Method for obtaining more-accurate covalently closed circular plasmid-to-chromosome ratios from bacterial lysates by dye-buoyant density centrifugation

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Abstract
A method that gives high recovery of deoxyribonucleic acid (DNA) from crude bacterial lysates using ethidium bromide-cesium chloride density gradient centrifugation is presented. After Pronase digestion and shearing of the lysate, essentially 100% recovery of chromosomal DNA and a reproducible recovery of covalently closed circular (CCC) plasmid DNA is obtained for a specific plasmid in a given strain. This method should be useful for comparing the CCC plasmid/chromosome ratio of various plasmid-host combinations.