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J. Bacteriol. doi:10.1128/JB.01126-06
Copyright (c) 2006, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Characterization of lptA and lptB, two essential genes implicated in lipopolysaccharide transport to the outer membrane of Escherichia coli

Dipartimento di Scienze biomolecolari e Biotecnologie, Università degli Studi di Milano, Italy; Dipartimento di Biotecnologie e Bioscienze, Università di Milano-Bicocca; Dipartimento di Biologia, Università degli Studi di Milano, Italy

^* To whom correspondence should be addressed. Email: alessandra.polissi{at}unimib.it.

	Abstract

The outer membrane (OM) of Gram negative bacteria is an asymmetric lipid bilayer that protects the cell from toxic molecules. Lipopolysaccharide (LPS) is an essential component of the OM in most Gram negative bacteria and its structure and biosynthesis is well known. Nevertheless, the mechanisms of transport and assembly of this molecule in the OM are poorly understood. To date the only proteins implicated in LPS transport are MsbA, responsible for LPS flipping across the inner membrane, and the Imp/RlpB complex, involved in LPS targeting to the OM.

Here we present evidence that two Escherichia coli essential genes, yhbN and yhbG, now renamed lptA and lptB, respectively, participate in LPS biogenesis. We show that mutants depleted of LptA and/or LptB not only produce an anomalous LPS form but also are defective in LPS transport to OM and accumulate de novo synthesized LPS in a novel membrane fraction of intermediate density between IM and OM. In addition we show that LptA is located in the periplasm and that expression of the lptA-lptB operon is controlled by the extracytoplasmic factor RpoE. Based on these data we propose that LptA and LptB are implicated in the transport of LPS from the inner to the outer membrane of E. coli.

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