Monod and Jacob proposed the operon model explain how the lac system is regulated. The term operon refers to two or more contiguous genes and the genetic elements that regulate their transcription in a coordinate fashion. Promoters had not yet been discovered when Monod and Jacob proposed the operon model but were readily incorporated into the operon model after their discovery. Lac operon mode shows a revised version of the original lac operon model that includes the lac promoter. The five major features of the model are:
    1. The products of the lacZ, lacY and lacA genes are encoded in a single polycistronic lac mRNA molecule.
    2. The promoter for this mRNA molecule is immediately adjacent to the lacO region. Promoter mutations (P-) that are completely incapable of making β-galactosidase, permease, and transacetylase have been isolated. The promoter is located between lacI and lacO.
    3. The operator is a sequence of bases (in the DNA) to which the repressor protein binds.
    4. When the repressor protein is bound to the operator; transcription of lac mRNA cannot take place.
    5. Inducers stimulate lac mRNA synthesis by binding to the repressor. This binding alters the repressor's conformation so it cannot bind to the operator. Thus, in the presence of an inducer the operator is unoccupied and the promoter is available for initiation of mRNA synthesis. This state is called derepression.
    This simple model explains many of the features of the lac system and of other negatively regulated genetic systems. However we will see in a later section that this explanation is incomplete as the lac operon is also subject to positive regulation.