The S mutation is an alteration in lacI such that the repressor protein binds to the operator, regardless of whether inducer is present or not. In other words, it is a mutation that inactivates the allosteric site, which binds to inducer, while not affecting the ability of the repressor to bind to the operator site. The dominance of the S mutation is due to the binding of the mutant repressor, even under circumstances when normal repressor does not bind to deoxyribonucleic acid (DNA) even in the presence of inducer.
The constitutive reverse mutations that map to lacI are mutational events, which inactivate the ability of this repressor to bind to the operator. The constitutive reverse mutations that map to the operator alter the operator DNA sequence such that it will not permit binding to any repressor molecules, which is either wild type or mutant repressor.