Major structural features of the cell wall arabinogalactans of Mycobacterium, Rhodococcus, and Nocardia spp.

The cell wall arabinogalactans of strains of Mycobacterium, Rhodococcus, and Nocardia were per-O-methylated, partially hydrolyzed with acid, and the resulting oligosaccharides were reduced and per-O-ethylated to yield per-O-alkylated oligoglycosyl alditol fragments. Analyses of these fragments by gas chromatography-mass spectrometry and of the intact solubilized polysaccharides by 1H and 13C NMR revealed the major structural features of the different arabinogalactans from representatives of the different genera. All of the mycobacterial products contained a homogalactan segment of alternating 5-linked alpha-galactofuranosyl (Galf) and 6-linked beta-Galf residues. The arabinan segment consisted of three major domains, linear 5-linked alpha-arabinofuranosyl (Araf) residues and branched (3-->5)-linked Araf units substituted with either 5-linked Araf or the disaccharide beta-Araf-(1-->2)-alpha-Araf at both branched positions. The recognition of these features in in vivo grown Mycobacterium leprae is an important development. The arabinan from strains of Nocardia contains a nonreducing-end motif composed of the linear trisaccharide, beta-Araf-(1-->2)-alpha-Araf-(1-->5)-Araf, attached to linear 5-linked alpha-Araf units. The galactan segment of the arabinogalactan of Nocardia sp. is composed of linear 5-linked beta-Galf units substituted in part at O-6 with terminal beta-glucosyl units. The two representative strains of Rhodococcus also differed in the composition of the galactan moiety; in addition to the 5-linked Galf, 2- and 3-linked beta-Galf units are present. The reducing end of the galactans, and therefore, apparently, of the entire arabinogalactans from all species from all genera, are apparently composed of the unit, rhamnosyl-(1-->3)-N-acetyl-glucosamine, which, in turn, is apparently attached to peptidoglycan via phosphodiester linkage.