Figure 2.

SPM analysis of plasmid clones. Approximately 5 μg of plasmid DNA was serially digested with Tsp509I (AATT), MseI (TTAA), BfaI (CTAG), and NlaIII (CATG) and loaded on an 8% polyacrylamide gel containing a linear gradient of chemical denaturant in a bath maintained at 59.5°C. The denaturant concentration was 9% at the top and 90% at the bottom; 100% denaturant is 7 M urea/40% formamide in 1× TAE buffer (40 mM Tris/20 mM NaOAc/1 mM EDTA, pH 7.4). An electric field of 10 V/cm was applied for 11 h. Lanes 1 and 25, HindIII digests of λ DNA; lanes 2–24, Tsp509I/MseI/BfaI/NlaIII digests of independent plasmid clones.