In bright light, little transducin is found in the outer segment, and abundant arrestin is found there; in this condition,vision is relatively insensitive to small changes in light.

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

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Pr chromophore undergoes a cis–trans isomerization between carbons 15 and 16 and rotation of the C14-C15 single bond on red light exposure.

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Phytochromobilin is synthesized in plastids from chlorophyll biosynthetic pathway and transported into cytosol by a passive process.

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Phytochromobilin is exported from the plastid to the cytosol where it autocatalytically attaches to the PHY apoprotein through a thioether linkage to a cysteine residue.