Operon

• An operon is a group of genes that are transcribed at the same time and usually control an important biochemical process.
• Operons are only found in prokaryotes.
• The promoter is the site where RNA polymerase binds to DNA and signals transcription of a polycystronic mRNA.
• The operator produces a binding site for repressor protein and controls the access of RNA polymerase.
• The regulator gene or repressor gene carries the genetic code for the production of repressor protein.
• Structural genes determine the amino acid sequences for enzyme production.
• The lac Operon consists of a promoter, operator, repressor gene (lac I gene), and three structural genes (lac Z, lac Y, lac A), each involved in processing the sugar lactose and producing B-galactosidase, galactose permease, and galactocide transacetylase respectively.
• E. coli can use either glucose, which is a monosaccharide, or lactose, which is a disaccharide.
• Lactose needs to be hydrolysed (digested) first before E. coli can use it.
• When glucose is present and lactose is absent, the E. coli does not produce β-galactosidase.
• The operon is not transcribed when both glucose and lactose are present because RNA polymerase is unstable and keeps falling off the promoter site.
• The change in shape of the repressor protein due to the allolactose causes it to no longer sit on the Operator site.
• The activator protein stabilises RNA polymerase and only works when glucose is absent.
• RNA polymerase can now reach its promoter site.
• This sugar allolactose fits onto the repressor protein at another active site, the allosteric site.
• When both glucose and lactose are present, the lac operon is transcribed only when lactose is present.
• In this way, E. coli only makes enzymes to metabolise other sugars in the absence of glucose.
• The repressor protein blocks the Promoter site where the RNA polymerase settles before it starts transcribing.
• When lactose is absent, a repressor protein is continuously synthesised and it sits on a sequence of DNA just in front of the lac operon, the Operator site.
• When lactose is present, a small amount of sugar allolactose is formed within the bacterial cell.
• Another protein, an activator protein, is needed when glucose is absent and lactose is present.
• When glucose is present and lactose is present, the E. coli does not produce β-galactosidase.
• When glucose is absent and lactose is absent, the E. coli does not produce β-galactosidase.
• When glucose is absent and lactose is present, the E. coli does produce β-galactosidase.