The Escherichia coli bacterium have a single circular DNA which consist of 4.6 million nucleotides. E. Coli genome encodes more than 4000 proteins. Since their environmental condition is regularly changing, bacterial organisms don't generally need to express their whole supplement of enzymes and proteins. For instance, if there is external tryptophan amino acids available, bacteria does not need to synthesize its amino acid anymore to prevent wasting energy. E. coli can have it direct from surrounding environment. This example showing microorganisms astounding capacities to adapt various environmental factors.
In 1953, Jacob and Monod had discovered first negative regulation of transcription in bacteria. Their studies showed that tryptophan operon was inhibited by tyrptophan amino acid. An operon system consist of the structural genes, the operator and the promoter regions.
Promoter region: The regulatory region located before the structural genes. Binding of regulatory proteins on this region control binding of RNA polymerase on the promoter. Subsequently, RNA polymerase enzyme express contiguous structural genes in operon region.
Operator region: A brief sequence of DNA which located just upstream of structural genes starting point. Occupation of the region controls whether the operon is active or not. Binding of repressor protein to the region prevents RNA polymerase to start transcription of structural genes and vice versa if its not bind.
Structural genes: These genes can express proteins and enzymes. The structural genes in tryptophan operon can code five different enzymes.
Regulator gene: The region located upstream of operon, and express repressor DNA-binding protein to control the operon.