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2.6% of the genome of the soil bacteriumPseudomonas putidaKT2440 encodes phage-related functions, but the burden of such opportunistic DNA on the host physiology is unknown. Each of the four apparently complete prophages borne by this strain was tested for stability, spontaneous excision and ability to cause lysis under various stressing conditions. While prophages P3 (PP2266–PP2297) and P4 (PP1532–1584) were discharged from the genome at a detectable rate, their induction failed otherwise to yield infective viruses. IsogenicP. putidaKT2440 derivatives bearing single and multiple deletions of each of the prophages were then subjected to thorough phenotypic analyses, which generally associated the loss of proviral DNA with an increase of physiological vigour. The most conspicuous benefit acquired by prophage-less cells was a remarkable improvement in tolerance to UV light and other insults to DNA. This was not accompanied, however, with an upgrade ofrecA-mediated homologous recombination. The range of tolerance to DNA damage gained by the prophage-free strain was equivalent to the UV resistance endowed by the TOL plasmid pWW0 to the wild-type bacterium. While theP. putida's prophages are therefore genuinely parasitic, their detrimental effects can be offset by acquisition of compensatory traits through horizontal gene transfer.