Computer Analysis of Phytochrome Sequences from Five Species: Implications for the Mechanism of Action

The amino acid sequences of phytochrome from Avena sativa, Oryza sativa, Curcurbita pepo, Pisum sativum and Arabidopsis thaliana have been analyzed with a variety of computer programs, with a view to identifying areas of the protein which contribute to the properties of this photoreceptor. A region at the C-terminus has been shown to be amphiphilic, and by analogy with surface-seeking peptides, may be responsible for interaction of phytochrome with lipid bilayers. Possible targeting sequences in phytochromes have been identified, including a series of four basic residues which correspond to those responsible for transport of nuclear-located proteins. Sites capable of post-translational modification have been found in monocot sequences, but not in dicot sequences. Areas of the phytochrome molecule which are exposed on the surface of the protein, and which are therefore capable of interaction with other cellular macromolecules, have been identified. Analogies with other biliproteins have been used to define minimum chromophore-protein interactions. Possible enzymic activities associated with phytochromes have been discussed with respect to local amino acid sequence similarity with enzymes.