Summary auto-generated
This study determined the complete nucleotide sequences of two gene clusters (cap5 and cap8) in Staphylococcus aureus that encode enzymes for synthesizing type 5 and type 8 capsular polysaccharides (CPs). Each cluster contains 16 open reading frames (ORFs) arranged in the same transcriptional orientation. The cap5 and cap8 loci are allelic, located on the same chromosomal region, with 12 of 16 genes showing nearly identical sequences (98% amino acid identity) and four type-specific genes in the central region showing significant sequence divergence. Based on sequence homology comparisons with characterized proteins from other bacteria, the researchers assigned putative functions to 15 of the 16 ORFs, including roles in amino sugar synthesis, sugar transferase activities, chain-length regulation, polymerization, and capsule export. The type-specific genes are hypothesized to encode transferases determining the distinctive linkages and O-acetylation patterns that differentiate CP5 from CP8. These findings provide a molecular framework for understanding how S. aureus generates the two most clinically prevalent capsule types, which account for approximately 80% of clinical isolates and represent potential vaccine targets.
Key findings
- Both cap5 and cap8 clusters contain 16 tightly clustered ORFs transcribed in one orientation; the loci are allelic with 12 common genes flanking 4 type-specific central genes
- Twelve ORFs show 98-99% amino acid identity between cap5 and cap8, while four central region ORFs (cap5H-K and cap8H-K) show less than 43% nucleotide homology, explaining the serotype specificity
- Putative functions assigned based on homology include amino sugar synthesis enzymes (epimerases, dehydrogenases), glycosyltransferases, chain-length regulators, O-acetyltransferases, and membrane-bound flippase/polymerase proteins
- Cap5A and Cap5B likely function as a complex regulating capsule chain-length, analogous to ExoP in Rhizobium, with Cap5B containing conserved ATP-binding motifs
- Type-specific genes cap5H-K and cap8H-K encode proteins that likely determine the distinct linkage patterns and O-acetyl group positioning that differentiate the two capsule types
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Abstract
Summary: The nucleotide sequences of two gene clusters, cap5 and cap8, involved in the synthesis of Staphylococcus aureus type 5 and type 8 capsular polysaccharides (CPs), respectively, were determined. Each gene cluster contained 16 ORFs, which were named cap5A through cap5P for type 5 CP and cap8A through cap8P for type 8 CP. The cap5 and cap8 loci were allelic and were mapped to the Smal-G fragment in the standard Smal map of Staph, aureus strain NCTC 8325. The predicted gene products of cap5A through cap5G and cap5L through cap5P are essentially identical to those of cap8A through cap8G and cap8L through cap8P, respectively, with very few amino acid substitutions. Four ORFs located in the central region of each locus are type-specific. A comparison of the predicted amino acid sequences of cap5 and cap8 with sequences found in the databases allowed tentative assignment of functions to 15 of the 16 ORFs. The majority of the capsule genes are likely to be involved in amino sugar synthesis; the remainder are likely to be involved in sugar transfer, capsule chain-length regulation, polymerization and transport.