There is a mutation in the lacI gene, called the Id mutation, which results in t

Question

There is a mutation in the lacI gene, called the Id mutation, which results in the lac operon being uninducible; that is, no -galactosidase is made in the presence or in the absence of lactose (these results are also unaffected by the presence or absence of glucose in the medium). In partial diploids (chromosome + plasmid), lacId acts as a dominant gene. In other words, even a good copy of the lacI gene on a plasmid cannot overcome the defect of the Id mutation on the genomic lacI gene.

What is the likely cause of this phenotype? DNA footprinting using DNaseI revealed much information about how the lac repressor functions. What would DNA footprinting reveal about the DNA binding sites of a normal Lac repressor protein? A normal RNA polymerase?

Explanation / Answer

The likely cause of this phenotype is Cis dominance.Due to mutations in LacI gene , the repressor changes into a super repressor that in turn becomes insensitive to an inducer.As a result of this,lac operon becomes uninducible.This Lac I mutant even becomes dominant over Lac I + because the Lac I mutant repressor combines with the Lac I+repessor to from a tetramer in which the Lac I mutant repressor interferes with functioning of normal repressor thus resulting in non inducibilty of lac operon as a whole.

DNA foot printing would reveal that a Lac repressor protein would bind at the operator site while a normal RNA polymerase would bind at the promoter site.This is because binding of Lac repressor and normal RNA polymerase to the operator and promoter respectively would not allow DNA ligase to attack on those sites of the DNA molecule.

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