UDP-galactofuranose precursor required for formation of the lipopolysaccharide O antigen of Klebsiella pneumoniae serotype O1 is synthesized by the product of the rfbDKPO1 gene

Novel selection for isoniazid (INH) resistance genes supports a role for NAD+-binding proteins in mycobacterial INH resistance

The genetic basis of isoniazid (INH) resistance remains unknown for a significant proportion of clinical isolates. To identify genes which might confer resistance by detoxifying or sequestering INH, we transformed the Escherichia coli oxyR mutant, which is relatively sensitive to INH, with a Mycobacterium tuberculosis plasmid library and selected for INH-resistant clones. Three genes were identified and called ceo for their ability to complement the Escherichia coli oxyR mutant. ceoA was the previously identified M. tuberculosis glf gene, which encodes a 399-amino-acid NAD+- and flavin adenine dinucleotide-requiring enzyme responsible for catalyzing the conversion of UDP-galactopyranose to UDP-galactofuranose. The proteins encoded by the ceoBC pair were homologous with one another and with the N terminus of the potassium uptake regulatory protein TrkA. Each of the three Ceo proteins contains a motif common to NAD+ binding pockets. Overexpression of the M. tuberculosis glf gene by placing it under the control of the hsp60 promoter on a multicopy plasmid in Mycobacterium bovis BCG produced a strain for which the INH MIC was increased 50% compared to that for the control strains, while similar overexpression of the ceoBC pair had no effect on INH susceptibility in BCG. Mycobacterial extracts containing the overexpressed Glf protein did not bind radiolabeled INH directly, suggesting a more complex mechanism than the binding of unmodified INH. Our results support the hypothesis that upregulated mycobacterial proteins such as Glf may contribute to INH resistance in M. tuberculosis by binding a modified form of INH or by sequestering a factor such as NAD+ required for INH activity.

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.