Serine-to-alanine substitutions at the amino-terminal region of phytochrome A result in an increase in biological activity.

We have used a tobacco transgenic plant system to assay the structure/function relationship of phytochrome A (phyA), a plant photoreceptor. The amino terminus of phyA from different plant species is very rich in serine residues. To investigate whether these serine residues are required for phytochrome function, the first 10 serine codons encoding amino acid residues 2-4, 10-14, 19, and 20 in the amino-terminal domain of the rice phyA gene (phyA) were changed to alanine codons. The mutant (S/A phyA), as well as the wild-type phyA cDNA, was placed under the control of the 35S promoter, and the chimeric genes were transferred into the tobacco genome by Agrobacterium-mediated transformation. Transgenic tobacco plants expressing either wild-type or S/A phyA showed similar phenotypic alterations, including dwarfism and dark-green leaves. However, hypocotyl elongation experiments revealed that transgenic seedlings expressing S/A phyA showed a higher amplitude of the red light response with respect to the inhibition of hypocotyl elongation. The observed difference is not correlated with expression levels of the transgene. The chromophore is attached to the mutant phyA apoprotein (PHY A), and the mutant photoreceptor is photoreversible, giving a difference spectrum indistinguishable from that of the rice phyA. Our results indicate that the S/A mutant has a higher biological activity as compared with the wild-type rice phyA.