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J. Bacteriol. doi:10.1128/JB.01389-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

HasB, the Serratia marcescens TonB paralog, is specific to HasR

Unité des Membranes Bactériennes Institut Pasteur (CNRS URA 2172) 25 rue du Dr Roux, Paris CEDEX 15, France

^* To whom correspondence should be addressed. Email: fbiville{at}pasteur.fr.

	Abstract

Serratia marcescens possesses two functional TonB paralogs, TonB_S.M. and HasB, for energizing TonB-dependent transport receptors (TBDT). Previous work had shown that HasB is specific to heme uptake in the natural host and in Escherichia coli expressing the S. marcescens TBDT receptor HasR, whereas TonB_S.M., and the E. coli TonB_EC protein function equally well with various TBDT receptors for heme and siderophores. This has raised the question of the target of this specificity. HasB could be specific either to heme TBDT receptors or only to HasR. To resolve this question, in the present work, we have cloned, in E. coli, another S. marcescens heme receptor, HemR, and we show that this receptor is TonB-dependent and does not work with HasB. This demonstrates that HasB is not dedicated to heme TBDT receptors, but rather, forms a specific pair with HasR. This is the first reported case of a specific TonB protein working with only one TBDT receptor in one given species. We discuss the occurrence, possible molecular mechanisms and selective advantages of such dedicated TonB paralogs.