Submitted to: Journal of Bacteriology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/13/2004
Publication Date: 7/1/2004
Citation: Ausmees, N., Kobayashi, H., Deakin, W.J., Marie, C., Krishnan, H.B., Broughton, W.J., Perret, X. 2004. Characterization of nopp, a type III secreted effector of rhizobium sp. ngr234. Journal of Bacteriology. 186(14):4774-4780.
Interpretive Summary: Rhizobia are soil-dwelling bacteria that form nodules on the roots of legume plants. The nodules are specialized structures where atmospheric nitrogen is fixed by the bacterium, which in turn, is utilized by soybean plants for growth and development. This process is termed biological nitrogen fixation and it enables legumes to grow in nitrogen-poor soils. Sinorhizobium fredii USDA257 and its closely related sibling NGR234 secrete proteins into the rhizosphere when they come into contact with legume root exudates. Some of these proteins are involved in regulating nodulation on soybean and other legumes. Currently, very little is known about how these proteins are exported to the rhizosphere. Such information is required for designing strategies targeted toward improving biological nitrogen fixation. We have demonstrated that Sinorhizobium fredii USDA257 and NGR234 use a type III transport system to secrete nodulation outer protein P into the rhizosphere. This protein either promotes nodulation or reduces nodulation in host-dependent manner. Information obtained from this basic study will help scientists to better understand the factors that limit the formation of nitrogen-fixing nodules on legumes. Such an understanding should enable scientists to manipulate biological nitrogen fixation so that farmers can increase yields with minimal use of nitrogen fertilizers.
Technical Abstract: The type three secretion system (TTSS) encoded by pNGR234a, the symbiotic plasmid of Rhizobium sp. NGR234, is responsible for the flavonoid- and NodD1-dependent secretion of nodulation outer proteins (Nop). Abolishing secretion of all or specific Nops significantly alters the nodulation abilities of NGR234 on many of its hosts. In the closely related strain, R. fredii USDA257, inactivation of the TTSS modifies the host range of the mutant to include the improved Glycine max variety McCall. To assess the impact of individual TTSS-secreted proteins on symbioses with legumes, various attempts were made to identify nop genes. Amino terminal sequencing of peptides purified from gels was used to characterize NopA, NopL and NopX, but failed to identify SR3, a TTSS-dependent product of USDA257. Using phage-display and antibodies that recognize SR3, the corresponding protein of NGR234 was identified as NopP. NopP, like NopL, is an effector secreted by the TTSS of NGR234 and, depending on the legume host, it may have deleterious, beneficial, or little effect on nodulation.