The most frequently occurring resistance of Gram-negative bacteria against tetracyclines is triggered by drug recognition of the Tet repressor. This causes dissociation of the repressor-operator DNA complex and enables expression of the resistance protein TetA, which is responsible for active efflux of tetracycline. The 2.5 angstrom resolution crystal structure of the homodimeric Tet repressor complexed with tetracycline-magnesium reveals detailed drug recognition. The orientation of the operator-binding helix-turn-helix motifs of the repressor is inverted in comparison with other DNA binding proteins. The repressor-drug complex is unable to interact with DNA because the separation of the DNA binding motifs is 5 angstroms wider than usually observed.
For the crystallization of proteins under microgravity conditions, a Chinese re-entry system was used, in which 101 experiments of 25 different biological macromolecules were accommodated. From the results obtained we conclude that under microgravity conditions crystal growth can only be expected under those crystallization conditions which also permit crystal growth on earth. A number of space-grown crystals were larger in size and of a better quality in their ability to diffract X-rays than the corresponding ground control crystals grown at the Chinese launch site. However, the space-grown crystals have not reached the X-ray diffraction quality of the crystals obtained under optimal conditions in the home laboratories.Protein crystallization; Microgravity
Scolochloa marchica sp. nova (Poaceae), ein neues Rohrichtgras aus MitteleuropaMit einer Karte, 4 Abbildungen und einer Tabelle Z u s a m m e n f a s s u n g Eine in Ost-Mitteleuropa 1998 und 1999 neu entdeckte Art der bisher als monotypisch geltenden Gattung Scolochloa wird mit Abbildungen als S. marchica beschrieben. Sie, unterscheidet sich deutlich von S. fesrucucea in Ahrchen-und Bliitenmorphologie, in der Blattspreite und in der Chromosomenzahl (2n = 42, hexaploid, statt 2n = 28 bei S. festucacea). Von den drei gegenwartigen Fundorten dieser neuen Art im Bundesland Brandenburg, Deutschland, werden deren sumpfige Standortc und die Vergesellschaftung der Populationen kurz gekennzeichnet. Andere Fundorte sind durch Aufsammlungen aus den Jahren 1867, 1878 (Brandenburg) und 1896 (Berlin) bekannt; ein altes Exemplar (ohne Datum) stammt von der Insel Wollin (Wolin) (Nordwestpolen). Alle Vorkommen der S. marchica in Europa befinden sich am auaersten westlichen Rand des eurasiatischen Arealanteils der disjunkt mittelnordamerikanisch-nordeurasiatisch verbreiten S. fesfucuceu. In einem Anhang sind alle untersuchten Herbarexemplare der S. fkmcacea, einschliealich des in Berlin gesammelten Typus des Namens (B) aufgelistet.
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