Genetics of a novel starch utilisation pathway present in Klebsiella oxytoca

A 14.3 kb DNA fragment from Klebsiella oxytoca M5a1 has been cloned and shown to provide Escherichia coli with the capacity for growth on alpha- and beta-cyclodextrins. This fragment is located immediately upstream of the previously identified cgt gene coding for cyclodextrin glycosyltransferase. It contains ten genes (cym) organised in two divergently oriented clusters separated by a non-coding region of 419 bp. Four of the genes code for products homologous to the maltose and linear maltodextrin uptake system, another one for a putative cytoplasmic cyclodextrinase. The cym genes of K. oxytoca are distinct and different from the mal genes; cym mutations do not affect maltose catabolism. On the other hand, whereas mutations in the maltose/maltodextrin-uptake genes do not influence cyclodextrin metabolism, a mutation inactivating the malPQ genes coding for maltodextrin phosphorylase and amylomaltase does. Cyclodextrin catabolism is independent of the presence of a functional cyclodextrin glycosyltransferase but degradation of starch and gamma-cyclodextrins requires the activity of this enzyme. The results indicate the existence of a novel starch degradation pathway which involves the extracellular conversion of starch into cyclodextrins by cyclodextrin glycosyltransferase, uptake of the cyclodextrins by a specific uptake system and intracellular linearisation by a cyclodextrinase. The malto-oligosaccharides produced are then channelled into the maltodextrin-degradation route involving the activity of maltodextrin phosphorylase and amylomaltase.