title: On the evolution of beta-galactosidase.
abstract: The amino acid sequence of beta-galactosidase (beta-D-galactoside galactohydrolase, EC 3.2.1.23) has been compared to itself and to other proteins. Two segments, each of about 380 amino acids, comprising the first three-fourths of the polypeptide chain, were found to be very similar to each other. It is concluded that they are homologous. The carboxyl-terminal fourth has a high percentage of amino acid identities with dihydrofolate reductase of Escherichia coli, suggesting these sequences also are homologous. A model for the origin of beta-galactosidase is presented. The overall similarity of beta-galactosidase to lac repressor does not appear to be significant.
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000235:genetics; D005075:Biological Evolution; D004794:Enzyme Repression; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics; D005696:Galactosidases / Q000235:genetics; D005809:Genes, Regulator; D007785:Lactose / Q000378:metabolism; D009876:Operon; D013762:Tetrahydrofolate Dehydrogenase / Q000235:genetics; D001616:beta-Galactosidase / Q000235:genetics
chemical_list: D001426:Bacterial Proteins; D013762:Tetrahydrofolate Dehydrogenase; D005696:Galactosidases; D001616:beta-Galactosidase; D007785:Lactose