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J. Mol. Micro. Biotechnol. 3: 309-318
The PecM Protein of the Phytopathogenic Bacterium
Erwinia chrysanthemi, Membrane Topology and Possible Involvement in
the Efflux of the Blue Pigment Indigoidine
Carine Rouanet and William Nasser
The pecS regulatory locus negatively modulates the expression of many virulence genes in
Erwinia chrysanthemi. This locus consists of two genes,
pecS and pecM, divergently transcribed. Previous studies have shown
that PecS down-regulates the expression of both
pecS and pecM genes and that PecM is required for full
PecS activity. Computer-aided hydropathy analysis of PecM predicted the presence of between 8 to 10
potential transmembrane segments. We analyzed the membrane topology of PecM using the
b-lactamase gene fusion system and obtained the following unique characteristics. PecM contains 10 membrane spanning
segments, with both the amino and carboxyl termini located in the cytoplasmic side of the inner membrane. The
fourth periplasmic loop, which has a relatively long hydrophilic domain containing 17 amino acid residues, may
play an important role in PecM function. The topological model obtained for PecM can be applied to PecM
homologues in other bacteria. Measurement of the extrusion of the blue pigment indigoidine by the
E. chrysanthemi derivative isogenic mutants
pecS, pecM and pecS-pecM revealed that PecM is required for complete efflux of the
pigment. Its relation to other efflux systems and its potential physiological role are discussed.
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