ARE THE PHYTOCHROMES PROTEIN-KINASES

The biochemical mechanism of phytochrome action is unknown. We have ex amined the proposal, based on sequence similarities to the sensor hist idine kinase components of bacterial two-component signaling systems, that the phytochromes may be functional homologs of these kinases. Fou r amino acids, three highly conserved between the phytochrome and bact erial kinase molecules and the other, the histidine residue putatively the target of autophosphorylation, were changed singly in the oat phy tochrome A sequence by in vitro site-directed mutagenesis, and the res ultant mutant photoreceptor molecules were assayed for activity by ove rexpression in transgenic Arabidopsis. Three of the four-mutant molecu les retained activity equivalent to that of the unmutagenized parent s equence, whereas the fourth mutant could not be evaluated because of l ow expression. The data show that the former three mutagenized residue s are not essential for phytochrome A function in transgenic Arabidops is, but, because of the negative nature of the results, the possibilit y cannot be precluded that the photoreceptor functions as a protein ki nase independent of these residues.