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Glycosyl hydrolases are important metabolic enzymes which catalyse the hydrolysis of glycosidic linkages in carbohydrates. They are widely distributed among plants, animals and microorganisms, and a large number of genes encoding these enzymes have been cloned and sequenced. Henrissat and co-workers have previously used comparative amino acid sequence analysis to classify a range of distinct families of glycosyl hydrolases [1,2]. Here we focus on two of the families defined by Henrissat and coworkers, families 27 and 36. We demonstrate that they comprise two subclusters of a single superfamily, show that an isomaltodextranase is also a member of this superfamily, and present a phylogenetic analysis of this new amalgamated superfamily, which we have designated the DAG [(isomalto)dextranase/oxgalactosidase)] family. Family 27 and family 36 have previously been shown to consist of a-galactosidases from eukaryotes and prokaryotes, respectively [1,2]. Comparison of representative members of these two families with the protein sequence databases, using the program BLAST [3], identified a number of new potential homologues, including several a-galactosidases, two a-N-acetylgalactosaminidases [4], and an isomaltodextranase from the bacterium Arthrobacter globiformis [5] (see Table 1). Homology between these enzymes was established by determining binary comparison scores, expressed in standard deviations (S.D.), by using the RDF2 program [6] with 500 random shuffles. For example, comparison of the Aspergillus niger a-galactosidase (a member of former family 27) with a Pediococcus pentosaceus agalactosidase (a member of former family 36) yielded a score of 10 S.D., whereas comparison of the A. globiformis isomaltodextranase (previously believed to be unique [5]) with the Cyamposis tetragonoloba a-galactosidase (a member of former family 27) gave an optimized score of 13 S.D. These and other scores obtained are of sufficient magnitude to establish evolutionary relatedness [7] between all of the fully sequenced members of this family (Table 1). Phylogenetic analysis revealed that this family consists of two primary clusters, with isomaltodextranase falling on a separate branch (Figure 1). The first cluster includes eukaryotic a-

The DAG family of glycosyl hydrolases combines two previously identified protein families