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NopA Is Associated with Cell Surface Appendages Produced by the Type III Secretion System of Rhizobium sp. Strain NGR234

¹LBMPS, Département de Biologie Végétale, Sciences III, 30 Quai Ernest-Ansermet, University of Geneva, CH-1211 Geneva 4, Switzerland; ²Plant Genetics Research Unit, United States Department of Agriculture-Agricultural Research Service and Department of Agronomy, University of Missouri, Columbia 65211, U.S.A.

Rhizobium sp. strain NGR234, which is capable of interacting with a large number of legumes, utilizes a variety of signaling molecules to establish nitrogen-fixing symbioses. Among these are nodulation outer proteins (Nops) that transit through a type III secretion system (TTSS). Abolition of Nop secretion affects nodulation of certain legumes. Under free-living conditions, the secretion of Nops can be induced by the addition of flavonoids. Here, we show that an in-frame deletion of nopA abolishes secretion of all other Nops and has the same impact on nodule formation as mutations that lead to a nonfunctional TTSS. This secretionminus phenotype of the nopA mutant, as well as bioinformatics analysis of NopA itself, suggests that NopA could be an external component of the TTSS. Electron microscopy showed that NGR234 synthesizes fibrillar structures on the cell surface in a flavonoid-inducible and NopA-dependent manner. Purification of the macromolecular surface appendages revealed that NopA is a major component of these structures.

Additional keywords: effector , pathogenicity , pilus , translocator .