Simultaneous transformation of Escherichia coli by pairs of compatible and incompatible plasmid DNA molecules.

Summary This paper describes experiments involving simultaneous transformation of Escherichia coli by DNA of two species of multicopy plasmid in order to study competence and DNA uptake in this organism. In transformation mixtures where separate species of plasmid DNA were present in equal amounts, selection for a single plasmid gave doseresponse curves with slopes of 1. These results indicate that uptake of a single molecule of plasmid DNA is sufficient to produce one transformant. Simultaneous selection for markers on both plamsids gave a dose-response curve with a slope of 2. The total numbers of transformants obtained in single or double transformation experiments at saturating DNA concentrations were the same, and represented the maximum number of transformable cells. However, the absolute frequency of double transformants was reduced 4–6 fold relative to frequencies obtained with single markers. Similar results were obtained using pairs of compatible or incompatible plasmids in rec ⁺ or recA strains. These findings may be explained either on the basis of interference between competing plasmid molecules for uptake or establishment or by assuming that competent cells of Escherichia coli vary in their ability to incorporate more than one plasmid DNA molecule. Our results are consistent with an interpretation where more than 80% of the transformable cells are only capable of establishing one plasmid moiecule.