Different terminators require different levels of rho factor for termination and therefore respond differently to the residual levels of rho factor in the mutants (rho mutants are usually leaky).

TLS Online TPP Program

#Id: 6394

#Unit 2. Cellular Organization

Major enzyme is Tryptophan Synthase

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).