During investigation of the lac operon, Jacob and Monod found that mutations in the same gene did not always lead to the same phenotypic outcome. For example, some mutations in the lacI gene led to constitutive lactose breakdown, whereas other mutations led to an inability to ferment lactose under any conditions. Which of the following statements are true about lacI^s mutations? Please select all that apply.

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Normally, lacI encodes the lac repressor protein. LacI binding at the operator represses the lac operon. Allolactose binding to LacI releases LacI from the operator and the lac operon is de-repressed. However, lacI^smutants encode a 'super-repressor' protein that cannot bind allolactose and is therefore always bound to DNA. This means that the lac operon is always repressed, even in the presence of lactose, and so lacI^s mutants cannot utilize lactose. The operator sequence is not changed in lacI^s mutants, though it would be possible for a mutation in the operator sequence to have a similar effect as a lacI^s mutation. Introduction of a functional operon on a plasmid (i.e., complementation in trans) is not effective at rescuing this phenotype. This is because the LacI^s protein never releases from the operator protein, so a 'good' copy of LacI cannot compensate for the 'bad' copy by replacing it at the operator.

During Investigation of the Lac Operon, Jacob and Monod Found