PLANT PHYSIOLOGY , Vol 101, Issue 1 97-103, Copyright © 1993 by American Society of Plant Biologists
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CELL BIOLOGY AND SIGNAL TRANSDUCTION |
Phytochrome Levels in the Green Alga Mesotaenium caldariorum Are Light Regulated
L. Z. Morand, D. G. Kidd and J. C. Lagarias
Department of Biochemistry and Biophysics, University of California, Davis, California 95616
Experiments undertaken in this investigation examine the influence of light
on the levels of phytochrome in the green alga Mesotaenium caldariorum and
also provide partial protein sequence of the algal phytochrome.
Immunochemical and spectrophotometric measurements reveal that phytochrome
levels increase nearly 4-fold upon transfer of light-grown algal cells to
total darkness during a 6- to 8-d adaptation period. Within 24 h after
return to continuous illumination, the level of phytochrome in dark-adapted
cells has decreased to that found in light-grown cells. Red or far-red
light experiments show that both effects of light, phytochrome accumulation
during dark adaptation and light-dependent decrease of phytochrome, do not
depend on the form of the phytochrome photoreceptor (i.e. far-red absorbing
or red absorbing) present in the algal cell. The light-dependent reduction
of phytochrome in dark-adapted cells is inhibited by the photosynthetic
electron transport inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethyl urea,
suggesting that this light effect is mediated by photosynthesis.
Microsequence analyses of internal peptides indicate that algal phytochrome
purified from dark-adapted cells shares the greatest sequence identity with
phytochrome from the fern Selaginella (74%). Compared with higher plant
photoreceptors, Mesotaenium phytochrome appears to be more closely related
to phyB gene products (i.e. 62 and 63% average sequence identity) than to
phyA gene products (i.e. 50 and 53% average sequence identity). Because
light regulation and the structure of Mesotaenium phytochrome do not
conform with either type I (light-labile) or type II (light-stable)
phytochromes from higher plants, these results support the hypothesis that
the lower green plant photoreceptors represent a distinct class of
phytochrome.
