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Center for Biofilm Engineering
Abstract:
"Evidence that the algI/algJ Gene Cassette, Required for O
Acetylation of Pseudomonas aeruginosa Alginate, Evolved by Lateral Gene
Transfer"
04-021 Pseudomonas aeruginosa strains, isolated from chronically
infected patients with cystic fibrosis, produce the O-acetylated extracellular
polysaccharide, alginate, giving these strains a mucoid phenotype. O acetylation
of alginate plays an important role in the ability of mucoid P. aeruginosa
to form biofilms and to resist complement-mediated phagocytosis. The O-acetylation
process is complex, requiring a protein with seven transmembrane domains (AlgI),
a type II membrane protein (AlgJ), and a periplasmic protein (AlgF). The
cellular localization of these proteins suggests a model wherein alginate is
modified at the polymer level after the transport of O-acetyl groups to the
periplasm. Here, we demonstrate that this mechanism for polysaccharide
esterification may be common among bacteria, since AlgI homologs linked to type
II membrane proteins are found in a variety of gram-positive and gram-negative
bacteria. In some cases, genes for these homologs have been incorporated into
polysaccharide biosynthetic operons other than for alginate biosynthesis. The
phylogenies of AlgI do not correlate with the phylogeny of the host bacteria,
based on 16S rRNA analysis. The algI homologs and the gene for their
adjacent type II membrane protein present a mosaic pattern of gene arrangement,
suggesting that individual components of the multigene cassette, as well as the
entire cassette, evolved by lateral gene transfer. AlgJ and the other type II
membrane proteins, although more diverged than AlgI, contain conserved motifs,
including a motif surrounding a highly conserved histidine residue, which is
required for alginate O-acetylation activity by AlgJ. The AlgI homologs also
contain an ordered series of motifs that included conserved amino acid residues
in the cytoplasmic domain CD-4; the transmembrane domains TM-C, TM-D, and TM-E;
and the periplasmic domain PD-3. Site-directed mutagenesis studies were used to
identify amino acids important for alginate O-acetylation activity, including
those likely required for (i) the interaction of AlgI with the O-acetyl
precursor in the cytoplasm, (ii) the export of the O-acetyl group across the
cytoplasmic membrane, and (iii) the transfer of the O-acetyl group to a
periplasmic protein or to alginate. These results indicate that AlgI belongs to
a family of membrane proteins required for modification of polysaccharides and
that a mechanism requiring an AlgI homolog and a type II membrane protein has
evolved by lateral gene transfer for the esterification of many bacterial
extracellular polysaccharides.
Franklin, M.J., S.A. Douthit, and M.A. McClure, "Evidence that the algI/algJ
Gene Cassette, Required for O Acetylation of Pseudomonas aeruginosa
Alginate, Evolved by Lateral Gene Transfer," J. Bacteriol., 186(14):4759-4773 |
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