The lac operon is an operon required for the transport and metabolism of lactose in Escherichia coli and many other enteric bacteria. Although glucose is the carbon source for most bacteria, the lac operon allows for the effective digestion of lactose when glucose is not available. Gene regulation of the lac operon was the first genetic regulatory mechanism to be understood clearly and it is often discussed in introductory molecular and cellular biology classes for this reason. Bacterial operons are polycistronic transcripts that are able to produce proteins from one mRNA transcript. In this case, when lactose is required as a source for the bacterium, the three genes of the lac operon can be expressed and their subsequent proteins translated, lacZ, lacY. The gene product of lacZ is β-galactosidase which cleaves lactose, a disaccharide, LacY encodes Beta-galactoside permease, a protein which becomes embedded in the cytoplasmic membrane to enable transport of lactose into the cell. It would be wasteful to produce the enzymes when there is no lactose available or if there is a more preferable energy source available, the lac operon uses a two-part control mechanism to ensure that the cell expends energy producing the enzymes encoded by the lac operon only when necessary. In the absence of lactose, the lac repressor halts production of the enzymes encoded by the lac operon and this dual control mechanism causes the sequential utilization of glucose and lactose in two distinct growth phases, known as diauxie. The lac operon consists of three genes, and a promoter, a terminator, regulator, and an operator. The three structural genes are, lacZ, lacY, and lacA. lacZ encodes β-galactosidase, an enzyme that cleaves the disaccharide lactose into glucose and galactose. LacY encodes Beta-galactoside permease, a transmembrane symporter that pumps β-galactosides including lactose into the cell using a gradient in the same direction. LacA encodes β-galactoside transacetylase, an enzyme that transfers an acetyl group from acetyl-CoA to β-galactosides, only lacZ and lacY appear to be necessary for lactose catabolism. Three-letter abbreviations are used to describe phenotypes in bacteria including E. coli. Examples include, Lac, His Mot SmR In the case of Lac, wild type cells are Lac+ and are able to use lactose as a carbon and energy source, while Lac− mutant derivatives cannot use lactose. The same three letters are used to label the genes involved in a particular phenotype, where each different gene is additionally distinguished by an extra letter. The lac genes encoding enzymes are lacZ, lacY, and lacA, the fourth lac gene is lacI, encoding the lactose repressor—I stands for inducibility. One may distinguish between structural genes encoding enzymes, and regulatory genes encoding proteins that affect gene expression, current usage expands the phenotypic nomenclature to apply to proteins, thus, LacZ is the protein product of the lacZ gene, β-galactosidase. Various short sequences that are not genes also affect expression, including the lac promoter, lac p
Layout of the lac operon.
Structure of lactose and the products of its cleavage.
lac operon in detail
Image: Annotated Theoretical Model of Bound Tetrameric Lac Repressor