InterPro domain: IPR044767

Canonical phytochromes have a conserved N-terminal photosensory core and a C-terminal regulatory region which typically includes a histidine-kinase-related domain (HKRD, also known as the HATPase domain) [ 1 ]. This entry represents the histidine-kinase-related domain found in Arabidopsis thaliana phytochrome phyA-phyE [ 2 , 3 ]. Following red light absorption, biologically inactive forms of phytochromes convert to active forms, which rapidly convert back to inactive forms upon far-red light irradiation. Phytochromes control the expression of a number of nuclear genes including those encoding the small subunit of ribulose-bisphosphate carboxylase, chlorophyll A/B binding protein, protochlorophyllide reductase or rRNA, and they also regulate temperature responses by associating with the promoters of key target genes in a temperature-dependent manner and subsequently repressing their expression probably in a PIF4-dependent manner [ 4 ]. Phytochromes can be considered as having an N-terminal photosensory region to which a bilin chromophore is bound, and a C-terminal output region, which includes the HATPase domain, and is involved in dimerisation and presumably contributes to relaying the light signal to downstream signalling events.

1. Phytochrome structure and signaling mechanisms. Annu Rev Plant Biol 57, 837-58
2. FAR-RED ELONGATED HYPOCOTYL1 and FHY1-LIKE associate with the Arabidopsis transcription factors LAF1 and HFR1 to transmit phytochrome A signals for inhibition of hypocotyl elongation. Plant Cell 21, 1341-59
3. Phytochromes function as thermosensors in Arabidopsis. Science 354, 886-889