Classification of phytochromes

Using DNA sequence to classify phytochromes

As more and more phytochrome and phytochrome like sequence information has become available from different organisms it has enabled them to be classified based upon common motifs. The structure of plant phytochromes has been conserved throughout the evolution of plants. Phytochromes from plants (plant phy) have three conserved domains on their N-terminal region; these are the PAS / P2 domain, the GAF / P3 domain and the PHY / P4 domain, and a C-terminal HKRD region. These domains are also present in the phytochromes of cyanobacteria and many bacteriophytochromes. Plant phytochromes can be distinguished from the organisms in that they have a domain known as P1; this domain is thought to be involved in the inhibition of dark reversion of P_fr to P_r. Additionally Plant phytochromes have a further two PAS domains (PAS1 and PAS2). The phytochromes from other organisms such as fungi keep the core Pas / GAF / PHY domain but have a C-terminal response regulator RR / REC motif. The PAS and GAF domains of phytochromes are synonymous with other signalling molecules, whereas the PHY domain seems to be more specific to phytochromes themselves.

Further information on phytochromes

Phytochromes make use of bilin chromophores, these chromophores photoisomerise when P_r is converted to P_fr.

Regulation of the process takes place through conserved N-terminal phytochrome photosensors that can fuse to regulatory domains. Phytochromone's are still evolving.