Partial redundancy in the synthesis of the D-arabinose incorporated in the cell wall arabinan of Corynebacterineae

摘要

The major cell wall carbohydrate of Corynebacterineae is arabinogalactan (AG), a branched polysaccharide that is essential for the physiology of these bacteria. Decaprenylphosphoryl-D-Arabinose (DPA), the lipid donor of D-arabinofuranosyl residues of AG is synthesized through a series of unique biosynthetic steps, the last one being the epimerization of decaprenylphosphoryl-β-D-ribose (DPR) into DPA that is believed to proceed via a sequential oxidation-reduction mechanism. Two proteins from Mycobacterium tuberculosis, (Rv3790 and Rv3791), were shown to catalyse the epimerization, in an in vitro system. The present study addressed the exact function of these proteins through the inactivation of the corresponding orthologs in Corynebacterium glutamicum (NCgl0187 and NCgl0186, respectively) and the analysis of the in vivo impact on AG biosynthesis. We showed that NCgl0187 is essential whereas NCgl0186 is not. Deletion of NCgl0186 led to a mutant possessing an AG that contains half arabinose and rhamnose, and less corynomycolates linked to AG but more trehalose mycolates, compared to the parental strain. A candidate gene that may encode a protein functionally similar to NCgl0186 was identified in both C. glutamicum (NCgl1429) and M. tuberculosis (Rv2073c). While the deletion of NCgl1429 had no effect on AG biosynthesis of the mutant, the gene could complement the mycolate defect of the AG of the NCgl0186 mutant strongly supporting the concept that two proteins play a similar function in vivo. Consistently, the NCgl1429 gene appeared to be essential in the NCgl0186 inactivated mutant. A detailed bioinformatics analysis showed that NCgl1429, NCgl0186, Rv3791 and Rv2073c could constitute with 52 other proteins belonging to actinomycetales, a group of closely related short chain reductases/dehydrogenases (SDRs) enzymes with atypical motifs. We proposed that the epimerization of DPR into DPA involved 3 enzymes that catalyse two distinct steps, each being essential for the viability of the bacterial cells.