SUMMARY: The gene, a member of the multiple sugar metabolism () operon, encodes an intracellular glucan 1,6-α-glucosidase which releases glucose from the non-reducing terminus of α-1,6-linked isomaltosaccharides and dextran. Comparison of primary amino acid sequences showed strong homology to oligo-1,6-glucosidases and, like these enzymes, DexB was able to release free glucose from the α-1,4,6-branch point in panose. This suggested a role for DexB in the metabolism of either starch or intracellular polysaccharide, which contain such branch points. However, purified intracellular polysaccharide from the wild-type strain LT11 and a mutant deficient in revealed no substantial differences in the extent of branching as demonstrated by iodine staining spectra and the degree of polymerization. Furthermore, thin layer chromatography of radiolabelled intracellular polysaccharide digested with wild-type and mutant cell extracts showed no differences in the products obtained. The involvement of DexB in dietary starch metabolism was investigated using α-limit dextrins produced from the action of α-amylase on starch. These induced the operon, including , and the DexB enzyme was able to act on the α-limit dextrins to give further fermentable substrates. The transport system encoded by the operon can also transport α-limit dextrin. DexB may therefore be important in the metabolism of extracellular starch.


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