Nucleotide binding domain serves as a built-in protein kinase that functions to phosphorylate the P domain.

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#Unit 6. System Physiology – Plant

Pfr, in turn, can be converted back to Pr by far-red light known as photoreversibility

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#Unit 6. System Physiology – Plant

When Pr molecules are exposed to red light, most of them absorb it and are converted to Pfr, but some of the Pfr also absorbs the red light and is converted back to Pr because both Pr and Pfr absorb red light. 

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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