Question

How would a mutation in the lac repressor protein that was missing the effector binding site behave in E. coli in the presence and absence of lactose? Would it produce B=-galactosidase?

Answer 1

The lac repressor is a DNA-binding protein that inhibits the expression of genes coding for proteins involved in the metabolism of lactose in bacteria. These genes are repressed when lactose is not available to the cell, ensuring that the bacterium only invests energy in the production of machinery necessary for uptake and utilization of lactose when lactose is present. When lactose becomes available, it is converted into allolactose, which inhibits the lac repressor's DNA binding ability, thereby increasing gene expression. Effector molecule are small molecule that interacts with the repressor and affects the affinity of the repressor for the operator. In this system lactose acts as an effector molecule. The repressor thus when lactose is available binds to it which leads to switching on the lac operon. The lacIS (superrepressor) mutants, shows no production of lac enzymes in the presence or absence of lactose. In this situation, the mutant repressor gene produces a superrepressor protein that can bind to the operator, but cannot recognize the inducer allolactose. Therefore, the mutant superrepressors bind to the operators even in the presence of the inducer, and the operons can never be transcribed. The presence of normal repressors in the cell has no effect, because, once a lacIS repressor is on the operator, the repressor cannot be induced to fall off. Cells with a lacIS mutation cannot use lactose as a carbon source.

Therefore, a mutation in the lac repressor protein that was missing the effector binding site will lead to repression of the operon eiher in presence or absence of lactose. As the operon is repressed, beta galactosidase and all other enzymes will not be produced.