Evidence for AHL Autoinducer Production by the Soybean Symbiont Bradyrhizobium Japonicum
During the rhizobium/legume symbiosis, the bacterial partner must make the transition from a free-living organism to an intracellular symbiont (bacteroid) capable of nitrogen fixation. Many bacterial genes are regulated in response to the transition from free-living bacterium to bacteroid. For example, attachment proteins would no longer be needed but nitrogenase would be needed. In addition, it would not be wise for the nitrogen-fixing bacteroid to begin fixing nitrogen until a sufficient cell density is achieved.
A number of symbiotic and pathogenic bacteria regulate the expression of symbiosis or virulence specific genes in response to cell density (quorum sensing). In a quorum sensing regulatory system, the bacterium produces an autoinducer molecule (AI) that is secreted to the surrounding medium. Once the AI reaches a high concentration, the AI interacts with regulatory proteins that either activate or repress specific genes. In gram-negative bacteria, two types of AIs have been observed (AI-1 and AI-2). AI-1 molecules are A'-acyl-homoserine lactones (AHL) and the structure of AI-2 has not yet been determined. Recently, evidence for a peptide AI molecule in B. japonicum has been presented. AHL AIs have been detected in Rhizobium leguminosarum and Rhizobium meliloti. However, to date, AHL autoinducers have not been detected in the soybean symbiont, B. japonicum.
Using the NTL4/pZLR4 indicator strain described by Piper et al., we screened twelve strains of B. japonicum. Three of the twelve strains (61A1186, 61A224, and 61A227) produce AHLs (Figure 1). To our knowledge, this is the first evidence of AHL autoinducer production in B. japonicum. The number and type of autoinducer molecule(s) is currently being pursued along with experiments to measure the time course of autoinducer production and optimization of autoinducer production. Autoinducer production by B. japonicum is a potential target for improving the competitiveness of inoculum strains and we are using the indicator strain to screen gene libraries for the genes responsible for autoinducer production and the genes that may be regulated by these molecules.
D. J. Westenberg, "Evidence for AHL Autoinducer Production by the Soybean Symbiont Bradyrhizobium Japonicum," Proceedings of the 13th International Congress on Nitrogen Fixation: Global Perspectives (2001, Hamilton, Ontario, Canada), CAB International Publishing, Jan 2002.
13th International Congress on Nitrogen Fixation: Global Perspectives (2001: Jul. 2-7, Hamilton, Ontario, Canada)
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© 2002 CAB International Publishing, All rights reserved.
01 Jan 2002