The putative permease PhlE of Pseudomonas fluorescens F113 has a role in 2,4-diacetylphloroglucinol resistance and in general stress tolerance

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Abstract
2,4-Diacetylphloroglucinol (PHL) is the primary determinant of the biological control activity ofPseudomonas fluorescensF113. The operonphlACBDencodes enzymes responsible for PHL biosynthesis from intermediate metabolites. ThephlEgene, which is located downstream of thephlACBDoperon, encodes a putative permease suggested to be a member of the major facilitator superfamily with 12 transmembrane segments. PhlE has been suggested to function in PHL export. Here the sequencing of thephlEgene fromP. fluorescensF113 and the construction of aphlEnull mutant, F113-D3, is reported. It is shown that F113-D3 produced less PHL than F113. The ratio of cell-associated to free PHL was not significantly different between the strains, suggesting the existence of alternative transporters for PHL. ThephlEmutant was, however, significantly more sensitive to high concentrations of added PHL, implicating PhlE in PHL resistance. Furthermore, thephlEmutant was more susceptible to osmotic, oxidative and heat-shock stresses. Osmotic stress induced rapid degradation of free PHL by the bacteria. Based on these results, we propose that the role ofphlEin general stress tolerance is to export toxic intermediates of PHL degradation from the cells.