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October, 2006
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Lac Operon: The genetic unit consisting of three structural genes, an operator and a regulatory gene. The regulatory gene controls the synthesis of the three structural genes: Beta-galactosidase and beta-galactoside permease (involved with the metabolism of lactose), and beta-thiogalactoside acetyltransferase.


JoVE 10984

Prokaryotes can control gene expression through operons—DNA sequences consisting of regulatory elements and clustered, functionally related protein-coding genes. Operons use a single promoter sequence to initiate transcription of a gene cluster (i.e., a group of structural genes) into a single mRNA molecule. The terminator sequence ends transcription. An operator sequence, located between the promoter and structural genes, prohibits the operon’s transcriptional activity if bound by a repressor protein. Altogether, the promoter, operator, structural genes, and terminator form the core of an operon. Operons are usually either inducible or repressible. Inducible operons, such as the bacterial lac operon, are normally “off” but will turn “on” in the presence of a small molecule called an inducer (e.g., allolactose). When glucose is absent, but lactose is present, allolactose binds and inactivates the lac operon repressor—allowing the operon to generate enzymes responsible for lactose metabolism. Repressible operons, such as the bacterial trp operon, are usually “on” but will turn “off” in the presence of a small molecule called a corepressor (e.g., tryptophan). When tryptophan—an essential amino acid—is abundant, tryptophan binds and activates the

 Core: Biology

Expression, Purification, Crystallization, and Enzyme Assays of Fumarylacetoacetate Hydrolase Domain-Containing Proteins

1Research Institute for Biomedical Aging Research, University of Innsbruck Austria, 2Center for Molecular Biosciences Innsbruck (CMBI), University of Innsbruck Austria, 3Division of Genetic Epidemiology, Medical University of Innsbruck Austria, 4Faculty of Chemistry, Department of Organic Chemistry, University of Vienna Austria

JoVE 59729

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