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Journal of Bacteriology, July 2004, p. 4759-4773, Vol. 186, No. 14
0021-9193/04/$08.00+0     DOI: 10.1128/JB.186.14.4759-4773.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Evidence that the algI/algJ Gene Cassette, Required for O Acetylation of Pseudomonas aeruginosa Alginate, Evolved by Lateral Gene Transfer

Michael J. Franklin,1,2* Stephanie A. Douthit,1,2 and Marcella A. McClure1,3

Department of Microbiology,1 Center for Biofilm Engineering,2 Center for Computational Biology, Montana State University, Bozeman, Montana 597173

Received 23 December 2003/ Accepted 19 April 2004

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.

* Corresponding author. Mailing address: Department of Microbiology, 109 Lewis Hall, Montana State University, Bozeman, MT 59717. Phone: (406) 994-2420. Fax: (406) 994-4926. E-mail: umbfm{at}montana.edu.

Journal of Bacteriology, July 2004, p. 4759-4773, Vol. 186, No. 14
0021-9193/04/$08.00+0     DOI: 10.1128/JB.186.14.4759-4773.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.



This article has been cited by other articles:

  • Qiu, D., Eisinger, V. M., Head, N. E., Pier, G. B., Yu, H. D. (2008). ClpXP proteases positively regulate alginate overexpression and mucoid conversion in Pseudomonas aeruginosa. Microbiology 154: 2119-2130 [Abstract] [Full Text]  
  • Dillard, J. P., Hackett, K. T. (2005). Mutations Affecting Peptidoglycan Acetylation in Neisseria gonorrhoeae and Neisseria meningitidis. Infect. Immun. 73: 5697-5705 [Abstract] [Full Text]  


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