A novel enzyme acting on starch and malto-oligosaccharides was identified and characterised. The non-hydrolytic enzyme, designated maltosyltransferase (MTase), of the hyperthermophilic bacterium Thermotoga maritima MSB8 disproportionates malto-oligosaccharides via glycosyl transfer reactions. The enzyme has a unique transfer specificity strictly confined to the transfer of maltosyl units. Incubation of MTase with starch or its constituents. i.e. amylose and amylopectin, led to the formation of a set of multiples of maltose (i.e. maltose, maltotetraose, maltohexaose etc.). Malto-oligosaccharides with a degree of polymerization (DP) X were disproportionated to products with a DP of X +/- 2n (with X > or = 3 and n = 0,1,2,...). Maximum activity in a 10-min assay was recorded at pH 6.5 and 85-90 degrees C. The enzyme displayed extraordinary resistance to thermal inactivation. For example, at 90, 85, and 70 degrees C (pH 6.5, 0.34 mg ml-1 protein), MTase half-lives of about 2.5 h, 17 h, and 21 days, respectively, were recorded. The gene for MTase, designated mmtA, was isolated from a gene library of T. maritima strain MSB8. Analysis of the MTase primary structure as deduced from the nucleotide sequence of mmtA revealed that the enzyme is not closely related to known protein sequences. However, low-level local similarity between MTase and the alpha-amylase enzyme family (glycosyl hydrolase family 13) was detected, including conserved acidic residues essential for catalysis. Therefore, MTase should be assigned to this family. Based on detailed sequence analyses and comparison with amylolytic enzymes of known crystal structure we propose that MTase contains a (beta/alpha)8-fold as the core supersecondary structure which is typical for the alpha-amylase family. On the other hand, MTase is unique in that it lacks several residues highly conserved throughout this family. Also, MTase possesses an extraordinarily large domain B (a domain typical for the alpha-amylase family, inserted between beta-strand 3 and alpha-helix 3 of the (beta/alpha)8-barrel fold).