A multiple plasmid-containing Escherichia coli strain: convenient source of size reference plasmid molecules.

A strain of Escherichia coli is described that contains eight plasmid species ranging in size from 1.36 × 106 to 35.8 × 106 daltons, which can be employed as a single source of covalently closed circular deoxyribonucleic acid molecules of different sizes for use as references in agarose gel electrophoretic analysis.