Title: Enzyme synthesis regulation in Escherichia coli
Objective(s): To study the enzyme synthesis regulation in Escherichia coli cells and determine which substance is the inducer for enzyme synthesis
The lactose operon encodes for three proteins namely:- LacZ- ? -galactosidase, LacY-the lactose permease, and LacA-lactose transacetylase; . The LacI repressor gene that close to lac operon represses the operon and the presence of an inducer will inactivate the repressor. Lactose cannot directly induce the lac operon , it undergoes isomerization to form allolactose; an isomer of lactose, which is an inducer of the Lac operon. It is generated in a side reaction by the low basal levels of ? -galactosidase(Wheatley, et al,. 2013). which are found before induction. IPTG (isopropyl-thio- ? -D-galactosidase) is frequently used as an inducer. But IPTG is not metabolized hence it is useless to cell as it is a gratuitous inducer.The use of less favoured substrates such as lactose is prevented when favoured carbon source such as glucose presents. Catabolite repression depends mainly on the intracellular level of cyclic AMP. Catabolite Activator Protein (CAP) binds with cyclic AMP and the complex is known as cyclic AMP receptor protein (CRP). The level of CRP is always at a constant.Transcription of catabolite sensitive operons like the Lac operon requires binding of CRP-cAMP complex to the promoter region. Hence, RNA polymerase can bind to and transcribe the operon. The regulation of cyclic AMP levels is as a result to changes in activity of adenylate cyclase which catalyses the conversion of both ATP to cyclic AMP and inorganic pyrophosphate. The presence of glucose will cause a drop in the activity of adenylate cyclase and hence there will be a drop in cyclic AMP levels. For the process to occur Glucose must be transported , not necessarily to be broken down and metabolized because non-metabolizable analogs of glucose, like 2-deoxyglucose, cannot be degraded but can be transported and also cause catabolite repression(Crombrugghe, et al,. 1969 ).
(Wheatley, et al,. 2013)
Figure 1: Shows the reactions of ?-galactosidase.