Determination of plasmid molecular weights from ultraviolet sensitivities

Abstract

MECHANISMS for the repair of ultraviolet (200–300 nm) irradiated DNA have been well studied in connection with cells and bacteriophages¹, but not with plasmids. During investigations of plasmid ultraviolet (UV) sensitivity by an Escherichia coli transformation assay², we found that the principal E. coli repair systems act on UV-irradiated plasmid DNA, exerting a major effect on plasmid survival. In host cells lacking these repair functions, however, the UV survival of transforming activity depends essentially on plasmid size alone, and decreases with increasing molecular weight. This gives an accurate and convenient method for determining plasmid size. In addition, the lower UV sensitivity of small plasmids in such conditions provides an effective way to select for mini-plasmids.