Fig 3. Liquid-based counter-selection using dP kinase activity.

(a) Experimental procedure. The selection cassette inserted in lacZ region of E. coli chromosome was replaced back to lacZ by eletroporating PCR-generated lacZ gene. Transformant cells were incubated at 37°C in dP-containing LB medium, and a portion of the culture were plated at each time points (0–8 hrs). The number of recombinants was estimated by counting the number of blue colonies per plate. (b) Ratio of recombinant cells (LacZ +) various periods after adding dP, with (open circles)/without (closed circles) dP treatment (1 μM).