The Function of the acr Genes in Phenazine Regulation and Transport in the Biological Control Strain Pseudomonas Chlororaphis 30-84
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The genes acrA and acrB, first identified in Escherichia coli encode components of a Resistance-Nodulation-Cell Division (RND) efflux system shown to be important for resistance to multiple antimicrobials. In Pseudomonas chlororaphis 30-84, transcriptional analysis demonstrated that the RpeA/RpeB two-component transduction system differentially regulated two genes immediately adjacent to rpeA and rpeB annotated as acrA and acrB. In the present study, I showed that constitutive expression of the 30-84 acrA/acrB genes conferred acriflavine resistance to E. coli and enhanced acriflavine resistance in 30-84 and also conferred resistance to tetracycline, chloramphenicol, and carbenicillin. Importantly, constitutive expression of acrA/acrB enhanced the growth of E. coli in the presence of ~300-800 ?g of total phenazines extracted from 30-84, demonstrating the function of AcrA and AcrB in phenazine transport. Constitutive overexpression of the acrAB operon in trans in wild type 30-84 resulted in enhanced phenazine production and transport. Phenazine production by the 30-84 rpeB mutant was not rescued by overexpression of acrA/acrB unless other RpeA/RpeB controlled traits involved in the regulation of phenazine biosynthesis such as quorum sensing also were restored. Earlier and greater production and diffusion of phenazines significantly increased the capacity of 30-84 to inhibit fungal growth. These data indicated that acrA/acrB enhanced RND-type transporter activities and phenazine transport. Based on the findings from this study, a model is proposed to illustrate how phenazine transport and biosynthesis are related in 30-84 and whether transport may contribute to positive catabolic feedback regulation of phenazine biosysnthesis.