced by GTP or its nonhydrolyzable analogue guanyl-5'-yl imidodiphosphate (Gpp(NH)p) but not by ATP, thus indicating that only the two guanosine nucleotides compete for the same binding site. The affinity of SsEF-2 for [3H]GDP is higher than that for GTP and Gpp(NH)p. On the contrary, in the presence of ribosomes the affinity of SsEF-2 for GDP is lower than that for Gpp(NH)p. SsEF-2 is endowed with an intrinsic hardly detectable GTPase activity that is stimulated by ribosomes up to 2000-fold. The ribosome-stimulated SsEF-2 GTPase (GTPaser) reaches a maximum at pH 7.8 and is not affected by ATP but is competitively inhibited by either GDP or Gpp(NH)p. Both Km for [gamma-32P]GTP and kcat of GTPaser increase with increasing temperature, and the highest catalytic efficiency is reached at 80 degrees C. The ADP-ribosylation of SsEF-2 does not significantly affect either the binding of GDP and GTP or the kinetics of the GTPaser. A hypothesis on the stimulation by ribosome of SsEF-2 GTPase is proposed
Molecular and functional properties of an archaeal phenylalanyl synthetase from the hyperthermophile Sulfolobus solfataricus
No full text
The A26G replacement in the consensus sequence A-X-X-X-X-G-K-[T,S] of the guanine nucleotide binding site activates the intrinsic GTPase of the elongation factor 2 from the archaeon sulfolobus solfataricus
No full textA recombinant form of the elongation factor 2 from the archaeon Sulfolobus solfataricus (SsEF-2), carrying the A26G substitution, has been produced and characterized. The amino acid replacement converted the guanine nucleotide binding consensus sequences A-X-X-X-X-G-K-[T,S] of the elongation factors EF-G or EF-2 into the corresponding G-X-X-X-X-G-K-[T,S] motif which is present in all the other GTP-binding proteins. The rate of poly(U)-directed poly(Phe) synthesis and the ribosome-dependent GTPase activity of A26GSsEF-2 were decreased compared to SsEF-2, thus indicating that the A26G replacement partially affected the function of SsEF-2 during translocation. In contrast, the A26G substitution enhanced the catalytic efficiency of the intrinsic SsEF-2 GTPase triggered by ethylene glycol [Raimo, G., Masullo, M., Scarano, G., & Bocchini, V. (1997) Biochimie 78, 832±837]. Surprisingly, A26GSsEF-2 was able to hydrolyse GTP even in the absence of ethylene glycol; furthermore, the alcohol increased the affinity for GTP without modifying the catalytic constant of A26GSsEF-2 GTPase. Compared to SsEF-2, the affinity of A26GSsEF-2 for [3H]GDP was significantly reduced. These findings suggest that A26 is a regulator of the biochemical functions of SsEF-2. The involvement of this alanine residue in the guanine nucleotide- binding pocket of EF-2 or EF-G is discussed
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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