The lac Operon in E. coli Bacteria adapt to changes in their surroundings by using regulatory proteins to turn groups of genes on and off in response to various environmental signals. The DNA of Escherichia coli is sufficient to encode about 4,000 proteins, but only a fraction of these are made at any one time. E. coli regulates the expression of many of its genes according to the food sources that are available to it. In 1961, Francois Jacob and Jacques Monod proposed the operon model of gene regulation in bacteria. The model was based on their study of the genes in E. coli that code for enzymes that affect the breakdown of lactose. Because of the pioneering work of Jacob and Monod, the lac operon is typically used to illustrate gene regulation in bacteria. An operon is a cluster of bacterial genes along with an adjacent promoter that controls the transcription of those genes. When the genes in an operon are transcribed, a single mRNA is produced for all the genes in that operon. The promoter contains short, specific nucleotide sequences that signal the start of a gene and is the site where RNA polymerase starts transcription. Operators are regions of DNA that interact with regulatory proteins that control the transcription operons. The regulatory gene of the lac operon produces an mRNA that produces a repressor protein, which can bind to the operator of the lac operon. The general term for the product of a regulatory gene is a regulatory protein. The lac regulatory protein is called a repressor because it keeps RNA polymerase from transcribing the structural genes. Thus the repressor inhibits transcription of the lac operon. When lactose is present, it binds to the repressor and changes its shape. As a result of this change, the repressor can no longer bind to the operator region. RNA polymerase can then bind to the promoter and transcribe the lac genes. When the enzymes encoded by the lac operon are produced, they break down lactose, eventually releasing the repressor to stop additional synthesis of lac mRNA. Messenger RNA also breaks down after a relatively short amount of time.