I NTRODUCTIQNPkytochrome is one of the most fasrinsring; proteins in plants. A great many striking photomorphoyenctie proccssex arc mediated by it (see [l]). Phytechromc is well charactcrizcd by biochemical and immunological techniques and by methods OF molecular biology, Dcspirc this fact, the mechanism of action of phytochrome, the molecular links between its conformational alterations and the ensuing biochemical and morphogenetic phenomena are still unknown, Many sophisticated experiments have not been able to unveil the secret of light-induced signal-transduction by phytochromc. Here we report on findings which we assume could be evidence of the mode of action and the phylogeny of this light-sensing protein.
MATERIALS AND METHODS I. Prepurct~ion of clones uttd scqrrctlcitrgLibraries of cDNA were constructed in the Xgti 1 expression vector.
Phages giving a positive response with a mo~wA~nal antibody (Z-3Bl
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.
We have screened a cDNA library of the moss Physcomitrella patens (Hedw.) for phytochrome sequences. The isolated sequences turned out to encode a phytochrome dissimilar to the phytochrome type postulated for the moss Ceratodon [(1992) Plant Mol. Biol. 20, 1003-1017] Physcomitrella phytochrome was completely alignable to fern phytochrome (Selaginella) and phytochromes of higher plants. The frequency of clones encoding this phytochrome indicated that a Ceratodon-like type should only be expressed, if at all, with lower frequencies than the sequenced phytochrome cDNA. Sequence differences between lower plant phytochromes are small as compared to phytochrome types of higher plants.
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