ExpR Coordinates the Expression of Symbiotically Important, Bundle-Forming Flp Pili with Quorum Sensing in Sinorhizobium meliloti

Type IVb pili in enteropathogenic bacteria function as a host colonization factor by mediating tight adherence to host cells, but their role in bacterium-plant symbiosis is currently unknown. The genome of the symbiotic soil bacteriumSinorhizobium meliloti contains two clusters encoding proteins for type IVb pili of the Flp (fimbrial low-molecular-weight protein) subfamily. To establish the role of Flp pili in the symbiotic interaction ofS. meliloti and its host,Medicago sativa , we deletedpilA1 , which encodes the putative pilin subunit in the chromosomalflp-1 cluster and conducted competitive nodulation assays. ThepilA1 deletion strain formed 27% fewer nodules than the wild type. Transmission electron microscopy revealed the presence of bundle-forming pili protruding from the polar and lateral region ofS. meliloti wild-type cells. The putative pilus assembly ATPase CpaE1 fused to mCherry showed a predominantly unilateral localization. Transcriptional reporter gene assays demonstrated that expression ofpilA1 peaks in early stationary phase and is repressed by the quorum-sensing regulator ExpR, which also controls production of exopolysaccharides and motility. Binding of acyl homoserine lactone-activated ExpR to thepilA1 promoter was confirmed with electrophoretic mobility shift assays. A 17-bp consensus sequence for ExpR binding was identified within the 28-bp protected region by DNase I footprinting analyses. Our results show that Flp pili are important for efficient symbiosis ofS. meliloti with its plant host. The temporal inverse regulation of exopolysaccharides and pili by ExpR enablesS. meliloti to achieve a coordinated expression of cellular processes during early stages of host interaction.