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The ''lac'' operon is an Operon required for the transport and Metabolism of Lactose in '' Escherichia Coli '' and some other Enteric Bacteria . It consists of three adjacent Structural Gene s, a Promoter , a Terminator , and an Operator . The ''lac'' operon is regulated by several factors including the availability of Glucose and of Lactose . Gene regulation of the ''lac'' operon was the first genetic regulatory mechanism to be elucidated and is often used as the Canonical example of Prokaryotic Gene Regulation . Structure of the operon The ''lac'' operon consists of three Structural Gene s, a Promoter , a Terminator , and an Operator . The three structural genes are:: ''lacZ'', ''lacY'', and ''lacA''.
Only ''lacZ'' and ''lacY'' appear to be necessary for lactose Catabolism . Specific control of the ''lac'' genes depends on the availability of the Substrate lactose to the bacterium. The proteins are not produced by the bacterium when lactose is unavailable as a carbon source. The ''lac'' genes are organized into an Operon ; that is, they are oriented in the same direction immediately adjacent on the chromosome and are co-transcribed into a single Polycistronic mRNA molecule. Transcription of all genes starts with the binding of the enzyme RNA Polymerase (RNAP), a DNA-binding Protein , to a specific DNA binding site immediately Upstream of the genes, the ''promoter''. From this position RNAP proceeds to transcribe all three genes (''lacZYA'') into mRNA. The DNA sequence of the '' E. Coli '' lac Operon , the lacZYA MRNA , and the ''lacI '' genes are available from GenBank (view) . 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 is always expressed (''constitutive''). 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 allolactose 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. The diagram below summarizes these statements. Genetic nomenclature Three-letter mnemonics are used to describe phenotypes in bacteria including ''E. coli''. Examples include:
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 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 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 gene expression, including the ''lac'' promoter, ''lac p'', and the ''lac'' operator, ''lac o''. Although it is not strictly standard usage, mutations affecting ''lac o'' are referred to as ''lac o''c, for historical reasons. Lactose analogues ]] ]] ]] ]] A number of lactose derivatives or analogs have been described that are useful for work with the lac operon. These compounds are mainly substituted galactosides, where the glucose moiety of lactose is replaced by another chemical group.
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