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THE PLANT CELL, Vol 8, Issue 5 859-871, Copyright © 1996 by American Society of Plant Biologists
Two Small Spatially Distinct Regions of Phytochrome B Are Required for Efficient Signaling Rates
D. Wagner, M. Koloszvari and P. H. Quail
Department of Plant Biology, University of California-Berkeley, Berkeley, California 94720
We used a series of in vitro-generated deletion and amino acid substitution
derivatives of phytochrome B (phyB) expressed in transgenic Arabidopsis to
identify regions of the molecule important for biological activity.
Expression of the chromophore-bearing N-terminal domain of phyB alone
resulted in a fully photoactive, monomeric molecule lacking normal
regulatory activity. Expression of the C-terminal domain alone resulted in
a photoinactive, dimeric molecule, also lacking normal activity. Thus, both
domains are necessary, but neither is sufficient for phyB activity.
Deletion of a small region on each major domain (residues 6 to 57 and 652
to 712, respectively) was shown to compromise phyB activity differentially
without interfering with spectral activity or dimerization. Deletion of
residues 6 to 57 caused a large increase in the fluence rate of continuous
red light (Rc) required for maximal seedling responsiveness, indicating a
marked decrease in efficiency of light signal perception or processing per
mole of mutant phyB. In contrast, deletion of residues 652 to 712 resulted
in a photoreceptor that retained saturation of seedling responsiveness to
Rc at low fluence rates but at a response level much below the maximal
response elicited by the parent molecule. This deletion apparently reduces
the maximal biological activity per mole of phyB without a major decrease
in efficiency of signal perception, thus suggesting disruption of a process
downstream of signal perception. In addition, certain phyB constructs
caused dominant negative interference with endogenous phyA activity in
continuous far-red light, suggesting that the two photoreceptors may share
reaction partners.
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