Yildirim (2003), lac Operon December 2006, model of the month by Melanie Stefan Show The preferred carbon source of E. coli is glucose. However, if glucose is unavailable, E. coli has an alternative option: It can break down lactose to produce glucose and galactose [1]. The switch between these two alternative pathways relies on the regulation of three structural genes by a control element. This functional unit, called "operon", was first described by Jacob et al. [2]. The lac operon contains three genes: lacZ codes for β-galactosidase, which is needed for breaking down lactose. lacA codes for galactoside permease, which is needed for lactose transport into the cell. lacA codes for thiogalactoside transacetylase, which does, however, not seem to play a role in this context [1]. Regulatory mechanisms within the lac operon are illustrated in figure 1: If glucose is present and lactose is absent, the lac repressor binds to the operator region. This prevents lac gene transcription. If both glucose and lactose are both present, lactose binds to the repressor and prevents it from binding to the operator region. The block of lac gene transcription is thus lifted, and a small amount of mRNA is produced. But since glucose is still available, the need for β-galactosidase and galactoside permease is limited. If, however, glucose is absent and lactose becomes the only available carbon source, the picture changes. Lactose still prevents the repressor from binding to the operator region. But in addition, the lack of glucose leads to a rise in cyclic AMP (cAMP) concentration. cAMP forms a complex with the catabolite activator protein (CAP). This complex binds to the promoter region and stimulates the transcription of the three lac genes. Large amounts of lac mRNA are produced. Shortly after the operon concept was first described, people started developing mathematical models for lac operon function (reviewed in [4]). More recent models (e.g. [5]) include more biological detail.
Bibliographic References
How does lactose affect gene expression?When a small amount of lactose is present the lac repressor will bind it causing dissociation from the DNA operator thus freeing the operon for gene expression.
What does lactose do to E. coli?E. coli are facultative anaerobic, Gram-negative bacilli that will ferment lactose to produce hydrogen sulfide. Up to 10% of isolates have historically been reported to be slow or non-lactose fermenting, though clinical differences are unknown.
What is lactose in gene regulation?The lactose operon (also known as the lac operon) is a set of genes that are specific for uptake and metabolism of lactose and is found in E. coli and other bacteria.
How is the lac operon regulated in E. coli?The operon is regulated by Lac repressor, the product of the lacI gene, which is transcribed from its own promoter (PI). The repressor inhibits transcription by binding to the lac operator (O).
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