A short guide to the E coli operonsThis page contains background material to the course Quantitative Systems Biology at KTH. Many of the examples used in the book followed in the 2012 year, and earlier years,
Uri Alon, 2007 The operon conceptThe term operon was coined by Monod and Jacob in 1960, and several E coli operons are described in detail in their still very readable review from 1961 (unfortunately this link can be slow, but it exists):
The lac operonThe lac operon regulates the transport and metabolism of lactose and is treated in the book in Fig 2.5 (page 17), Fig 4.9 (page 55), and in the evolutionary experiments described in Fig 10.1 (page 196), Fig 10.2 (page 197) and in Figs 10.4 and 10.5 (page 200). Lactose is a sugar of two carbon rings (a disaccharide sugar) that is found most notably in milk and made from galactose and glucose. Glucose and galactose are simple sugars (monosaccharides) where glucose is very common while galactose is also mainly found in milk products. The lac operon codes lacZ, which cleaves lactose into glucose and galactose, lacY, which pumps lactose into the cell, and lacA. The lac operon is regulated by the lac repressor which represses transcription of the lac genes when it binds to a binding site in the operon. The lac operon is also regulated by the catabolite activator protein (CAP) (also known as cAMP receptor protein) which enhances the expression of the lac genes when it binds to its binding site in the operon. lac repressor depends on the signal allolactose (of which IPTG is an experimental substitute) and only binds DNA when allolactose is not present. Similarly, CAP depends on the signal cAMP and only binds DNA when cAMP is present. The concentration of cAMP varies inversely to that of glucose. The lac operon therefore (roughly) executes the logical function NOT(glucose) AND NOT(NOT(lactose)). In words this means that the lac genes are mainly expressed when glucose is not present in the medium but lactose is.The gal operonThe gal operon regulates the metabolism of galactose and is mentioned in the book in Fig 4.14 (page 64). The gal operon is regulated by CRP-cAMP as the lac operon, and also by the gal repressor (galR) and gal isorepressor (galS). Regulation is quite complex and involves looping of DNA between two different repressor sites. The regulation is galR and galS is described in
The ara operonThe ara operon regulates the transport and metabolism of arabinose and is treated in the book in Fig 4.9 (page 55). Arabinose is a sugar of one five-carbon ring and has its name from gum arabic, a natural gum made of hardened sap historically cultivated in Arabia. The ara operon codes for araB, araA and araD and is regulated by CRP-cAMP and the araC repressor. This system was reviewed quite recently
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