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 5-aminolevulinate dehydratase (ALAD; EC 2.4.1.24) of a plant. A fulllength cDNA clone (1727 bp) encoding this enzyme has been identified by immunoscreening a lambda gt 11 cDNA library of spinach. ALAD is not a plant-specific enzyme; however, the plant enzyme differs from the well known ALAD enzymes of bacteria, yeast and animals in structural and biochemical properties and in that it is located in the plastid. Differences and homologies can be traced back to the molecular level. The mature ALAD subunit, whose N-terminus was determined by automatic Edman degradation, is a protein of 367 amino acid residues and has a Mr of 40,132. This figure is in the range of molecular weights of non-plant ALADs. The active centre is highly conserved and the same is true for the ion-binding domain, except that 4 cysteines of the non-plant enzymes (binding Zn2+) have disappeared and a total of 6 aspartic acids meets the demands of Mg2+-binding. However, there are more distinct differences. Apart from a transit sequence of 56 amino acids targeting the plastid, the N-terminal part of the mature plant enzyme differs considerably from non-plant ALAD enzymes. It is rich in prolines and hydroxylated amino acids. The apparent Mr on SDS-PAGE is 45,000 or higher, but up to now posttranslational modifications have not been found.