Several gram-negative plant and animal pathogenic bacteria have evolved a type III secretion system (TTSS) to deliver effector proteins directly into the host cell cytosol. Sinorhizobium fredii USDA257, a symbiont of soybean and many other legumes, secretes proteins called Nops (nodulation outer proteins) into the extracellular environment upon flavonoid induction. Mutation analysis and the nucleotide sequence of a 31.2-kb symbiosis (sym) plasmid DNA region of USDA257 revealed the existence of a TTSS locus in this symbiotic bacterium. This locus includes rhc (rhizobia conserved) genes that encode components of a TTSS and proteins that are secreted into the environment (Nops). The genomic organization of the TTSS locus of USDA257 is remarkably similar to that of another broad-host range symbiont, Rhizobium sp. strain NGR234. Flavonoids that activate the transcription of the nod genes of USDA257 also stimulate the production of novel filamentous appendages known as pili. Electron microscope examination of isolated pili reveals needle-like filaments of 6 to 8 nm in diameter. The production of the pili is dependent on a functional nodD1 and the presence of a nod gene-inducing compound. Mutations in several of the TTSS genes negate the ability of USDA257 to elaborate pili. Western blot analysis using antibodies raised against purified NopX, Nop38, and Nop7 reveals that these proteins were associated with the pili. Mutations in rhcN, rhcJ, rhcC, and ttsI alter the ability of USDA257 to form nodules on Glycine max and Macroptilium atropurpureum.

The type III secretion system (TTSS) of plant- and animal-pathogenic bacteria is involved in translocation of virulence factors into the host cell cytosol where they modulate cellular processes. Sinorhizobium fredii USDA257 is a gram-negative soil bacterium that forms nitrogen-fixing nodules on specific soybean cultivars (Glycine max (L.) Merr.). This microsymbiont is known to secrete at least five nodulation outer proteins (Nops) in response to flavonoid induction. Some of these Nops have been shown to be secreted by TTSS in this symbiotic bacterium. We have isolated and purified an 18-kDa extracellular protein from flavonoid-induced cultures of USDA257. The N-terminal amino acid sequence of this purified protein was identical to the published sequence of the soybean cultivar-specificity protein, NopB (formerly NoIB). Inactivation of rhcN, which encodes an ATPase, abolished secretion of NopB. Similarly, a nonpolar nopB deletion mutant was compromised in its ability to secrete several Nops. A construct containing the coding region of nopB under control of a T7 promoter was expressed successfully in Escherichia coli and, subsequently, the recombinant NopB was purified by nickel-affinity column chromatography. Polyclonal antibodies raised against purified recombinant NopB were used in Western blot analysis to demonstrate the association of NopB with pilus-like surface appendages. Deletion analysis indicated that the first 33 N-terminal residues of NopB were necessary and sufficient to mediate the secretion of a green fluorescent protein reporter. Introduction of plasmid-borne extra copies of nopB into USDA257 resulted in lower accumulation of native NopB. We also show that USDA257 and its nonpolar nopB deletion mutant exhibited discernible differences in their ability to nodulate legume hosts.

The symbiosis between Sinorhizobium fredii USDA257 and soybean [Glycine max (L.) Merr.] exhibits a high degree of cultivar specificity. USDA257 nodulates primitive soybean cultivars but fails to nodulate agronomically improved cultivars such as McCall. In this study we provide evidence for the involvement of a new genetic locus that controls soybean cultivar specificity. This locus was identified in USDA257 by Tn5 transposon mutagenesis followed by nodulation screening on McCall soybean. We have cloned the region corresponding to the site of Tn5 insertion and found that it lies within a 1.5-kb EcoRI fragment. DNA sequence analysis of this fragment and an adjacent 4.4-kb region identified an operon made up of three open reading frames encoding proteins of deduced molecular masses of 41, 13, and 104 kDa, respectively. These proteins revealed significant amino acid homology to glycine cleavage (gcv) system T, H, and P proteins of Escherichia coli and other organisms. Southern blot analysis revealed the presence of similar sequences in diverse rhizobia. Measurement of ␤-galactosidase activity of a USDA257 strain containing a transcriptional fusion of gcvT promoter sequences to the lacZ gene revealed that the USDA257 gcvTHP operon was inducible by glycine. Inactivation of either gcvT or gcvP of USDA257 enabled the mutant to nodulate several agronomically improved North American soybean cultivars. These nodules revealed anatomical features typical of determinate nodules, with numerous bacteroids within the infected cells. Unlike for the previously characterized soybean cultivar specificity locus nolBTUVW, inactivation of the gcv locus had no discernible effect on the secretion of nodulation outer proteins of USDA257.Sinorhizobium fredii USDA257 is a fast-growing rhizobium that forms nitrogen-fixing nodules on Glycine max, Glycine soja, Neonotonia wightii, Phaseolus vulgaris, and several other legumes (24, 36). The symbiotic relationship between S. fredii USDA257 (here called USDA257) and soybean is of particular scientific and economic interest because this bacterium nodulates soybean in a cultivar-specific manner (13,19). USDA257 effectively nodulates the primitive soybean cultivar Beijing but fails to nodulate agronomically improved cultivars such as McCall (2, 13). Histological studies have shown that USDA257 is able to infect McCall root hairs and induce cortical cell divisions but fails to form infection threads (5).Previously, we have demonstrated the presence of negatively acting nodulation genes by creating Tn5 mutants (14). In one of the mutants (DH4) the Tn5 insertion was located in sym plasmid, while in the second mutant (DH5), the mutation was located in the bacterial chromosome. In DH4, the Tn5 insertion was located in an 8.0-kb EcoRI fragment which harbors the nolXWBTUV locus (34). Disruption of any these genes expands the host range of S. fredii USDA257. Subsequent studies have demonstrated that this locus is part of a USDA257 type III secretion system (T3SS) (27,34). Both DH4 and DH5 were able to form ...

Sinorhizobium fredii USDA257, a soybean symbiont, exports several nodulation outer proteins (Nops) into the rhizosphere. These proteins, which are exported by a type III secretion system (TTSS), have a pivotal role in host-specific nodulation. The entire TTSS of S. fredii lies within a 31-kb region that includes conserved genes that code for secretion machinery proteins, Nops, and several open reading frames (ORF) of unknown function. Identifying the functions of these ORF is essential to understand fully the role of TTSS in nodulation. Here, we report the characterization of y4xP, an ORF of previously unknown function. Southern blot analysis revealed that USDA257 contains two copies of y4xP, while a sibling, USDA191, contains a single copy. The amino acid sequence of Y4XP is homologous to both eukaryotic and prokaryotic cysteine synthase, a key enzyme in sulfur assimilation. The coding region of USDA257 y4xP under control of T7 promoter was expressed in Escherichia coli, and the recombinant protein was purified by nickel-affinity chromatography. Antibodies generated against soybean cysteine synthase cross-reacted with the recombinant protein. A nonpolar mutant of y4xP of USDA191 showed a marked reduction in cysteine synthase activity. Enzyme activity was completely restored when the mutant was complemented with a plasmid containing the y4xP sequence. Cysteine synthase activity was confined to the cell cytosol. Extracellular protein fraction from genistein-induced USDA191 showed no cysteine synthase activity. This observation indicates that cysteine synthase, which is located in the TTSS locus, is not a type III secreted protein. A nonpolar cysteine synthase mutant was able to export all the Nops to the rhizosphere, albeit in reduced amounts compared with the wild-type USDA191. Interestingly, USDA191 cysteine synthase mutant was able to initiate nodules on 'McCall' soybean more efficiently than the wild-type. Our results demonstrate that y4xP encodes a cysteine synthase and inactivation of this gene enhances the ability of USDA191 to form nodules on 'McCall' soybean by regulating Nops production.