1) In the wildtype (the DNA sequence most commonly found in nature) lac operon,

Question

1) In the wildtype (the DNA sequence most commonly found in nature) lac operon, transcription is severely down-regulated. A mutation in lacI that causes an early stop codon in the sequence (called a lacI - strain) results in an operon that has high levels of transcription of the polycistronic m-RNA. What type of regulatory protein is the LacI protein

2) If a lacI - bacterium is grown in the presence of the sugar X-gal, a blue dye appears; however, the concentration of X-gal decreases over time. What has happened to the X-gal?

Explanation / Answer

1) Normally (wild type) lac operon is an example of negative inducible operon. In negative inducible operon regulatory gene produce regulatory protein which in active form acts as a repressor & binds to the operator resulting in prevention of binding of RNA polymerase to the operator & thus inhibit the transcription. When this regulatory protein is inactivate by the substrate it's unable to bind to the operator & thus unable to prevent transcription. This is happen in the case of lac operon. In the absence of lactose, the regulator protein is active & binds to the operator & inhibits transcription. When lactose is present, some of the lactose is converted to allolactose which binds with the regulator protein & prevents its binding with the operator & thus transcription can takes place. So, the regulatory protein is a repressor.

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