Operon
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| Lac Operon |
| If E.Coli is grown in a medium containing both glucose and lactose, which carbon source will it prefer and why? Explain your answer with reference to operon concept using suitable diagram. |
Triptophan Operon
There is 3 structural genes in the lac operon Lac Z codes for the enzyme β galactosidase. This enzyme breaks a β galactosidase into its component sugars, ( Disaccharide lactose into monosacchardies glucose & galactose)
Lac Y codes for the β galactosidase permease. This transports β galactosidase into the cell.
Lac A codes for β-galactosidase transcetylase an enzyme that transfers an acetyl group from acetyl CoA to β-galactosidase.
Because of their close linkage , all the three genes are transcribed as a single unit, resulting in a polycistronic mRNA. A single message serves for the translation of all three gene products. Monod proposed that adjacent to the lac ZYA structural genes , there is an operator gene, which interacts
with a trans-acting gene product called the repressor molecule ( that is coded by lac I) lac I gene product i.e, the repressor protein is allosteric i.e, it reversibly interacts with another molecule causing the conformational change in three-dimensional shape & a change in chemical activity.
This repressor protein normally interacts with the DNA sequence of operator region & inhibits the action of RNA polymerase . Thus, the transcription of structural genes is repressed.
When lactose is introduced into the culture medium , lactose binds to the repressor molecule & causes an allosteric conformational change. This change alters the binding site of the repressor rendering it
incapable with operator DNA. Then RNA polymerase transcribes the structural genes. So, when the repressor fails to bind operator , transcription occurs. This is negative control.
In wild type bacteria ( genotype is 1+ O+ Z+ Y+ A+) if lactose is present is present , it binds to repressor
& inactivates it, operator is free from repressor , RNA polymerase then transcribes the genes into mRNA & then translates into protein. If in these wild type bacteria , lactose is absent , the repressor binds to the operator . The transcription & translation is inhibited.