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Type IVb pili in enteropathogenic bacteria function as a host-colonization factor by mediating tight adherence to host cells, but their role in bacteria-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 of S. meliloti and its host Medicago sativa, we deleted pilA1 that encodes the putative pilin subunit in the chromosomal flp1 cluster and conducted competitive nodulation assays. ThepilA1 deletion strain formed 27% fewer nodules than wild type. Transmission electron microscopy revealed the presence of bundle-forming pili protruding from the polar and lateral region of S. 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 of pilA1 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 the pilA1 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 detected by footprinting analyses. Our results show that Flp pili are important for efficient symbiosis of S. meliloti with its plant host. The temporal inverse regulation of exopolysaccharides and pili by ExpR enables S. meliloti to achieve a coordinated expression of cellular processes during early stages of host interaction.
Zatakia HM, Nelson CE, Syed UJ, Scharf BE. (2014). Appl Environ Microbiol. Feb 7. [Epub ahead of print]
Via IvanOresnik, Sebastian López
