Abstract
BackgroundLipopolysaccharide (LPS) is a pathogen associated molecular pattern (PAMP) of animal and plant pathogenic bacteria. Variation at the interstrain level is common in LPS biosynthetic gene clusters of animal pathogenic bacteria. This variation has been proposed to play a role in evading the host immune system. Even though LPS is a modulator of plant defense responses, reports of interstrain variation in LPS gene clusters of plant pathogenic bacteria are rare.ResultsIn this study we report the complete sequence of a variant 19.9 kb LPS locus present in the BXO8 strain of Xanthomonas oryzae pv. oryzae (Xoo), the bacterial blight pathogen of rice. This region is completely different in size, number and organization of genes from the LPS locus present in most other strains of Xoo from India and Asia. Surprisingly, except for one ORF, all the other ORFs at the BXO8 LPS locus are orthologous to the genes present at this locus in a sequenced strain of X. axonopodis pv. citri (Xac; a pathogen of citrus plants). One end of the BXO8 LPS gene cluster, comprised of ten genes, is also present in the related rice pathogen, X. oryzae pv. oryzicola (Xoc). In Xoc, the remainder of the LPS gene cluster, consisting of seven genes, is novel and unrelated to LPS gene clusters of any of the sequenced xanthomonads. We also report substantial interstrain variation suggestive of very recent horizontal gene transfer (HGT) at the LPS biosynthetic locus of Xanthomonas campestris pv. campestris (Xcc), the black rot pathogen of crucifers.ConclusionOur analyses indicate that HGT has altered the LPS locus during the evolution of Xanthomonas oryzae pathovars and suggest that the ancestor of all Xanthomonas oryzae pathovars had an Xac type of LPS gene cluster. Our finding of interstrain variation in two major xanthomonad pathogens infecting different hosts suggests that the LPS locus in plant pathogenic bacteria, as in animal pathogens, is under intense diversifying selection.
Highlights
Lipopolysaccharide (LPS) is a pathogen associated molecular pattern (PAMP) of animal and plant pathogenic bacteria
As in Xanthomonas oryzae pv. oryzae (Xoo) strain BXO1 [21], the Xoo strain BXO8 LPS locus is flanked by the genes for cystathionine gamma lyase at one end and electron transport flavoprotein at the other end
The size, number of genes encoded, and the organization of the BXO8 LPS locus are in contrast to the LPS locus that is present in Xoo strain BXO1 [21]
Summary
Lipopolysaccharide (LPS) is a pathogen associated molecular pattern (PAMP) of animal and plant pathogenic bacteria. Variation at the interstrain level is common in LPS biosynthetic gene clusters of animal pathogenic bacteria. This variation has been proposed to play a role in evading the host immune system. Eleven highly divergent gene clusters were reported to occupy an LPS locus in Pseudomonas aeruginosa, an opportunistic human pathogen [3]. This variation is attributed to selection to evade immune responses; in Vibrio cholera changes involving LPS loci within different strains have been implicated in major epidemics [4]. Apart from being a Pathogen Associated Molecular Pattern (PAMP), LPS acts as a receptor for bacteriophages, with mutations that cause altered LPS resulting in phage resistance [5,6]
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