PROPOSAL ON THE NATURE OF PHYTOCHROME ACTION BASED ON THE C-TERMINAL SEQUENCES OF PHYTOCHROME

Analyzing a new phytochrome sequence, we came across a striking 23 to 26 % homology between a Cterminal section of this phytochrome sequence, spanning about 240 amino acids and the C-terminal part of bacterial sensor proteins responsive to environmental stimuli. When e.g. NtrB (protein sensing nitrogen limitation) of Bradyrhizobium was compared to this and other phytochrome sequences, it turned out that more than 40 % of the amino acids of NtrB coincided in the right position with an amino acid of at least one phytochrome. In addition, there were also clusters of amino acids that were conserved in all of the screened proteins. The pattern of conservation is even reflected in hydropathy profiles. These findings suggest that the structural homologies between bacterial sensor proteins (which are known to confer information on the environment by conformational changes on a fitting regulator protein) and phytochromes may also mean functional homologies. The new phytochrome sequence, which was isolated by means of a monoclonal anti-phytochrome antibody from a cDNA library of a green plant (Selaginella) in the expression vector lambda gt 11, consists of 1471 nucleotides coding for the 406 C-terminal amino acids of a phytochrome. The residual nucleotides are 3'-untranslated. On the amino acid level, the new sequence shares homologies with other phytochromes that range between 57,6 % (phytochrome B of Arabidopsis)and 45.3 % (phytochroma of Oryza). © 1991, Gustav Fischer Verlag, Stuttgart. All rights reserved.