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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.

Phytochrome Levels in the Green Alga Mesotaenium caldariorum Are Light Regulated