FIG. 5.

Secondary CsCl gradients containing bis-benzimide allow detection of small amounts of ¹⁵N-labeled DNA in the presence of excess unlabeled DNA. In separate experiments, either 2 μg (▵) or 200 ng (⋄) of ¹⁵N-labeled E. coli DNA was mixed with 10 μg of unlabeled P. aeruginosa DNA, and following primary centrifugation, 0.8 to 1.0 μg of this DNA with density of 1.725 g ml⁻¹ was resolved in secondary CsCl gradients containing bis-benzimide. The peaks corresponding to ¹⁵N-labeled E. coli DNA and unlabeled P. aeruginosa DNA are labeled for clarity.