Lac operon

The lac operon consists of three adjacent genes required for the transport and of lactose (milk sugar) in the Escherichia coli (E. coli) and some other bacteria. The term operon is used when genes (in this case lacZYA) are co-transcribed into a single messenger RNA. The lac operon is regulated by several factors, one of which is the availability of lactose as an energy source. Control of the lac genes was the first genetic regulatory mechanism to be elucidated, one reason for this is that it is one of the simplest, at least in outline, consisting of simple negative (lac repressor) and positive (CAP) regulatory elements. The lac operon has been considered the canonical example of prokaryotic gene regulation.

Structure of the operon

The genes of the lac operon include two important ones: lacZ encodes an intracellular enzyme (LacZ, ?-galactosidase), that cleaves the disaccharide lactose into glucose and galactose. A second gene, lacY, encodes a transport protein (LacY, lactose permease) that pumps lactose into the cell. Specific control of the lac genes depends on the presence of the substrate lactose in the growth medium. The genes are highly transcribed into messenger RNA (mRNA), and the mRNA then translated into protein, only when lactose is present.

Related Topics:
Enzyme - Permease

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The lac genes are organized into an operon---they are oriented in the same direction immediately adjacent on the chromosome and are co-transcribed into a single mRNA molecule. Transcription of all genes starts with the binding of the enzyme RNA polymerase (RNAP) to a specific binding site on the DNA next to the gene called the promoter. The lac promoter is just upstream of the lacZ gene, from which position RNAP proceeds to copy all three genes (lacZYA) into mRNA.

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The regulatory response to lactose requires an intracellular regulatory protein called the lactose repressor. The lacI gene encoding repressor lies nearby the lac operon and it is always expressed (referred to as constitutive expression). If lactose is missing from the growth medium, the repressor binds very tightly to a short DNA sequence just downstream of the promoter near the beginning of lacZ called the lac operator. Repressor bound to the operator interferes with binding of RNAP to the promoter, and therefore mRNA encoding LacZ and LacY is only made at very low levels. When cells are grown in the presence of lactose, a lactose metabolite called allo-lactose binds to the repressor, causing a change in its shape. Thus altered, the repressor is unable to bind to the operator, allowing RNAP to transcribe the lac genes and thereby leading to high levels of the encoded proteins.

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Genetic nomenclature

Three-letter mnemonics are used to describe phenotypes in bacteria including E. coli. Examples include Lac (the ability to use lactose), His (the ability to synthesize the amino acid histidine), Mot (swimming motility), and Str (response to the antibiotic streptomycin). In the case of Lac, wild type cells are Lac+ and are able to use lactose as a carbon and energy source, Lac- mutant derivatives cannot use lactose. The same three letters are typically used (lower-case, italicized) 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 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, ?-galactosidase. Various short sequences that are not genes also affect gene expression, including the lac promoter, lac p, and the lac operator, lac o. Although it is not strictly standard usage, mutants affecting lac o are referred to as lac oc, for historical reasons.

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