Biof 3124 Apni5, 2018 Instructor E. Glona C. Regisford, Ph D tam 3 Name 6. (10 p

Question

Biof 3124 Apni5, 2018 Instructor E. Glona C. Regisford, Ph D tam 3 Name 6. (10 points) We have discussed how the lac operon (sce Figure E3-2) is controlled by both the CAP activator protein and the lac repressor. You create cells that are mutant in the g coding for the lac repressor so that now these cells lack the lac repressor under all conditions. For these mutant cells, state whether the lac operon will be ON or OFF under the following situations and explain why in each case CAP bindingbinding site site RNA polymerase start site for RNA synthesis promoter) operator lacz gone

Explanation / Answer

The lac operon will be ON.

However, level of expression is regulated by the concentration of glucose.

The lac operon is expressed only when two conditions are met: lactose is available, and glucose is not available.

In the given case of lac repressor mutant, lac repressor will not be expressed, and hence it will not bind to the promoter. The situation mimics the first condition of lac operon expression (i.e., lactose is available). RNA polymerase can transcribe the operon. But the level of expression depends on binding of RNA polymerase which further depends on the concentration of glucose sensed via cAMP and CAP (catabolite activator protein).

Now the expression will depend upon the second condition (i.e., glucose availability). When glucose levels are low, cAMP is produced. The cAMP attaches to CAP allowing it to bind DNA. The CAP further helps RNA polymerase bind to the promoter resulting in high level of expression. When glucose level is high, no cAMP is made, CAP cannot bind DNA without cAMP, so transcription occurs only at a low level.

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