Table 1. Plasmid selection by repressor titration.

	Transformation efficiency: colonies/fmol DNA
	No additives	Ap	Ap + IPTG
pUC18	618.02	587.36	575.73
pUC18ΔlacO	9.90	0	632.39
pORT1a	589.69	0	0

Calcium-competent DH1lacdapD (100 µl aliquots) were transformed with pUC18, pUC18ΔlacO and pORT1a by adding 0.5 µg plasmid DNA, heat shocking at 42°C for 45 s, and incubating at 37°C in 900 µl LB broth containing IPTG for 1 h. Cells were spun down and washed to remove IPTG and resuspended in 1.0 ml LB broth, then plated on LB agar (100 µl of a 1/100 dilution of the transformation cultures) containing no additives, Ap or Ap + IPTG. Results represent the means of three separate experiments in which transformations were plated in triplicate. Transformation efficiencies are displayed in colonies per femtomole of plasmid DNA. A negative control (no DNA) had the same number of colonies as those for pUC18ΔlacO with no additives, but no colonies with Ap present.