Phytochromes as light-modulated protein kinases

Many phytochrome responses in plants are induced by red light and inhibited by far-red light. To explain the biochemical basis of these observations, it was speculated that plant phytochromes are light-regulated enzymes more than 40 years ago. The search for such an enzymatic activity has a long and rather tumultuous history.(1,2) Biochemical data in the late 1980s had sug gested that oat phytochrome might be a light-regulated protein kinase. The topic was the subject of intense debate, but solid experimental data backin g the kinase model has been published recently. Two lines of research playe d a key role in this finding: the production of biologically active highly purified recombinant phytochrome and the discovery of phytochromes in proka ryotes. This review discusses the key steps of this discovery and suggests some hypotheses for the role of protein kinase activity in photomorphogenes is.