The Role of the embA and embB Gene Products in the Biosynthesis of the Terminal Hexaarabinofuranosyl Motif of Mycobacterium smegmatisArabinogalactan

Abstract

The emb genes are conserved among different mycobacteria. In Mycobacterium smegmatis and Mycobacterium tuberculosis, they belong to an operon comprising three genes, embC, embA, and embB. The EmbB protein has been proposed to be the target of ethambutol, a drug which is known to inhibit the synthesis of the arabinan portion of the mycobacterial cell wall arabinogalactan (AG). To further define the role of EmbB protein in arabinan biosynthesis, embA, -B, and -C genes were inactivated individually by homologous recombination in M. smegmatis. All three mutants were viable, and among the three, the slowest growing embB ؊ mutant encountered profound morphological changes and exhibited a higher sensitivity to hydrophobic drugs and detergents, presumably due to an increase in cell wall permeability. Furthermore, chemical analyses showed that there was a diminution in the arabinose content of arabinogalactan from the embA ؊ and embB ؊ mutants. Specifically, in comparison with the wild-type strain, the crucial terminal hexaarabinofuranosyl motif, which is a template for mycolylation, was altered in both embA ؊ and embB

Identification of a Novel Arabinofuranosyltransferase AftB Involved in a Terminal Step of Cell Wall Arabinan Biosynthesis in Corynebacterianeae, such as Corynebacterium glutamicum and Mycobacterium tuberculosis

“…We recently identified a unique DPA-dependent ␣-D-arabinofuranosyltransferase (AftA) that is responsible for the deposition of the first Araf residue onto the galactan moiety of AG, thus "priming" the polysaccharide for further extension by the Emb proteins (19). However, our current understanding of further downstream arabinan biosynthesis of AG is limited to that of the Emb proteins and is poorly defined (9,39). M. tuberculosis possesses three Emb proteins encoded within the embCAB operon, of which EmbA and EmbB have been implicated in cell wall arabinan biosynthesis (39), whereas EmbC is involved in lipoarabinomannan biosynthesis (31,40,41).…”

Fluorescent Phospholipid Analogs as Microscopic Probes for Detection of the Mycolic Acid-Containing Layer inCorynebacterium glutamicum: Detecting Alterations in the Mycolic Acid-Containing Layer Following Ethambutol Treatment

“…By inhibiting arabinogalactan synthesis, EMB prevents formation of its covalently-bound mycolate layer. 11,12,27,31) The visible effects of EMB on mycolate-layer formation were observed mainly at cell division planes and cell poles (Fig. 2).…”

“…10) Biochemical studies show that Emb proteins are arabinofuranosyl transferases. [11][12][13] Some discrepancies remain, in that disruption of any one emb gene is not lethal. 12) C. glutamicum has a cell-wall structure and composition similar to mycobacteria: the cytoplasmic membrane is covered with a thick peptidoglycan layer, and an arabinogalactan layer surrounds the peptiodoglycan.…”

“…[11][12][13] Some discrepancies remain, in that disruption of any one emb gene is not lethal. 12) C. glutamicum has a cell-wall structure and composition similar to mycobacteria: the cytoplasmic membrane is covered with a thick peptidoglycan layer, and an arabinogalactan layer surrounds the peptiodoglycan. As in mycobacteria, mycolic acid, which is covalently bound to the terminal hexa-arabinosyl motif of arabino-galactan, forms the most exterior layer.…”

Molecular structure of EmbR, a response element of Ser/Thr kinase signaling in Mycobacterium tuberculosis

“…EmbR has originally been cloned from Mycobacterium avium in the context of identifying genes that confer ethambutol resistance (13). Ethambutol, a front-line antitubercular drug, targets a set of membraneembedded arabinosyltransferases (M. avium EmbA and -B and Mtb EmbC, -A, and -B) that are involved in arabinogalactan and lipoarabinomannan biosynthesis, a critical component of the mycobacterial cell wall (13)(14)(15)(16)(17). In M. avium, the embR gene is located upstream of embA and -B, leading to the hypothesis that embR might control expression of the Emb arabinosyltransferases.…”