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J. Bacteriol. doi:10.1128/JB.00883-06
Copyright (c) 2006, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Functional analysis of nine putative chemoreceptor proteins in Sinorhizobium meliloti

Lehrstuhl für Genetik, Universität Regensburg, D-93040 Regensburg, Germany

^* To whom correspondence should be addressed. Email: birgit.scharf{at}biologie.uni-regensburg.de.

	Abstract

The genome of the symbiotic soil bacterium Sinorhizobium meliloti contains eight genes coding for methyl-accepting chemotaxis proteins (McpS-Z) and one gene coding for a transducer-like protein (IcpA). Seven of the MCP proteins are localized in the cytoplasmic membrane via two membrane spanning regions, whereas McpY and IcpA are lacking such hydrophobic regions. The periplasmic regions of McpU, McpV and McpX contain the small-ligand binding domain Cache. In addition, McpU possesses the ligand-binding domain TarH. By probing gene expression with lacZ fusions, we have identified mcpU and mcpX as being highly expressed. Deletion of any one of the receptor genes caused impairments in the chemotactic response towards most organic acids, amino acids and sugars in a swarm plate assay. The data imply that chemoreceptor proteins in S. meliloti can sense more than one class of carbon source and suggest that many or all receptors work as an ensemble. Tactic responses were virtually eliminated for a strain lacking all nine receptor genes. Capillary assays revealed three important sensors for the strong attractant proline, namely McpU, McpX and McpY. Receptor deletions variously affected free swimming speed and attractant-induced chemokinesis. Noticeably, cells lacking mcpU were swimming 9% slower than the wild-type control. We infer that McpU inhibits the kinase activity of CheA in the absence of an attractant. Cells lacking one of the two soluble receptors were impaired in chemokinetic proficiency by more than 50%. We propose that the internal sensors, IcpA and the PAS-domain containing McpY, monitor the metabolic state of S. meliloti.

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