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Capsule biosynthesis and basic metabolism in Streptococcus pneumoniae are linked through the cellular
G G Hardy1, M J Caimano, J Yother
1Department of Microbiology, University of Alabama at Birmingham, Birmingham, Alabama 35294, USA.
Journal of Bacteriology
|March 14, 2000
Summary
Most phosphoglucomutase (PGM) activity for Streptococcus pneumoniae type 3 capsule synthesis comes from a cellular PGM, not capsule locus genes. A mutation in this cellular PGM significantly reduced capsule production.
Area of Science:
- Microbiology
- Biochemistry
- Genetics
Background:
- Streptococcus pneumoniae type 3 capsule synthesis requires UDP-glucose (UDP-Glc) and UDP-glucuronic acid (UDP-GlcUA).
- UDP-Glc is generated via glucose-6-phosphate to glucose-1-phosphate to UDP-Glc, mediated by phosphoglucomutase (PGM) and UDP-glucose pyrophosphorylase.
- The type 3 capsule locus contains genes for UDP-glucose pyrophosphorylase (cps3U) and a PGM homologue (cps3M), but these are not essential for capsule production.
Purpose of the Study:
- To investigate the genetic basis for reduced capsule production in a Streptococcus pneumoniae type 3 mutant.
- To identify the specific phosphoglucomutase (PGM) responsible for the majority of PGM activity in type 3 capsule biosynthesis.
Main Methods:
- Characterization of a spontaneous mutant with fourfold less capsule production.
- Genetic analysis to locate the mutation outside the type 3 capsule locus.
- Enzymatic assays and complementation studies to confirm the function of the identified PGM homologue (pgm).
- Insertional inactivation of the pgm gene to assess its role in capsule production and cell growth.
Main Results:
- The mutation responsible for reduced capsule production was located in a distant gene (pgm) encoding a second PGM homologue, not within the capsule locus.
- Enzymatic and complementation studies confirmed the identified gene product functions as a PGM.
- Insertional inactivation of pgm drastically reduced capsule production to less than 10% of the parental level.
- Loss of PGM activity led to growth defects and selection for suppressor mutations.
Conclusions:
- The primary source of phosphoglucomutase (PGM) activity essential for Streptococcus pneumoniae type 3 capsule biosynthesis is the cellular PGM encoded by the pgm gene.
- Capsule locus genes (cps3M) contribute minimally to the overall PGM activity required for capsule production.
- The cellular PGM is critical for both bacterial growth and capsule polysaccharide synthesis.