Iron is an essential element for most organisms, including bacteria. The oxidized form is insoluble, and the reduced form is highly toxic for most macromolecules and, in biological systems, is generally sequestrated by iron- and heme-carrier proteins. Thus, despite its abundance on earth, there is practically no free iron available for bacteria whatever biotope they colonize. To fulfill their iron needs, bacteria have multiple iron acquisition systems, reflecting the diversity of their potential biotopes. The iron/heme acquisition systems in bacteria have one of two general mechanisms. The first involves direct contact between the bacterium and the exogenous iron/heme sources. The second mechanism relies on molecules (siderophores and hemophores) synthesized and released by bacteria into the extracellular medium; these molecules scavenge iron or heme from various sources. Recent genetic, biochemical, and crystallographic studies have allowed substantial progress in describing molecular mechanisms of siderophore and hemophore interactions with the outer membrane receptors, transport through the inner membrane, iron storage, and regulation of genes encoding biosynthesis and uptake proteins.

In gram-negative bacteria, type I secretion is carried out by a translocator made up of three proteins that span the cell envelope. One of these proteins is a specific outer membrane protein (OMP) and the other two are cytoplasmic membrane proteins: an ATP-binding cassette (ABC) and the so-called membrane fusion or adaptor protein (MFP). Type I secretion is sec-independent and bypasses the periplasm. This widespread pathway allows the secretion of proteins of diverse sizes and functions via a C-terminal uncleaved secretion signal. This C-terminal secretion signal specifically recognizes the ABC protein, triggering the assembly of the functional trans-envelope complex. This report will mainly deal will recent data concerning the structure and assembly of the secretion complex as well as the effects and role of substrate folding on secretion by this pathway.

Secretion ofEscherichia coli a-hemolysin into the medium does not require the cleavage of an N-terminal signal peptide. The specific secretion apparatus was shown to consist of two proteins, HlyB and HlyD, both located in the inner membrane and encoded by genes contiguous to the hemolysin structural gene (hlyA). It was proposed that these two proteins constitute a membrane-bound translocator for hemolysin [Mackman, N., Nicaud, J. M., Gray, L. & Holland, I. B. (1986) Curr. Top. Microbiol. Immunol. 125,. We show here that an E. coli outer membrane protein, the TolC protein, encoded by a gene not located in the hly cluster, is specifically required for hemolysin secretion. This result suggests that an outer membrane protein might be a component of the secretion apparatus allowing a specific interaction between the inner and the outer membrane.

Gram-negative bacteria use specific heme uptake systems, relying on outer membrane receptors and excreted heme-binding proteins (hemophores) to scavenge and actively transport heme. To unravel the unknown molecular details involved, we present 3 structures of the Serratia marcescens receptor HasR in complex with its hemophore HasA. The transfer of heme over a distance of 9 Å from its high-affinity site in HasA into a site of lower affinity in HasR is coupled with the exergonic complex formation of the 2 proteins. Upon docking to the receptor, 1 of the 2 axial heme coordinations of the hemophore is initially broken, but the position and orientation of the heme is preserved. Subsequently, steric displacement of heme by a receptor residue ruptures the other axial coordination, leading to heme transfer into the receptor.heme binding ͉ iron uptake ͉ membrane protein ͉ membrane transport ͉ protein complex

A 5.5 kb DNA fragment carrying the functions necessary for the specific secretion of the extracellular metalloproteases B and C produced by the Gram‐negative phytopathogenic bacterium Erwinia chrysanthemi has been sequenced. The fragment contains four transcribed and translated genes: inh, which codes for a protease inhibitor and is not required for protease secretion, and prtD, prtE and prtF, which share significant homology with the hlyB, hlyD and tolC genes required for alpha‐haemolysin secretion in Escherichia coli. Mutations in any of the three prt genes abolish protease secretion. The prtD and prtE products (60 and 50 kd) contain at least one hydrophobic segment and the prtF gene product contains a signal sequence.