Showing papers by "Lei Wang published in 2001"

Molecular Characterization of Streptococcus pneumoniae Type 4, 6B, 8, and 18C Capsular Polysaccharide Gene Clusters

Abstract: Capsular polysaccharide (CPS) is a major virulence factor in Streptococcus pneumoniae. CPS gene clusters of S. pneumoniae types 4, 6B, 8, and 18C were sequenced and compared with those of CPS types 1, 2, 14, 19F, 19A, 23F, and 33F. All have the same four genes at the 5' end, encoding proteins thought to be involved in regulation and export. Sequences of these genes can be divided into two classes, and evidence of recombination between them was observed. Next is the gene encoding the transferase for the first step in the synthesis of CPS. The predicted amino acid sequences of these first sugar transferases have multiple transmembrane segments, a feature lacking in other transferases. Sugar pathway genes are located at the 3' end of the gene cluster. Comparison of the four dTDP-L-rhamnose pathway genes (rml genes) of CPS types 1, 2, 6B, 18C, 19F, 19A, and 23F shows that they have the same gene order and are highly conserved. There is a gradient in the nature of the variation of rml genes, the average pairwise difference for those close to the central region being higher than that for those close to the end of the gene cluster and, again, recombination sites can be observed in these genes. This is similar to the situation we observed for rml genes of O-antigen gene clusters of Salmonella enterica. Our data indicate that the conserved first four genes at the 5' ends and the relatively conserved rml genes at the 3' ends of the CPS gene clusters were sites for recombination events involved in forming new forms of CPS. We have also identified wzx and wzy genes for all sequenced CPS gene clusters by use of motifs.

Sequence analysis of four Shigella boydii O-antigen loci: implication for Escherichia coli and Shigella relationships.

Abstract: Lipopolysaccharide (LPS) is a key component of the outer membranes of gram-negative bacteria. It comprises three distinct regions: lipid A, an oligosaccharide core, and, commonly, a repeat unit polysaccharide O antigen. The O antigen is one of the most variable cell constituents, with variation in the types of sugars present, their arrangement within the O unit, and the linkages between O units. The highly variable nature of the O antigen provides the basis for serotyping, and 187 O-antigen forms (serotypes) have been recognized in Escherichia coli (including Shigella strains) (11, 23, 25). The genes for O-antigen synthesis are normally in a gene cluster which maps between galF and gnd in E. coli and Salmonella enterica. The differences between the many forms of O antigen are almost entirely due to genetic variation in this gene cluster. It has been proposed that inter- and intraspecies lateral transfer of O-antigen genes played an important role in redistributing the polymorphic forms (e.g., references 20, 24, 51, and 53). In regard to the origin of the polymorphism, it has been found that new forms can be formed by homologous recombination or recombination mediated by a transposable element (e.g., references 8, 15, 52, 53, and 57). The O antigen is on the cell surface and appears to be a major target of both the immune system and bacteriophages, which must apply intense selection. Selection is probably a major factor in the origin and maintenance of the high level of variation. Each strain expresses only one O-antigen form, and the variation is thought to allow each of the various clones of a species to present a surface that offers a selective advantage in the niche occupied by that clone. It has been estimated that a selective advantage of only 0.1% for one O antigen over another in a given niche is more than sufficient to maintain different alleles in different clones (45). Analysis of sequence variation in housekeeping genes showed that most of the 46 Shigella serotypes fall into three clusters within E. coli, with five outlier strains (see reference 43). It is important to note that although 46 Shigella serotypes are recognized, there are only 33 distinct O antigens, the others being modifications that in E. coli or S. enterica would not be given separate status. There are only two distinct O-antigen forms for the 14 Shigella flexneri serotypes (see reference 43), and also Shigella boydii serotype 15 and Shigella dysenteriae serotype 2 have identical O antigens (11). Most O-antigen variation is in clusters 1 and 2, with 19 and 7 O-antigen forms, respectively (43). Based on sequence diversity, it was estimated that strains within these two clusters diverged over 50,000 to 270,000 years. Of the 33 O-antigen forms found in the two clusters, 12 are identical to other known E. coli O antigens and 21 are unique to Shigella clones. This determination is based on cross-reactions summarized by Ewing (11). In many cases the conclusions have been confirmed by other structure data or the extensive restriction fragment length polymorphism analysis of the O-antigen gene cluster reported by Coimbra et al. (6), although there are a few discrepancies that might lead to minor adjustments when they are resolved. If the unique forms were gained in Shigella rather than lost by other E. coli strains, the 21 new O antigens gained by Shigella clones in the last 50,000 to 270,000 years represent 11% of the total number of E. coli O antigens, a very rapid expansion by interspecies lateral transfer. To start analysis of this phenomenon, we sequenced gene clusters for S. boydii O antigens 4, 5, 6, and 9. S. boydii O antigens 4 and 6 are in cluster 1, while O antigens 5 and 9 are in cluster 2. S. boydii O antigens 4 and 5 are identical to O antigens 53 and 79, respectively, of traditional E. coli strains, and we also studied the O-antigen gene clusters for these E. coli O antigens.