title: Construction, isolation and implications of repressor-galactosidase - beta-galactosidase hybrid molecules.
abstract: Escherichia coli heterogenotes, which produce hybrid molecules between the chimaeric protein repressor-galactosidase and the enzyme beta-galactosidase, were constructed. Repressor-galactosidase in which fully active lac repressor is covalently linked to active beta-galactosidase, is an aggregate with a core structure of four beta-galactosidase parts and two peripheral lac repressor dimers. The lac repressor dimers, which are separated by tetrameric beta-galactosidase, retain all the biological activities of tetrameric lac repressor. Substitution of repressor-galactosidase subunits with beta-galactosidase subunits leads to hybrid molecules with y beta-galactosidase subunits aggregated with (4-y) repressor-galactosidase subunits (where y = 1, 2 or 3). A 2:2 hybrid, i.e. a tetrameric beta-galactosidase core with one lac repressor dimer grafted to it, binds at least 100 times less strongly to 32P-labelled  lambdaplac DNA than pure lac repressor or repressor-galactosidase. The data suggest a model in which lac repressor binds with two subunits to lac operator and with the other two subunits elsewhere on the DNA, possibly on sequences like the lac operator.
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism; D004269:DNA, Bacterial / Q000378:metabolism*; D004794:Enzyme Repression; D004926:Escherichia coli / Q000235:genetics; D005696:Galactosidases / Q000235:genetics*; D007785:Lactose / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D055503:Protein Multimerization; D013329:Structure-Activity Relationship; D001616:beta-Galactosidase / Q000235:genetics*
chemical_list: D001426:Bacterial Proteins; D004269:DNA, Bacterial; D005696:Galactosidases; D001616:beta-Galactosidase; D007785:Lactose