Pseudomonas fluorescens F113 Can Produce a Second Flagellar Apparatus, Which Is Important for Plant Root Colonization

Abstract

The genomic sequence of <i>Pseudomonas fluorescens</i> F113 has shown the presence of a 41 kb cluster of genes that encode the production of a second flagellar apparatus. Among 2,535 pseudomonads strains with sequenced genomes, these genes are only present in the genomes of F113 and other six strains, all but one belonging to the <i>P. fluorescens</i> cluster of species, in the form of a genetic island. The genes are homologous to the flagellar genes of the soil bacterium <i>Azotobacter vinelandii</i>. Regulation of these genes is mediated by the <i>flhDC</i> master operon, instead of the typical regulation in pseudomonads, which is through <i>fleQ</i>. Under laboratory conditions, F113 does not produce this flagellum and the <i>flhDC</i> operon is not expressed. However, ectopic expression of the <i>flhDC</i> operon is enough for its production, resulting in a hypermotile strain. This flagellum is also produced under laboratory conditions by the <i>kinB</i> and <i>algU</i> mutants. Genetic analysis has shown that <i>kinB</i> strongly represses the expression of the <i>flhDC</i> operon. This operon is activated by the Vfr protein probably in a c-AMP dependent way. The strains producing this second flagellum are all hypermotile and present a tuft of polar flagella instead of the single polar flagellum produced by the wild-type strain. Phenotypic variants isolated from the rhizosphere produce this flagellum and mutation of the genes encoding it, results in a defect in competitive colonization, showing its importance for root colonization.