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Abstract

The successful nitrogen-fixing symbiosis between the Gram-negative soil bacterium and its leguminous plant host alfalfa () requires the bacterial exopolysaccharide succinoglycan. Succinoglycan and flagellum production, along with the ability to metabolize more than 20 different carbon sources and control the expression of a large number of genes, is regulated by the ExoR–ExoS/ChvI signalling pathway. The ExoR protein interacts with and suppresses the sensing activities of ExoS, the membrane-bound sensor of the ExoS/ChvI two-component regulatory system. Here we show that expression is clearly upregulated in the absence of any functional ExoR protein. This upregulation was suppressed by the presence of the wild-type ExoR protein but not by a mutated ExoR protein lacking signal peptide. The levels of expression could be directly modified in real time by changing the levels of total ExoR protein. The expression of was also upregulated by the constitutively active sensor mutation , and blocked by two single mutations, and , in the ExoS sensing domain. Presence of the wild-type ExoS protein further elevated the levels of expression in the absence of functional ExoR protein, and reversed the effects of , and mutations. Altogether, these data suggest that ExoR protein autoregulates expression through the ExoS/ChvI system, allowing cells to maintain the levels of expression based on the amount of total ExoR protein.

Keyword(s): CF, calcofluor white
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2010-07-01
2019-12-13
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vol. , part 7, pp. 2092 - 2101

Supplementary tables ( PDF, 41 kb): Oligonucleotide primers used for plasmid constructions Oligonucleotide primers used for sequencing



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