GalA residue

The order of elution of the 24 different standard RG oligomers is shown in table I. The Rha residues are indicated by and the GalA residues by O, whereas the us-GalA units, which are introduced by RG-lyase action, are indicated by . The symbols are explained below the table, and will be used throughout the paper, although the shades of the symbols are different in the figures. 

Figure 1. Time-course measurement of polygalacturonase (PG) activity in the culture filtrate of Colletotrichum lindemuthianum grown on pectin and HPLC-Dionex analysis of mono-, di- and tri-galacturonic acid residues simultaneously released in the culture medium. The data are the mean of three independent experiments or represent one typical experiment in the case of galacturonic acid (GalA) residues. DP= Galacturonic acid degree of polymerization.

Bacteria of the genus Citrobacter from the family Enterobacteriaceae are inhabitants of the intestinal tract and are present in sewage, surface waters, and food contaminated with fecal material. In accordance with close phylogenetic relationship to Salmonella, Citrobacter has a so-called Salmonella-type enterobacterial LPS core (inner Hep3Kdo2 pentasaccharide with both HepI and HepII phosphorylated at 0-4 and HepII substituted at 0-3 with a Glc residue). Two core structures recently fully characterized (C. werkmanii PCM 1548 and PCM 1549) (Kondakova et al., 2009) also contain a GalA residue linked at position 7 of HepIII, which is known to occur in both Salmonella-type (Providencia, see below) and non-Salmonella-type core (Proteus, Holst, 2002, 2007). 

We have already described that a number of rhizobial LPS mutants are sym-biotically defective because they likely induce an increased defense response by the host and/or are more sensitive to the host defense response. One structural feature of rhizobial LPS that appears to be important is the presence of OPS since its absence appears to result in a more robust plant defense response. We have also suggested (above) that the lack of OPS exposes the anionic COS on the bacterial surface which may make the rhizobial cell more sensitive to antimicrobial cationic peptides. Recent work in our laboratory (Brown, unpublished) has shown that a mutation of R. leguminosarum biovar viciae 3841 which specifically results in the loss of GalA residues from the core increases resistance to cationic peptides. It has also recently been shown that R. etli CE3 mutants in IpxE and IpxF, which are unable to remove the 1 and 4 -phosphates and, therefore have LA with increased anionic character, show increased sensitivity to cationic peptides (Ingram et al., 2010). 

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