EUKARYOTIC PHYTOCHROMES - LIGHT-REGULATED SERINE THREONINE PROTEIN-KINASES WITH HISTIDINE KINASE ANCESTRY/

The discovery of cyanobacterial phytochrome histidine kinases, togethe r with the evidence that phytochromes from higher plants display prote in kinase activity, bind ATP analogs, and possess C-terminal domains s imilar to bacterial histidine kinases. has fueled the controversial hy pothesis that the eukaryotic phytochrome family of photoreceptors are light-regulated enzymes. Here we demonstrate that purified recombinant phytochromes from a higher plant and a green alga exhibit serine/thre onine kinase activity similar to that of phytochrome isolated from dar k grown seedlings. Phosphorylation of recombinant oat phytochrome is a light- and chromophore-regulated intramolecular process. Eased on com parative protein sequence alignments and biochemical cross-talk experi ments with the response regulator substrate of the cyanobacterial phyt ochrome Cph1, we propose that eukaryotic phytochromes are histidine ki nase paralogs with serine/threonine specificity whose enzymatic activi ty diverged from that of a prokaryotic ancestor after duplication of t he transmitter module.