In the absence of lactose, this operon is off because an active repressor binds to the operator and prevents transcription. Lactose metabolism begins with hydrolysis of lactose into its component monosaccharides, glucose and galactose. This reaction is catalyzed by the enzyme ß-galactosidase. Only a few molecules of -galactosidase are present in an E. coli cell grown in the absence of lactose. If lactose is added to the bacterium’s environment, the number of ß-galactosidase molecules increases by a thousandfold within 15 minutes. The gene for ß-galactosidase is part of the lac operon, which includes two other genes coding for enzymes that function in lactose metabolism. The regulatory gene, lacI , located outside the operon, codes for an allosteric repressor protein that can switch off the lac operon by binding to the operator. ○ Unlike the
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This note was uploaded on 01/04/2012 for the course BSC BSC1005 taught by Professor Orlando,rebecca during the Fall '10 term at Broward College.