A biological containment system for Rhizobium meliloti based on the use of recombination-deficient (recA−) strains

Abstract Using a newly developed integration vector, the Escherichia coli gusA gene conferring GUS-activity or the firefly ( Photinus pyralis ) luc gene mediating bioluminescence were integrated into a non-essential site of the chromosome of Rhizobium meliloti 2011. The integration of the constitutively expressed marker genes into the chosen site per se did not affect the strains' ability to perform homologous recombination, its growth characteristics or its symbiotic nitrogen fixation. Comparative microcosm analyses between the bioluminescent, recombination-proficient (RecA⁺) R. meliloti strain L33 whose construction is reported in this paper, and its previously described recombination-deficient (RecA^?) isogenic counterpart L1 indicate that RecA^? strains of Rhizobium are safe hosts for deliberate release experiments.