Molecular Biology of the Gene 7th Edition by Richard Losick, James Watson, Michael Levine, Tamara Baker, Alexander Gann

Exercise 2

The regulation of a novel operon in E. coli involves two operators that sandwich a promoter and three structural genes. RNA polymerase transcribes the structural genes from the promoter, and a specific repressor represses transcription from that promoter. In the presence of the relevant signal, the repressor binding to DNA is disrupted and repression is alleviated.
To study the mechanism of repression, researchers created a reporter containing the promoter but replaced the operators with lac operator sites. Wild-type lac repressor was able to repress expression from this construct in the cell. In vitro, reseachers visualized protein-DNA complexes in electron micrographs using the reporter construct after incubation with either wild-type Lac repressor or a mutant of the Lac repressor that binds to the operator sites, dimerizes, but fails to tetramerize. The scored data of their observations are given in the table below.

In addition, it was shown that the mutant Lac repressor was not able to repress expression from the reporter construct in vivo, although it was able to repress the endogenous lac operon.
Given the data in the table, propose a model for repression stating how the data supports your model. What other information would you want to know about the novel repression protein to support this model?