In a negative (repressible) operon, the repressor and co-repressor complex binds with the operator. The free repressor cannot bind to the operator. Immediately upstream of the three lac genes is a fourth gene that is responsible for the negative regulation of the operon. This gene expresses a protein called the lac repressor. The repressor can bind to a short sequence of the lac promoter. When repressor binds to that sequence it physically blocks access by RNA polymerase to the DNA.