8/18/2019 Light Regulation of Phytochrome Functions

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Light regulation of phytochrome functions. A, light-regulated His kinase activity

initiates phytochrome-mediated light signaling in prokaryotes. The cyanobacterial

phytochrome 1 (Cph1) has light-modulated protein kinase activity.

Autophosphorylation is attenuated by red light and increased by far-red light.

hosphorylated r could trigger a light signal. The phosphorylated r transfers its

phosphate to the !cp response regulator" this could in turn initiate light signaling.hosphorylated !cp could transfer this phosphate to a yet to be determined

acceptor ( X ). #ote that for most plant phytochrome responses fr is the active

isoform. B,nuclear events in plant phytochrome signaling. hytochrome $ only

translocates into the nucleus in its far-red light absorbing form (fr$). %n the nucleus

fr$ interacts &ith the '#A-bound % transcription factor. hytochrome A

translocates into the nucleus in its red light absorbing form* r+ (r that has been

cycled through fr" this occurs preferentially in far-red rich light). Translocation also

occurs for fr (the far-red absorbing form)" ho&ever* irradiation of plants &ith red

light triggers rapid phyA degradation. %n the nucleus phyA presumably a,ects !1-

mediated gene activation. 1 is a negative regulator of phytochrome signaling" it

has been proposed that 1 inhibits nuclear translocation of phytochromes. C,

other light-regulated properties of plant phytochrome. /ight a,ects the er0Thr

protein kinase activity of phytochromes. Contrary to Cph1* oat phyA has higher

auto- and trans-phosphorylation activity in the fr form. %nteraction &ith signaling

intermediates is also isoform-dependent" this is true for %* &hich is nuclear* and

for #'* &hich is found both in the nucleus and the cytoplasm. The protein

stability of phyA is very much reduced in its far-red absorbing form (fr).