Construction, isolation and implications of repressor-galactosidase - beta-galactosidase hybrid molecules

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.