An clone was detected in a NCIMB 7423 plasmid library capable of degrading soluble amylose. Deletion subcloning of its recombinant plasmid indicated that the gene(s) responsible for amylose degradation was localized on a 1.8 kb fragment. This region was sequenced in its entirety and shown to encompass a large ORF capable of encoding a protein with a calculated molecular mass of 57184 Da. Although the deduced amino acid sequence showed only weak similarity with known amylases, significant sequence identity was apparent with the 4-α-glucanotransferase enzymes of Streptococcus pneumoniae (46.9%), potato (42.9%) and (16.2%). The clostridial gene (designated ) was followed by a second ORF which, through its homology to the equivalent enzymes of and was deduced to encode maltodextrin phosphorylase (MalP). The translation stop codon of overlapped the translation start codon of the putative gene, suggesting that the two genes may be both transcriptionally and translationally coupled. 4-α-Glucanotransferase catalyses a disproportionation reaction in which single or multiple glucose units from oligosaccharides are transferred to the 4-hydroxyl group of acceptor sugars. Characterization of the recombinant enzyme demonstrated that glucose, maltose and maltotriose could act as acceptor, whereas of the three only maltotriose could act as donor. The enzyme therefore shares properties with the MalQ protein, but differs significantly from the glucanotransferase of which is unable to use maltotriose as donor or glucose as acceptor. Physiologically, the concerted action of 4-α-glucanotransferase and maltodextrin phosphorylase provides with a mechanism of utilizing amylose/maltodextrins with little drain on cellular ATP reserves.


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