Question

1. If you isolated a mutation that disrupted a positive regulatory gene for the thr operon (which contains the genes needed to synthesize the amino acid threonine), what would the phenotype be?

2. Compare and contrast the positive regulation of expression of the lac operon by the cAMP-CAP complex with the negative regulation of expression of the lac operon by the LacI protein. You should have several similarities and parallel differences.

3. All human beings have a rich growth of E. coli bacteria in their large intestine. Will the lac operon in the bacteria present in a lactose-intolerant individual who is careful never to consume anything containing lactose be activated or repressed? Explain.

Answer 1

1. Positive regulatory genes are regulatory elements that permit RNA polymerase binding to the promoter region, thus allowing transcription to occur. Now if mutation disrupts that gene then it simply means that RNA polymerase would not be able to bind to the promoter region and there will be no transcription. Thus the phenotype formed will be unable to metabolize the amino acid threonine as there will be no enzymes formed due to the absence of transcription and thus no translation.

2.  In terms of the lac operon, the positive regulator would be the CRP-cAMP complex that must be bound close to the site of the start of transcription of the lac genes. The binding of this positive regulator allows RNA polymerase to bind successfully to the promoter of the lac gene sequence which advances the transcription of lac genes; lac Z, lacY, and lac A.

In terms of the lac operon, the negative regulator would be the lac repressor lac I which binds to the promoter in the same site that RNA polymerase normally binds. The binding of the lac repressor to RNA polymerase's binding site inhibits the transcription of the lac genes.

3. In the humans that are lactose intolerant and who do not consume lactose, the lac operon will be in the repressed state as there is no lactose present so there is no need for lactose-digesting enzymes.  the lac operon is activated only when lactose is present and glucose is absent.