FIG 1.

Principle of ICEclc detection in individual and transfer-competent Pseudomonas putida cells. (a) ICEclc remains integrated and silent in exponentially growing cells (EXPO, white bars in gray cells). Three to 5% of cells under nongrowing conditions (STAT) activate the core ICE promoters for its transfer competence program (magenta cells). Upon new nutrient addition, the transfer-competent cells (tc) excise (white circle) and transfer the ICE (black protrusions from cells). tc cells are impaired for cell division and frequently lyse (dashed cell outline). Non-tc cells continue to divide normally in exponential phase. (b) Schematic outline of ICEclc, its recombination boundaries, positions of genes mutated in this study, and insertion position of the lacO_ARRAY. (c and d) LacI-CFP single or LacI-CFP/TetR-YFP double fluorescent focus formation by ectopic expression of a single-copy chromosomally inserted arabinose-inducible lacI-cfp or lacI-cfp/tetR-yfp gene construct. Fluorescent protein binding to the 240-fold-copied cognate DNA binding site results in visible foci, as illustrated schematically. Cells are colabeled with a single-copy chromosomally inserted fusion of echerry to the P_inR promoter of ICEclc, which is active exclusively in tc cells. tc cells losing the ICE upon excision are depicted in gray.