A strain of E. coli is genetically engineered in which the lacZ and lacI genes are removed and replaced with a gene encoding a fluorescent protein. A copy of the operon in which the protein product fluoresces green is inserted into the chromosome, and a copy in which the protein product fluoresces red is inserted into a plasmid. Both copies carry a wild-type lacO gene, and the cells of the strain have a single copy of the lacI repressor gene. If both fluorescent proteins are expressed, the cells fluoresce yellow (because a combination of red and green fluorescence appears as yellow), and if neither of the fluorescent proteins is expressed, the cells show no fluorescence.

The cells would fluoresce ___ in the presence of an inducer and in the absence of an inducer.

Explanation:

The cells would fluoresce yellow in the presence of an inducer and yellow in the absence of an inducer

Normally,

In the presence of an inducer:

Inducer binds to the lac repressor and induce conformational changes causing dissociation of lac repressor from operator

In absence of lac repressor RNA Polymerase binds to the promoter and starts transcription of genes which catabolize lactose

In the absence of an inducer:

Without an inducer lac I gene produces lac repressor which binds to operator region and prevent binding of RNA Polymerase and thus blocks transcription

In case of fluorescence,

If there is mutation in lac operon,the operon becomes constitutive(continuous)

Mutation in lac I gene

Due to mutation in lac I gene lac repressor becomes defective and unable to bind with lac operator

Mutation in operator region

Due to mutation in operator region it prevents binding of lac repressor

Hence operon in the given conditions will be constitutive and yellow fluorescence will be observed in both cases(with and without inducer)