DNA trapping electrophoresis

Attempts to improve the size separation of single-stranded DNA in polyacrylamide gels by field-inversion gel electrophoresis (FIGE) have met with limited success. Here we show that attaching a neutral globular protein, streptavidin, to one end of a single-stranded DNA molecule profoundly alters the DNA mobility pattern and increases the band separation manyfold within a size range controlled by voltage and pulse cycle. In constant field, short modified fragments are only slightly retarded but long molecules are retarded dramatically above a 'threshold size' of 0.6 kilobases at 60 V per cm. At this voltage, molecules above a 1.2-kilobase 'cut-off' do not enter the gel. Both the threshold and the cut-off sizes decrease as the voltage increases. In FIGE, the longer the reverse pulse, the larger the modified fragments that enter the gel. We interpret these results as the trapping by the gel matrix of the protein attached to the DNA. The probability of release then depends on the balance between the electric field and thermal motion: the larger the DNA and the higher the voltage, the harder it is to release.     

Morphology and intracortical projections of functionally characterised neurones in the cat visual cortex

The neuronal structure and connectivity underlying receptive field organisation of cells in the cat visual cortex have been investigated. Intracellular recordings were made using a micropipette filled with a histochemical marker, which was injected into the cells after their receptive fields had been characterised. This allowed visualisation of the dendritic and axonal arborisations of functionally identified neurones.     

The structures of five newAspidosperma alkaloids related to uleine

Zusammenfassung Aus der Rinde des brasilianischen BaumesAspidosperma dasycarpon A. DC. wurden fünf neue Alkaloide (II–VI) isoliert, die das selten vorkommende Uleinskelett besitzen. Die Strukturen wurden durch chemische Korrelation mit Ulein (I) sichergestellt.

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Paper XLVII in the seriesAlkaloid Studies. For paper XLVI seeK. S. Brown Jr. andC. Djerassi, J. Am. chem. Soc.86, in press (1964).     

Pore-forming toxins

Pore-forming toxins are widely distributed proteins which form lesions in biological membranes. In this review, bacterial pore-forming toxins are treated as a paradigm and discussed in terms of the structural principles on which they work. Then, a large family of bacterial toxins, the cholesterol-binding toxins, are analyzed in depth to provide an overview of the processes involved in pore formation. The ways in which the cholesterol-binding toxins (cholesterol-dependent cytolysins) interact with membranes and form pores, the structure of the monomeric soluble and oligomeric pore-forming states, and the effects of the toxin on membrane structure are discussed. By surveying the range of work which has been done on cholesterol-binding toxins, a working model is elaborated which reconciles two current, apparently diametrically opposed, models for their mechanism. Received 7 September 2001; received after revision 12 November 2001; accepted 5 December 2001