Question

I need to compare the Lac, Trp, Gal and Arabinose operons

Answer 1

Lac operon

• The Lac operon is the classic operon and is responsible for the degradation of the milk protein lactose.
• The Lac operon is an inducible operon; in the absence of lactose the operator is blocked by a repressor protein.
• The operon is made up of a promoter with operator, and three genes (lacZ, lacY, and lacA) which encode β-galactosidase, permease, and transacetylase.
• Thethree genes are involved in the breakdown of lactose into its metabolites:
• β-galactosidase breaks lactose down into glucose and galactose, while the other two proteins aid in the metabolic process.
• The expression of the Lac operon is controlled by the regulatory gene lacI, located immediately adjacent to the promoter region
• In order for the Lac operon to be turned on, an allolactose molecule must inactivate the repressor protein.

Trp operon

• The Trp operon is responsible for synthesis of the amino acid tryptophan.
• The Trp operon is made up of a promoter with an operator, and five genes that encode enzymes for tryptophan synthesis.
• The Trp operon is regulated by the regulatory gene trpR
• It is a repressible operon
• it is turned off by a repressor protein.
• The repressor protein is synthesized by trpR.
• When a corepressor is present, in this case tryptophan, it binds to the repressor protein in an allosteric site.
• Thischanges the conformation of the protein such that it can bind to the operator and block transcription by preventing the binding of RNA polymerase to the promoter

Arabinose operon

• The L-arabinose operon, also called ara operon, encodes enzymes needed for the catabolism of arabinose to xylulose 5-phosphate.
• It is both inducer and repressor
• In th presence of arabinose, the AracC protein binds to the ara I region and, when bound to cAMP, the CAP protein binds to a site adjacent to araI.
• This stimulates the transcription of the araB, araA, and araD genes.
• In the absence of arabinose, the AraC protein binds to both the araI and araO regions, forming a DNA loop. This prevents transcription of the ara operon.

Gal operon

• The gal operon is a prokaryotic operon, which encodes enzymes necessary for galactose metabolism
• It's works via binding of repressor molecules to two operators.
• These repressors dimerize, creating a loop in the DNA.
• The loop as well as hindrance from the external operator prevent RNA polymerase from binding to the promoter, and thus prevent transcription.
• The gal operon of E. coli consists of 4 structural genes galE, galT, galK ,and galM