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Genetic basis for the structural difference between Streptococcus pneumoniae serotype 15B and 15C capsular
Saskia van Selm1, Lisette M van Cann, Marc A B Kolkman
1Department of Infectious Diseases and Immunology, Utrecht University, 3584 CL Utrecht, The Netherlands.
Infection and Immunity
|October 24, 2003
Summary
The genetic basis for Streptococcus pneumoniae serotype switching between 15B and 15C lies in a variable TA repeat within the O-acetyltransferase gene. This repeat length difference causes a frameshift, leading to distinct capsule structures and reversible serotype transitions.
Area of Science:
- Microbiology
- Genetics
- Molecular Biology
Background:
- Streptococcus pneumoniae serotypes 15B and 15C exhibit structural differences in their capsular polysaccharides.
- Reversible switching between these serotypes has been previously reported, suggesting an underlying genetic mechanism.
Purpose of the Study:
- To identify the genetic basis for the structural variation between Streptococcus pneumoniae serotypes 15B and 15C.
- To elucidate the mechanism behind the reversible serotype switching between these related serotypes.
Main Methods:
- Investigated the capsular polysaccharide synthesis (cps) loci of serotypes 15B and 15C.
- Utilized PCR amplification and DNA sequencing to analyze the cps locus.
- Examined the O-acetyltransferase gene (cps15bM) for variations in clinical isolates.
Main Results:
- A short tandem TA repeat in the O-acetyltransferase gene (cps15bM) was identified as the key difference.
- Serotype 15B possesses eight TA units, while serotype 15C has nine, causing a frameshift and truncated protein in 15C.
- The number of TA repeat units (7-9) correlated perfectly with the serotype in 17 clinical isolates.
Conclusions:
- Structural differences and serotype switching between Streptococcus pneumoniae 15B and 15C are caused by variations in the TA repeat length within the O-acetyltransferase gene.
- Slipped-strand mispairing leading to deletion or insertion of TA units in the cps15bM gene drives the reversible serotype transitions.