In dark-grown or etiolated plants, phytochrome is present in a red light–absorbing form, referred to as Pr because it is synthesized in this form.

TLS Online TPP Program

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#Unit 2. Cellular Organization

Trp operon is negatively regulated by two ways

1. Repression

2. Attenuation

Why is this extra control of two layers on trp operon needed? 

TLS Online TPP Program

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#Unit 2. Cellular Organization

The answer probably lies in the fact that repression of the trp operon is weak—much weaker.

TLS Online TPP Program

#Id: 612

#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: 613

#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

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#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: 615

#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