The thermal degradation of plasmid pUC18 held at temperatures between
100 and 135 degrees C was examined by measuring the ability of heat-tr
eated plasmid preparations to transform Escherichia coli to ampicillin
resistance using electroporation. Substantial protection against loss
of transforming ability during heating was provided by concentrations
of NaCl between 0.25 and 2.0 mol l(-1). For example, the addition of
1.0 mol l(-1) NaCl to samples heated at 100 degrees C for 15 min incre
ased transformation frequency about 200-fold compared with samples hea
ted without NaCl. In the presence of 0.5-2.0 mol l(-1) NaCl, transform
ing capacity was not destroyed even by heating at 121 degrees C for 15
min, i.e. after a typical sterilization treatment. These findings may
have implications for the safe disposal of genetically modified micro
-organisms and recombinant DNA preparations.