Cyclodextrin glycosyltransferase (CGTase) catalyzes the formation of -, - and -cyclodextrins (cyclic -(1,4) linked oligosaccharides of 6, 7 or 8 glucose residues, respectively) from starch. Nine substrate binding subsites were observed in an X-ray structure of the CGTase from Bacillus circulans strain 251 complexed with a maltononaose substrate. Subsite -6 is conserved in CGTases, suggesting its importance for the reactions catalyzed by the enzyme. To investigate this in detail, we made six mutant CGTases (Y167F, G179L, G180L, N193G, N193L and G179L/G180L). All subsite -6 mutants had decreased kcat values for -cyclodextrin formation, as well as for the disproportionation and coupling reactions, but not for hydrolysis. Especially G179L, G180L and G179L/G180L effected the transglycosylation activities, most prominently for the coupling reactions. The results demonstrate that (i) subsite -6 is important for all three CGTase catalyzed transglycosylation reactions. (ii) Gly180 is conserved because of its importance for the circularization of the linear substrates. (iii) It is possible to independently change cyclization and coupling activities. (iv) Substrate interactions at subsite -6 activate the enzyme in catalysis, via an induced-fit mechanism. This article provides for the first time definite biochemical evidence for such an induced-fit mechanism in the -amylase family.