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Journal of Bacteriology, June 2006, p. 4474-4486, Vol. 188, No. 12
0021-9193/06/$08.00+0     doi:10.1128/JB.00246-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Proteins Exported via the PrsD-PrsE Type I Secretion System and the Acidic Exopolysaccharide Are Involved in Biofilm Formation by Rhizobium leguminosarum

Daniela M. Russo,¹ Alan Williams,² Anne Edwards,² Diana M. Posadas,¹ Christine Finnie,^2, Marcelo Dankert,¹ J. Allan Downie,² and Angeles Zorreguieta^1,2*

Fundación Instituto Leloir, CONICET, and Inst. de Investigaciones Bioquímicas, FCEyN, University of Buenos Aires, Patricias Argentinas 435, (C1405BWE) Buenos Aires, Argentina,¹ John Innes Centre, Norwich Research Park, Colney, Norwich, NR4 7 UH, United Kingdom²

Received 16 February 2006/ Accepted 27 March 2006

The type I protein secretion system of Rhizobium leguminosarum bv. viciae encoded by the prsD and prsE genes is responsible for secretion of the exopolysaccharide (EPS)-glycanases PlyA and PlyB. The formation of a ring of biofilm on the surface of the glass in shaken cultures by both the prsD and prsE secretion mutants was greatly affected. Confocal laser scanning microscopy analysis of green-fluorescent-protein-labeled bacteria showed that during growth in minimal medium, R. leguminosarum wild type developed microcolonies, which progress to a characteristic three-dimensional biofilm structure. However, the prsD and prsE secretion mutants were able to form only an immature biofilm structure. A mutant disrupted in the EPS-glycanase plyB gene showed altered timing of biofilm formation, and its structure was atypical. A mutation in an essential gene for EPS synthesis (pssA) or deletion of several other pss genes involved in EPS synthesis completely abolished the ability of R. leguminosarum to develop a biofilm. Extracellular complementation studies of mixed bacterial cultures confirmed the role of the EPS and the modulation of the biofilm structure by the PrsD-PrsE secreted proteins. Protein analysis identified several additional proteins secreted by the PrsD-PrsE secretion system, and N-terminal sequencing revealed peptides homologous to the N termini of proteins from the Rap family (Rhizobium adhering proteins), which could have roles in cellular adhesion in R. leguminosarum. We propose a model for R. leguminosarum in which synthesis of the EPS leads the formation of a biofilm and several PrsD-PrsE secreted proteins are involved in different aspects of biofilm maturation, such as modulation of the EPS length or mediating attachment between bacteria.

Present address: Department of Chemistry, Carlsberg Laboratory Gamle, Carlsberg Vej 10, DK 2500, Valby, Denmark.

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