Biotechnology and Bioinformatics (1+1)

1.3.4.1. Bacterial operons

Lac operon

Lac operon consists of 3 gene s, lac Z, Y and A coding for β-galactosidase, permease and transacetylase catalyzing a catabolic pathway.

• The genes for these three enzymes occur adjacent to each other and thus are linked. These are referred to as structural gene s, since they have the information to code for the amino acid sequence and thus directly decide the structure and function of the individual protein s of the pathway.
• These 3 genes are regulated as a unit by a single switch operator O. This entire unit is referred to as an operon.
• RNA polymerase binds to the promoters region P and initiates transcription . However, under normal conditions transcription cannot proceed, since a repressor protein coded by the i gene binds to the operator and blocks RNA polymerase movement.
• In the presence of the inducer lactose, the repressor protein structure is modified such that the repressor cannot bind to the operator any more.
• This leads to the transcription of the operon and induction of β-galactosidase and the other two enzymes.
• β-galactosidase cleaves lactose into glucose and galactose and once this happens induction will cease. The genes are expressed or not expressed depending on whether the operator switch is on or off. When the switch is on, the three genes are transcribed by RNA polymerase into a single stretch of mRNA covering all the three genes.
• Each gene segment is referred to as a cistron and the long messenger RNA covering all the cistrons is known as polycistronic.