The pioneering studies of Jacob and Monod on the Lac operon in the 1950s led to a body of work on the regulation of gene expression in bacteria. These studies established for the first time that regulatory proteins could bind to DNA and influence gene transcription either positively or negatively. This allows, for example, operons containing genes encoding proteins required to break down particular substrates to be transcriptionally activated in response to the presence of the substrate. Similarly, operons containing genes encoding proteins needed to synthesize a particular substance could be transcriptionally silenced when that substance was available to the bacteria from its environment (for the full discussion of these mechanisms see Chapters 2 and 3).<\/p>\n\n\n\n