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The F1-ATP synthase atp operon in the proteobacterium Rhodobacter blasticus contains six open reading frames, encoding six hypothetical proteins. Five of these subunits, in the stoichiometry (alphabeta)3gamma delta epsilon make up the catalytic F1-ATP synthase complex similarly in bacteria, chloroplasts and mitochondria. The sixth gene of the R. blasticus atp operon, urf6, shows very little sequence homology to any protein of known structure or function. The gene has previously been cloned, the product (called majastridin) has been heterologously expressed in Escherichia coli, and purified to high homogeneity [M. Brosché, I. Kalbina, M. Arnfelt, G. Benito, B.G. Karlsson, A. Strid, Occurrence, overexpression and partial purification of the protein (majastridin) corresponding to the URF6 gene of the Rhodobacter blasticus atp operon, Eur. J. Biochem. 255 (1998) 87-92]. We have solved the X-ray crystal structure and refined a model of majastridin to atomic resolution. Here we present the crystal structures of apo-majastridin and the complex of majastridin with Mn2+ and UDP and show that it has extensive structural similarity to glycosyltransferases (EC 2.4). This is the first structure determined from a new group of distantly related bacterial proteins of at least six members. They share the identical amino acids that bind Mn2+ and a triplet of amino acids in the putative sugar-binding site.