A bacterium has a faulty lac operon in which there is a structural defect in the operator. In this bacterium there is a structural problem with a segment of DNA that binds a repressor.
A protein known as a repressor binds to an operator site. The operator in this instance (and numerous others) is a section of DNA that lies just downstream of the RNA polymerase binding site or overlaps with it (promoter). In other words, it lies between the operon's promoter and its genes.
The repressor protein inhibits the synthesis of messenger RNA by attaching to the promoter region of the gene(s) (mRNA). The control of gene expression in cells requires repressor proteins.
Thus, by physically impeding the RNA polymerase's ability to operate, these repressor proteins stop the gene from being transcribed.
