The effect of MSMEG_6402 gene disruption on the cell wall structure of Mycobacterium smegmatis

Arabinogalactan (AG) of mycobacterial cell wall consists of arabinan region, galactan region and disaccharide linker. The arabinan is composed of D-arabinofuranose residues, and decaprenyphosphoryl-D-arabinose (DPA) is the donor of the D-arabinofuranose residues. DPA is formed from phosphoribose diphosphate (PRPP) in a four-step process catalyzed by transferase, phosphatase and epimerase, respectively. Mycobacterium tuberculosis Rv3806c has been identified as PRPP: decaprenyl-phosphate 5-phosphoribosyltransferase, and heteromeric Rv3790/Rv3791 has epimerase activity. Rv3807c is putative phospholipid phosphatase. However, there is no direct biochemical evidence since expression of Rv3807c has been unsuccessful. Mycobacterium smegmatis MSMEG_6402 is ortholog of Rv3807c. To investigate the function of MSMEG_6402 on AG biosynthesis, a conditional MSMEG_6402 gene knock out (M. sm-ΔM_6402) strain was constructed through homologous recombination technique. The morphological and compositional changes of cell wall were examined in the M. sm-ΔM_6402 strain. The M. sm-ΔM_6402 strain grew at non-permissive temperature slower than that at permissive temperature, indicating that MSMEG_6402 is non-essential for growth of M. smegmatis. The change of cell shape and detectable bulging on the cell surface of M. sm-ΔM_6402 strain were observed by scanning electron microscopy, and curled as well as deformed cell wall of M. sm-ΔM_6402 strain was revealed by transmission electron microscopy. Analysis of sugar composition in the cell wall by HPLC indicated that the ratio of arabinofuran to galactofuran in M. sm-ΔM_6402 strain was changed to 1.7:1 comparing with 2:1 in the wild type. It demonstrates that the lacking MSMEG_6402 interferes the biosynthesis of arabinan. Analyzing 5' P-DPR and DPR from both M. sm-ΔM_6402 strain and wild type M. smegmatis is undergoing in this lab.