Evolution of Lipopolysaccharide Biosynthesis Genes
Lipopolysaccharide (LPS) is a highly polymorphic structure that differs within and between genera, and contains three main components: lipid A, core oligosaccharide (OS), and O-specific antigen in the order in which they occur in LPS, which correlates wit
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Monica M. Cunneen and Peter R. Reeves
11.1
Introduction
Lipopolysaccharide (LPS) is a highly polymorphic structure that differs within and between genera, and contains three main components: lipid A, core oligosaccharide (OS), and O-specific antigen in the order in which they occur in LPS, which correlates with increasing structural diversity for each component. In Escherichia coli, for example, there are five core OS types known and over 180 O-antigen forms (including Shigella), and in Salmonella enterica, 2 and 46 respectively. The diversity of O-antigen forms has been widely studied for some species although the forms known may be underestimates as most of the isolates typed are from humans or domestic animals and their associated environments. Further examples for well-documented species are 20 O-antigen forms recognised in Pseudomonas aeruginosa [1], 21 in Yersinia pseudotuberculosis [2] and about 200 in Vibrio cholerae [3]. Such structural diversity is linked with genetic diversity, and in this chapter, the evolution and diversity of the genes required for the synthesis of these LPS structural components will be explored. We start with an overview of the structure and genetics of each LPS component and discussion of the characteristics of the genes involved in biosynthesis, followed by case studies to highlight particular evolutionary aspects, such as the variation between genetic loci among species, how the clusters involved can be grouped by species or the pathways involved, and also the evidence for gene transfer events on a whole-cluster, gene-block and individual gene scale. We will then conclude with a discussion on the evolutionary forces driving the immense diversity of LPS.
M.M. Cunneen • P.R. Reeves (*) Division of Microbiology, School of Molecular and Microbial Biosciences, University of Sydney, Sydney, NSW 2006, Australia e-mail: [email protected]; [email protected] Y.A. Knirel and M.A. Valvano (eds.), Bacterial Lipopolysaccharides, DOI 10.1007/978-3-7091-0733-1_11, # Springer-Verlag/Wien 2011
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11.2
M.M. Cunneen and P.R. Reeves
Overview of LPS Structure and Gene Clusters
Biosynthesis of the basic 2-keto-3-deoxy-octulosonic acid (Kdo)-lipid A structure is generally well conserved among Gram-negative bacteria, with nine genes (lpxA-D, lpxH, waaA and lpxK-M), required for its synthesis (see also Chaps. 6 and 8). These genes are scattered throughout the genome: some individually, and others in clusters, in contrast to the genes for core OS and O-antigen, which are generally in specific gene clusters for each component. The genes for lipid A are in effect part of the core OS gene cluster, and a study in Campylobacter indicated sequence variation in the lpxA gene correlated with species divisions [4], as is expected for housekeeping genes. The variation in lipid A involves for example, the type (length and number) of fatty acids present, the degree of phosphorylation, and the presence of substitutes like phosphoethanolamine and 4-amino-4-deoxy-L-arabinose. Genes
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