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J. Bacteriol. doi:10.1128/JB.01590-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Chemotaxis of Escherichia coli to Pyrimidines: a New Role for the Signal Transducer Tap

Section of Microbiology, University of California, Davis, CA, 95616

^* To whom correspondence should be addressed. Email: reparales{at}ucdavis.edu.

	Abstract

Escherichia coli is chemotactic to sugars, amino acids, and dipeptides, and the responses are mediated by methyl-accepting chemotaxis proteins (MCPs). Using capillary assays, we demonstrated that E. coli RP437 is attracted to the pyrimidines thymine and uracil and the response was constitutively expressed under all tested growth conditions. All MCP mutants lacking the MCP Tap showed no response to pyrimidines, suggesting that Tap, which is known to mediate dipeptide chemotaxis, is required for pyrimidine chemotaxis. In order to confirm the role of Tap in pyrimidine chemotaxis, we constructed chimeric chemoreceptors (Tapsr and Tsrap), in which the periplasmic and cytoplasmic domains of Tap and Tsr were switched. When Tapsr and Tsrap were individually expressed in an E. coli strain lacking all four native MCPs, Tapsr mediated chemotaxis towards pyrimidines and dipeptides, but Tsrap did not complement the chemotaxis defect. Addition of the C-terminal 19 amino acids from Tsr to the C-terminus of Tsrap resulted in a functional chemoreceptor that mediated chemotaxis to serine but not pyrimidines or dipeptides. These results indicate that the periplasmic domain of Tap is responsible for detecting pyrimidines and the Tsr signaling domain confers Tapsr with the ability to mediate efficient chemotaxis. A mutant lacking dipeptide binding protein was wild type for pyrimidine taxis, indicating that dipeptide binding protein, which is the primary chemoreceptor for dipeptides, is not responsible for detecting pyrimidines. It is not yet known whether Tap detects pyrimidines directly or via an additional chemoreceptor protein.