GENE REGULATION –PART 2

TRYPTOPHAN OPERON  (Trp Operon)

• It is a gene control unit present in E.coli
• Contains a cluster of genes that control and regulate the synthesis of the Amino acid Tryptophan
• It has two regions – i) Control or regulatory region and  ii) Structural region

Control region

• Control region has 3 genes- i) Trp R   ii)  a region called P and  iii) Trp L along with an attenuator sequence
• Trp R may be present upstream of the operon and is responsible for a  m-RNA that produces a repressor protein
• P refers to the promoter that also has the operator sequence
• RNA polymerase binds to Promoter
•  Repressor binds to Operator
• Trp L is the leader  sequence that also has the attenuator sequence that can cause looping near the  structural genes to bring about Attenuation or antitermination
• Attennuation is premature termination of transcription taking place in Prokaryotes
• It happens when there are too many charged t-RNAs
• But transcription is halted temporarily by looping thus blocking  the RNA polymerase activity

Structural Region

• Structural region has 5 genes namely TrpE, TrpD, TrpC, TrpB and TrpA
• All the 5 genes are involved in the metabolic pathway for the synthesis of Tryptophan
• These genes produce enzymes that catalyse various reactions in the biosynthetic pathway of Tryptophan synthesis
• TrpE gene contains genetic information for antharanilate synthetase subunit I
• TrpD gene for anthranilate synthetase subunit II
• TrpC for Glycerol phosphate synthase
• TrpB for Tryptophan synthase Unit I and
• Trp A for Tryptophan synthase Unit II
• These 5 genes transcribe a polycistronic m- RNA that will produce the above 5 polypeptides
• These polypeptides will ultimately lead to the production of Tryptophan 

Mode of action

Operon Off

• The repressor protein produced by gene Trp R  can bind to the effector (also called co-repressor) namely Tryptophan
• The repressor now  becomes activated by undergoing a conformational change and is able to bind at the operator  region and thus  transcription is stopped
• This happens when Tryptophan concentration in the cell is very high and no more synthesis is required
• Thus the Operon is switched off   

• When Tryptophan levels in the cell become very low,   binding of effector and repressor becomes loose and the effector is released
• The repressor’s conformation is altered and it  no longer binds at the operator
• Hence RNA polymerase is able to move along the Operon and transcription starts
• Operon is switched On
• Enzymes are produced
• Tryptophan is synthesised   
• This Operon shows alternating switching On and Off based on the levels of Tryptophan in the cell