Jacob and Monod demonstrated that , in the bacterium E. coli ) , there are specific proteins that repress the transcription of the DNA to its product ( RNA . Jacob and Monod made key experimental and theoretical discoveries that demonstrated that in the case of the lactose system outlined above ( in the bacterium E. coli ) , there are specific proteins that are devoted to repressing the transcription of the DNA to its product ( RNA , which in turn is decoded into protein ) .