title: The functional repressor parts of a tetrameric lac repressor-beta-galactosidase chimaera are organized as dimers.
abstract: The chimaeric protein repressor-galactosidase, 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. Electron micrographs revealed repressor-galactosidase to be a tetrameric aggregate. When lac repressor, alone, was crosslinked with dimethyl suberimidate, dimers, trimers, tetramers, and oligomers of the protein subunit were produced, whereas crosslinking of the tetrameric repressor-galactosidase resulted in the production of only dimers of the chimaera. 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, whereas iodine still produced a covalent linkage. These results are interpreted as evidence that the lac repressor parts of the tetrameric repressor-galactosidase-chimaera are organized as dimers on the tetrameric-beta-galactosidase core. Because this chimaera has been previously shown to have normal repressor activity [B. Müller-Hill and J. Kania (1974) Nature, 249,561-563], we conclude that lac repressor still is biologically active as a dimeric aggregate.
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism; D055598:Chemical Phenomena; D002621:Chemistry; D002918:Chymotrypsin; D004269:DNA, Bacterial / Q000378:metabolism; D004926:Escherichia coli; D005696:Galactosidases; D005809:Genes, Regulator; D007094:Imides; D007455:Iodine; D007785:Lactose / Q000378:metabolism; D008854:Microscopy, Electron; D009876:Operon; D011487:Protein Conformation; D013329:Structure-Activity Relationship; D014357:Trypsin
chemical_list: D001426:Bacterial Proteins; D004269:DNA, Bacterial; D007094:Imides; D007455:Iodine; D005696:Galactosidases; D002918:Chymotrypsin; D014357:Trypsin; D007785:Lactose