Editing Lac operon (section)

== Structure ==
[[Image:Lactose etc.png|thumb|300px|Structure of lactose and the products of its cleavage.]]
*The ''lac'' operon consists of 3 [[structural gene]]s, and a [[promoter (biology)|promoter]], a [[terminator (genetics)|terminator]], [[Regulator gene|regulator]], and an [[operator (biology)|operator]]. The three structural genes are: ''lacZ'', ''lacY'', and ''lacA''.
**''lacZ'' encodes [[Β-Galactosidase|β-galactosidase]] (LacZ), an intracellular [[enzyme]]  that cleaves the [[disaccharide]] [[lactose]] into [[glucose]] and [[galactose]].
** ''lacY'' encodes [[Beta-galactoside permease|β-galactoside permease]] (LacY),  a transmembrane [[symporter]] that pumps [[galactoside|β-galactosides]] including lactose into the cell using a proton gradient in the same direction. Permease increases the permeability of the cell to [[galactoside|β-galactosides]].
** ''lacA'' encodes [[Beta-galactoside transacetylase|β-galactoside transacetylase]] (LacA), an enzyme that transfers an [[acetyl group]] from acetyl-CoA to thiogalactoside.

Only ''lacZ'' and ''lacY'' appear to be necessary for lactose [[catabolic pathway]].

By numbers, ''lacI'' has 1100 bps, ''lacZ'' has 3000 bps, ''lacY'' has 800 bps, ''lacA'' has 800 bps, with 3 bps corresponding to 1 amino acid.<ref>{{Cite web |date=2019-01-03 |title=2.6: The lac Operon; CAP site; DNA footprinting |url=https://bio.libretexts.org/Bookshelves/Biochemistry/Supplemental_Modules_(Biochemistry)/2%3A_Bacteria/2.6%3A_The_lac_Operon_CAP_site_DNA_footprinting |access-date=2024-06-20 |website=Biology LibreTexts |language=en}}</ref>

===Genetic nomenclature===
Three-letter abbreviations 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)
* Sm<sup>R</sup> (resistance to the antibiotic [[streptomycin]])

In the case of Lac, wild type cells are Lac<sup>+</sup> and are able to use lactose as a carbon and energy source, while Lac<sup>−</sup> 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''<sup>c</sup>, for historical reasons.