The operon model, as proposed in 1961 by Jacob and Monod

Step 1:    The regulator gene R encodes a repressor molecule, which can bind    to the    operator (O) and thereby inhibit transcription of the adjacent structural genes.

Step 2:    A small-molecule inducer complexes with the repressor, thereby altering the    equilibrium between the conformational states of the repressor.

Step 3:    The repressor–inducer complex binds less tightly to the operator.

Step 4:    This loosening facilitates transcription of the structural genes, resulting in production of messenger RNA—that is, an RNA copy of the structural genes.

Step 5:    The mRNA sequence is translated into proteins.