Enzymatic synthesis of UDP-galactofuranose and an assay for UDP-galactopyranose mutase based on high-performance liquid chromatography

Characterization of the type 8 capsular gene cluster of Streptococcus pneumoniae

The complete nucleotide sequence of the capsular gene cluster (cap8) responsible for the biosynthesis of the capsular polysaccharide of Streptococcus pneumoniae type 8 has been determined. The cap8 gene cluster, located between the genes dexB and aliA, is composed of 12 open reading frames. A 14.7-kb DNA fragment embracing the cap8 genes was sufficient to transform an unencapsulated type 3 S. pneumoniae strain to a strain with the type 8 capsule. A possible scenario for the evolution of pneumococcal types 2 and 8 is outlined.

Biosynthetic origin of mycobacterial cell wall galactofuranosyl residues

SETTING

Mycobacterial galactofuran is essential to the linking of the peptidoglycan and mycolic acid cell wall layers. Galactofuran biosynthesis should thus be essential for viability.

OBJECTIVE

The objective was to determine the pathway of galactofuranosyl biosynthesis and to clone a gene encoding an essential enzyme necessary for its formation.

DESIGN

Specific enzymatic conversions involved in formation of galactopyranose and galactofuranose residues in other bacteria were tested for in Mycobacterium smegmatis. M. tuberculosis deoxyribonucleic acid (DNA) was identified by homology.

RESULTS

It was shown that the de novo synthesis of the galactose carbon skeleton occurred in M. smegmatis by the transformation of UDP-glucopyranose to UDP-galactopyranose via the enzyme UDP-glucose 4-epimerase (E.C. 5.1.3.2). The N-terminal sequence of this enzyme was obtained after purification. The galactose salvage pathway enzyme, UDP-glucose-galactose-1-phosphate uridylyltransferase (E.C. 2.7.7.12), was also shown to be present. The critical biosynthetic transformation of the galactopyranose to galactofuranose ring form was shown to occur at the sugar nucleotide level via the enzyme UDP-galactopyranose mutase (E.C. 5.4.99.9). The M. tuberculosis DNA encoding this enzyme was sequenced, the gene expressed in Escherichia coli, and the expected enzymatic activity demonstrated.

CONCLUSION

Galactofuranose biosynthesis can now be pursued as a potential drug target in M. tuberculosis.