The location and nature of the linkage between peptidoglycan and oligoglucans in the cell wall of Mesorhizobium loti HAMBI 1148 have been defined by the analysis of nitrous acid deamination of peptidoglycan glucosaminyl residues. The MurNH2-Glcn fraction was obtained after converting deaminoacylated and N-deacetylated muramyl residues in the cell wall preparation to lactam forms which were stable during subsequent deamination, followed by reduction and opening of the lactams. GC/MS analysis of this material, subjected to partial hydrolysis and reduction or to methanolysis followed by peracetylation, confirmed the presence of glucosyl residues glycosidically attached to muramic acid. The MALDI-TOF spectroscopic analysis of the deaminated material also revealed the presence of [M-H]- or [M+Na-2H]- ions representing fragments containing muramic acid with one to three linked glucose residues. The analysis of fully methylated neutral oligosaccharides released from the peptidoglycan with lysozyme followed by borohydride reduction showed the presence of di- and trisaccharides lacking the reducing end.
Periplasmic and extracellular glucans of Mesorhizobium huakuii were isolated and characterized by compositional and MALDI-TOF analyses, as well as 1H and 13C NMR spectroscopy. It was shown that M. huakuii produces a cyclic β-glucan composed entirely of nonbranched glucose chains and unmodified by nonsugar substituents. The degree of polymerisation of the cyclic oligosaccharides was estimated to be in the range from 17 to 28. The most abundant glucan molecules contained 22 glucose residues. Glucose residues within the glucan were connected by β-(1,2) glycosidic linkages. The cyclic glucan produced by M. huakuii is quite similar to the periplasmic β-(1,2) glucans synthesized by Agrobacterium and Sinorhizobium genera. The synthesis of β-glucan in M. huakuii is osmoregulated and this glucan could function as an osmoprotectant in free living cells.
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