Formation of the closed complex, in contrast, is readily reversible: polymerase can as easily dissociate from the promoter as make the transition to the open complex.

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#Unit 6. System Physiology – Plant

Proportion of phytochrome in the Pfr form after saturating irradiation by red light is only about 85%. 

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#Unit 6. System Physiology – Plant

Very small amount of far-red light absorbed by Pr makes it impossible to convert Pfr entirely to Pr by broad spectrum far-red light. An equilibrium of 97% Pr and 3% Pfr is achieved. This equilibrium is termed the photostationary state.

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#Unit 6. System Physiology – Plant

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#Unit 6. System Physiology – Plant

The N-terminal a PAS domain 

a GAF domain with bilin-lyase activity, which binds the chromophore, 

PHY domain, which stabilizes phytochrome in the Pfr form

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#Unit 6. System Physiology – Plant

PAS-related domain (PRD) repeats mediate phytochrome dimerization. 

The PRD domain targets the Pfr form of phyB to the nucleus, although it lacks a canonical nuclear localization signal (NLS).

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#Unit 6. System Physiology – Plant

In higher plants the phytochrome chromophore is a linear tetrapyrrole called phytochromobilin