We have sequenced cDNA and genomic clones coding for phytochrome of the fern Selaginella. On the amino acid level, this phytochrome shares sequence homologies with phytochromes of higher plants which range between 62 (phytochrome B of Arabidopsis) and 55 (56)% [phytochrome C of Arabidopsis (Avena)]. Introns in the Selaginella gene are short and occupy positions known from phytochrome sequences of higher plants. A rooted phylogenetic tree based on mutation distances puts Selaginella phytochrome closest to the hypothetical ancestor. A similar tree arises if the tree is constructed with partial sequences (about 200 amino acids) around the chromophore attachment site. An extension of this tree by sequences of other cryptogamic plants (Mougeotia, Ceratodon, Psilotum) shows all these sequences including those of the phytochromes B and C of Arabidopsis on a branch, well separated from the branch formed by phytochromes known to accumulate in etiolated plants. The rooted phytochrome phylogenetic tree, however, is difficult to reconcile with the fossil record.
We have isolated phytochrome genes from the moss Physcomitrella, the fern Psilotum and PCR‐generated phytochrome sequences from a few other ferns. The phytochrome gene of the moss Physcomitrella turned out not to contain the aberrant C‐terminal third of the phytochrome from the moss Ceratodon, but the transmitter module‐like sequences found in other phytochromes. A series of different phytochrome genes was detected in Psilotum. Differences between the amino acid sequences derived from them ranged from about 5 to more than 22%. Some of these genes are likely pseudogenes. Analysis by phylogenetic tree constructions revealed that higher and lower plant phytochromes evolved with different velocities. Lower plant phytochromes form a separate family characterized by a high degree of similarity. The amino acid differences between phytochrome types detected in a single species of higher plants are about two‐fold higher than the differences between phytochromes of species of lower plants belonging to different divisions (Physcomitrella and Selaginella). Future studies on phytochrome sequences may eventually also throw light on the significance of Psilotum in the evolution of vascular plants.
Phytochrome evolution: Phytochrome genes in ferns and mosses. -Physiol. Plant. 91: 241-250.We have isolated phytochrom'e genes from the moss Physcomitrella, the fern Psilotum and PCR-generated phytochrome sequences from a few other ferns. The phytochrome gene of the moss Physcomitretla tumed out not to contain the aberrant C-terminal third of the phytochrome from the moss Ceratodon, but the transmitter module-like sequences fonnd in other phytochromes. A series of differeni phytochrome genes was detected in Psilotum, Differences between the amino acid sequences derived from them ranged from about 5 to more than 22%. Some of these genes are likely pseudogenes. Analysis by phylogenetic tree constructions revealed that higher and lower plant phytocbromes evolved with different velocities. Lower plant phytochromes form a sepai'ote family characterized by a high degree of similarity. The amino acid differences bettt'een phytochrome types detected in a single species of higher plants are about two-fold higher than the differences between phylochromes ' Of species of lower plants belonging to different 'divisions (Physcomitrella and Selaginelki), Future studies on phytochrome sequences may eventually also throw light on the significance of P.'iilotum in the evolution of vascular plants.
C-terminal amino acid sequences of two phytochromes sharing less than 40 per cent homology and two transmitter modules of bacterial sensor proteins that share homologies with phytochromes were examined by hydrophobic cluster analysis. Striking coincidences in distribution, size, shape and orientation of hydrophobic and hydrophilic domains of the bacterial transmitter modules and phytochrome sequences were revealed. The results corroborate the view that the folding of the homologous regions of the two groups of proteins is similar. Since the mode of action of phytochrome is not known, the structural coincidences may be indicative of how phytochrome transmits signals, although distinct differences should not be over looked. Some coinciding structural features in phytochromes and bacterial sensors may tentatively be extracted from secondary structure predictions.
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