Jacob Lebowitz

A simple electrophoretic method for the determination of superhelix density of closed circular DNAs and for observation of their superhelix density heterogeneity.

Abstract A simple method for directly determining the superhelix density of closed circular DNAs by ethidium bromide titration using agarose gel electrophoresis in tubes containing increasing concentrations of the dye has been developed. Since the positively charged dye migrates toward the cathode in a direction opposite to that of the DNA, ethidium bromide need not be present in the buffer chambers if electrophoresis is stopped before the dye boundary meets the DNA band. In this way, a constant concentration of ethidium bromide is present throughout the path of the DNA band. Consequently, by avoiding the presence of dye in the buffer chambers, many gels containing different concentrations of ethidium bromide can be run at the same time in a conventional gel-tube electrophoresis apparatus. A dye titration can be performed in less than a day by using this method. At several dye concentrations, the closed circular DNA is resolved into distinct bands which probably reflect intrinsic heterogeneity in superhelical density.

Through the looking glass: the discovery of supercoiled DNA

The discovery of supercoiled DNA was first reported 25 years ago in a paper entitled, The Twisted Circular Form of Polyoma Viral DNA by Vinograd, Lebowitz, Radloff, Watson and Laipis. This personal reflection describes the different experimental and conceptual processes that eventually led to the discovery as they actually occurred.

A revised equation relating DNA buoyant density to guanine plus cytosine content

Abstract An equation relating DNA buoyant density of CsCl to G + C content is given which uses the correct density value of Escherichia coli DNA as the reference. This is done to eliminate the current confusion brought about by two references states.

CHEMICAL MODIFICATION OF SUPERCOILED DNA: EFFECT ON THE RATE OF TRANSCRIPTION

ABSTRACT Superhelical SV40 DNA I can be modified with the single strand specific reagent N-cyclohexyl-N′-β(4, methylmorpholinium) ethylcarbodiimide (CMC). A limited reaction, less than 2% of the base pairs, results in almost total inhibition of in vitro transcription by DNA-dependent-RNA polymerase from E. coli . The inhibition is shown to be primarily a loss of the ability to the enzyme to bind to the super-coiled DNA.