Are the phytochromes protein kinases?

The biochemical mechanism of phytochrome action is unknown. We have examined the proposal, based on sequence similarities to the sensor histidine kinase components of bacterial two-component signaling systems, that the phytochromes may be functional homologs of these kinases. Four amino acids, three highly conserved between the phytochrome and bacterial kinase molecules and the other, the histidine residue putatively the target of autophosphorylation, were changed singly in the oat phytochrome A sequence by in vitro site-directed mutagenesis, and the resultant mutant photoreceptor molecules were assayed for activity by overexpression in transgenic Arabidopsis. Three of the four-mutant molecules retained activity equivalent to that of the unmutagenized parent sequence, whereas the fourth mutant could not be evaluated because of low expression. The data show that the former three mutagenized residues are not essential for phytochrome A function in transgenic Arabidopsis, but, because of the negative nature of the results, the possibility cannot be precluded that the photoreceptor functions as a protein kinase independent of these residues.