A substrate-dependent biological containment system for Pseudomonas putida based on the Escherichia coli gef gene

A model substrate-dependent suicide system to biologically contain Pseudomonas putida KT2440 is reported. The system consists of two elements. One element carries a fusion between a synthetic lac promoter (PA1-04/03) and the gef gene, which encodes a killing function. This element is contained within a transposaseless mini-Tn5 transposon so that it can be integrated at random locations on the Pseudomonas chromosome. The second element, harbored by plasmid pCC102, is designed to control the first and bears a fusion between the promoter of the P. putida TOL plasmid-encoded meta-cleavage pathway operon (Pm) and the lacI gene, encoding the Lac repressor, plus xylS2, coding for a positive regulator of Pm. In liquid culture under optimal growth conditions and in sterile and nonsterile soil microcosms, P. putida KT2440 (pWWO) bearing the containment system behaves as designed. In the presence of a XylS effector, such as m-methylbenzoate, the LacI protein is synthesized, preventing the expression of the killing function. In the absence of effectors, expression of the PA1-04/03::gef cassette is no longer prevented and a high rate of cell killing is observed. Fluctuation test analyses revealed that mutants resistant to cell killing arise at a frequency of around 10(-5) to 10(-6) per cell per generation. Mutations are linked to the killing element rather than to the regulatory one. In bacteria bearing two copies of the killing cassette, the rate of appearance of mutants resistant to killing decreased to as low as 10(-8) per cell per generation.