Hydrolysis of pyrophosphate makes phosphodiester bond a highly favorable reaction with a DG of –7 kcal/mol, which corresponds to an equilibrium constant (Keq) of 105.

TLS Online TPP Program

#Id: 6397

#Unit 2. Cellular Organization

Why is this extra control of two layers on trp operon needed?  The answer probably lies in the fact that repression of the trp operon is weak—much weaker.

TLS Online TPP Program

#Id: 6398

#Unit 2. Cellular Organization

Repression-Trp aporepressor dimer binds high tryptophan, changes conformation with high affinity for trp operator

Tryptophan is a corepressor

TLS Online TPP Program

#Id: 6399

#Unit 2. Cellular Organization

trp operon can run even in the presence of repressor. The attenuation system permits another 10-fold control over the operon’s activity.  Thus, the combination of repression and attenuation controls the operon over a 700-fold range, from fully inactive to fully active: (70-fold [repression] X 10-fold [attenuation] = 700-fold). 

TLS Online TPP Program

#Id: 6400

#Unit 2. Cellular Organization

Attenuation is primarily based on riboswitch (Leader sequence) that function in co-translationally in trp operon

TLS Online TPP Program

#Id: 6401

#Unit 2. Cellular Organization

Attenuator riboswitch (Leader sequence) is made up of almost 140 nt which further can be divided into four Intra -complementary regions 

TLS Online TPP Program

#Id: 6402

#Unit 2. Cellular Organization

Attenuator riboswitch (Leader sequence) contains two major check-points in region 1

UGG codons on position 10, 11

A stop codon UGA between Region 1 and 2