MEDizzy - While studying the lac operon in bacteria, a scientist

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Regulation of Gene Expression

While studying the lac operon in bacteria, a scientist isolates mutants of Escherichia coli, which always express the genes of the lac operon (constitutive synthesis). The scientist creates partial diploids of the regulatory elements of the lac operon in these mutants of E. coli. In one partial diploid, expression of the lac operon is still constitutive (synthesis of the genes is observed even in the absence of an inducer). A likely explanation for this result is which of the following?

ExplanationConstitutive synthesis can occur by either of the two mechanisms. The first is an inability to synthesize lac repressor; the second is to have a mutation in the operator region that renders repressor binding impossible. An inability to synthesize lac repressor can be repaired in trans; if the partial diploid contains a functional lac repressor gene, functional protein will be synthesized from the gene, which can bind to the chromosomal operator region, and regulate lac gene expression. If, however, an oc mutation occurred, the operator region is in cis with the operon and can only regulate regions of DNA that are adjacent to the operator. Thus, if a partial diploid contains a normal operator region on the extrachromosomal region of DNA, that operator region cannot regulate the operon on the chromosomal DNA. Thus, the constitutive mutant that was not rescued in a partial diploid is most likely an oc mutation. If the inducer could no longer bind to the repressor, then no expression would occur, as the repressor would not leave the operator region. In addition, if this were the case, then adding normal repressor to the cell (via the partial diploid) should allow expression of the operon. Similarly, if inducer bound too tightly to the repressor, the introduction of normal repressor should reverse the effects of the mutated repressor. The lac repressor does not contain a transactivation domain.