When glucose is present, the lac genes are not fully expressed, even in the pres

Question

When glucose is present, the lac genes are not fully expressed, even in the presence of inducer. This is called catabolite repression. 1. Why does it make biological sense to have the lactose operon under negative control by Lac repressor? 2. CAP is necessary to turn on several sugar operons (including the arabinose, lactose, maltose, and galactose operons). Cells with mutations in CAP cannot efficiently metabolize any of these sugars. On plates that contain a sugar and tetrazolium (an indicator dye), colonies are white if that sugar is metabolized and red if it is not. This kind of plate is often used to screen for cells that cannot metabolize a particular sugar. How could you use these plates to isolate CAP mutants?

Explanation / Answer

i) Catabolite repression lets bacteria to switch to rapidly metabolizable carbon and energy source first. Since glucose is preferred over lactose, synthesis of enzymes involved in catabolism of lactose is inhibited due to negative control over lac operon.

ii) Take one 'red' colony from bacterial culture with particular sugar and dye with possibility of mutations in CAP . Culture the same colony on plate with different sugar and dye and isolate red colony with possible mutation in CAP. Repeat the process with other sugar plates.

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