The functional repressor parts of a tetrameric lac repressor-beta-galactosidase chimaera are organized as dimers.

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Abstract
The chimeric protein repressor-galactosidase [from Escherichia coli], in which fully active lac repressor is covalently linked to the active enzyme .beta.-galactosidase, was used as a system for probing the quaternary structure of lac repressor. EM showed that repessor-galactosidase is a tetrameric aggregate. When lac repressor alone was crosslinked with dimethyl suberimidate, dimers, trimers, tetramers and oligomers of the protein subunit were produced, but crosslinking of the tetrameric repressor-galactosidase resulted in the production of only dimers of the chimera. Treatment of lac repressor with iodine resulted in the formation of protein dimers; the same result was obtained with repressor-galactosidase. After limited proteolysis of lac repressor, no crosslinking was obtained after treatment with dimethyl suberimidate, but iodine still produced a covalent linkage. The lac repressor parts of the tetrameric repressor-galactosidase-chimera are apparently organized as dimers on the tetrameric-.beta.-galactosidase core. Because this chimera was previously shown to have normal repressor activity, lac repressor still is biologically active as a dimeric aggregate.