FIG. 4.

IPTG dose response of DNA transformation competence in regulatable and control strains. Strains were induced as indicated during incubation with plasmid DNA that confers resistance to nalidixic acid (Nal^r) when recombined into the chromosome. Efficiency was expressed as the number of Nal^r transformants per CFU. The mean and standard error of 8 to 10 identical experiments are shown. Each variant with its wild-type promoter is shown without IPTG. The recA6 version of RM21 was used because the pilE sequence in wild-type RM21 was found to be unstable (data not shown).