If the planarian is cut in half, the anterior portion of the lower half regenerates a head while the posterior of the upper half regenerates a tail. 

TLS Online TPP Program

#Id: 6395

#Unit 2. Cellular Organization

Major enzyme is Tryptophan Synthase whose activity is reduced when trp concentration is high

TLS Online TPP Program

#Id: 6396

#Unit 2. Cellular Organization

Trp operon is negatively regulated by two ways

 Repression

Attenuation

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